Lightning
Ed Huesers
> > Ed Huesers wrote:
> >>Would an igloo provide an adequate faraday
> >>shield? How large can the door/opening be?
> >> Would one strike melt a hole through an 8' thick snow wall?
Gary Coffman wrote:
> Ice and snow are very poor conductors. Ion mobility is nearly zip.
An article in Scientific American made reference to a quazi liquid
layer existing on the surface of ice "until tens of degrees below
freezing".
> An effective Faraday cage has to be a *very good* conductor of
> electricity. The very high conductivity forces the current to remain
> on the outside of the Faraday cage, leaving the potential inside
> effectively zero. So an igloo won't act as an effective Faraday cage.
> (A steel bodied car will, though.)
A Faraday cage has openings compared to an igloo and I've heard that
the high voltage flow charges the surface of an object somewhat like a
capacitor.
But this seems flawed as what would make a trees sap boil and then
have the tree explode if the current didn't flow through the mass of the
tree.
> It is a truism that you can't insulate against lightning. A bolt that
> has passed through miles of air, itself a pretty good insulator,
> won't be stopped by any feasible amount of insulation you can
> interpose between cloud and Earth.
Yes, the misconception that the rubber tires of a vehicle insulate
when it is actually the Faraday cage principal that makes a vehicle
safe.
Make you wonder about the plastic cars they're making nowa days.
> Picture lightning as a constant current source backed by hundreds
> of millions of volts. The ionized air channel seems to act as a current
> regulator.
Much the same as dielectric in an EDM.
> Nearly all lightning bolts have an air channel of about the
> same diameter, about 6 inches, and this seems to serve to fix the
> current at about 20 kiloamps. (Magnetic pinch effects in the plasma
> are probably at work here.) There are atypical "superbolts" with
> currents up to 200 kA, but they are aberrations, usually the result
> of multiple step leaders merging.
>
> What you want to do instead of trying to block the bolt, which you
> effectively can't with any practical thickness of insulation (miles of
> air couldn't do it), is to offer the lightning a more attractive path to
> Earth. That's what lightning rods do.
Well, the lightning rod is not much of an option for backpackers.
Maybe, if they were made of *unobtainium*.;o)
> By streaming early (point discharge causing a stream of ions to rise
> from the tip of the rod), they effectively form the tallest and best
> conductive paths for the bolt to travel to Earth. Lightning has to follow
> electrical laws just like any other electrical current flow, and the majority
> of the current will flow along the lowest impedance path offered to it.
So, standing next to a tree is not a good idea... blood conducts
better than sap?
> You've probably noticed that lightning bolts tend to assume a jagged
> (fractal) shape in air. That's because minute differences in conductivity
> of the air in front of the bolt (due to dust, stray ions, etc, in the air) causes
> the step leaders to wander in order to travel over the lowest impedance
> path. But offer the bolt a better low impedance path, and it will take it. A
> lightning rod and downlead does this.
> Note too that lightning is effectively RF. Because it is a sharp risetime
> pulse, Fourier analysis shows it to consist of odd harmonics with significant
> energy reaching all the way up into UHF. So this has to be kept in mind
> when designing lightning mitigation systems. Reactance in the path is at
> least as important as conductivity. That's why you want to avoid any sharp
> bends in the downlead of a lightning rod system. Even small amounts of
> inductance will cause large potentials to be expressed across them when
> currents are in the 20 kA range.
>
> For example, the inductance of a 10 foot section of Rohn 25 tower is
> sufficient to cause a 20 kV potential to be expressed across it when
> a 20 kA pulse is flowing down it. If your antenna feedline is swung off
> the tower at the 10 foot height, it will carry that 20 kV into your house
> and to your equipment. Best practice is to always bring feedlines all
> the way down to Earth before bringing them over to equipment. That
> allows the potential to be shorted out at the tower base with a suppressor.
>
> The real world is a very complicated place, and the currents and fields
> involved with the lightning discharge are huge. This makes effective
> lightning mitigation design very difficult. We simply can't model every
> interaction precisely. So we have to follow best practice rules of thumb
> and hope we haven't overlooked anything significant.
I'm still curious as to what determines the maximum opening size in a
Faraday cage. The door opening in an igloo needs to be a least shoulder
width and could make the whole thing a moot point.
Thanks to you and Mark for playing.
Ed Huesers
http://www.grandshelters.com
Jerry M. Wright
wrote:
>
> I'm still curious as to what determines the maximum opening size in a
>Faraday cage.
The frequency of the electromagnetic radiation you are trying to
block. For high frequency stuff, very fine mesh - for lower frequency
larger openings can be tolerated. The opening should be at least less
than 1/2 wavelength and much smaller is preferable if you need to a
large degree of attenuation.
H. Paul Jacobson
>
> Gary Coffman wrote:
> > Ice and snow are very poor conductors. Ion mobility is nearly zip.
>
> An article in Scientific American made reference to a quazi liquid
> layer existing on the surface of ice "until tens of degrees below
> freezing".
But the layer does not provide much electrical conductivity. The
conductivity of ice is low enough that a battery powered radar system can
image the ground under a 3 km thick ice sheet. Flowable Liquid water
(such as at the base of the ice) is a strong reflector to such
systems. Impurities in the ice alter the conductivity enough to serve as
markers for events like volcanic eruptions, both when imaged by radar, and
when measured with probes in an ice core.
Paul
Ron Wong
Wright) wrote:
If Jerry's right then you shouldn't have to worry about a door opening -
at least not if it's part of a Faraday cage.
Lightning tends to generate electromagnetic waves in the kilohertz range
(thus all the static on AM radio compared to FM when an electrical storm
occurs). 740 khz (KSFO here in San Francisco) has a wavelength of about
400 m. 1550 khz has a wavelength of about 200 m.
If the doorway was 0.5 m wide, then the largest wavelength that would get
through would be 1 m and its corresponding frequency would be off the
charts - that is, not the type generated by a lightning strike (more in
the microwave region).
ron
Greg Rose
"Ed Huesers" <e...@grandshelters.com> wrote:
>
> Well, the lightning rod is not much of an option for backpackers.
> Maybe, if they were made of *unobtainium*.;o)
Well, then there's that double barreled shotgun I was carrying once on a
ridge when a lightening storm struck.
Maybe I should have planted it in the ground and got away.
As it is I just ran in panic.
Hey, it worked.
;^)
Greg Rose
JJVW
mid-1800's it was trendy in women's fashions to have a chain attached to the back of
your hat that dragged on the ground behind you. This was to protect you from all those
lightning strikes that were taking people down all over the streets of Paris at the
time.
Ed Huesers wrote:
>
>
> Well, the lightning rod is not much of an option for backpackers.
> Maybe, if they were made of *unobtainium*.;o)
> Ed Huesers
> http://www.grandshelters.com
Lou
"JJVW" <jj...@earthlink.net> wrote in message
news:3B26F850...@earthlink.net...
> Don't let that stop the French. I remember reading a while back that
sometime in the
> mid-1800's it was trendy in women's fashions to have a chain attached to
the back of
> your hat that dragged on the ground behind you. This was to protect you
from all those
> lightning strikes that were taking people down all over the streets of
Paris at the
> time.
>
> Ed Huesers wrote:
After reading this I could not help but think of Galen and his
"Bear-Pruf"
tent line. Maybe he could incorporate the waste from tent manufacturing
into a new line of hats.
Gary Coffman
>If Jerry's right then you shouldn't have to worry about a door opening -
>at least not if it's part of a Faraday cage.
>
>Lightning tends to generate electromagnetic waves in the kilohertz range
>(thus all the static on AM radio compared to FM when an electrical storm
>occurs). 740 khz (KSFO here in San Francisco) has a wavelength of about
>400 m. 1550 khz has a wavelength of about 200 m.
>
>If the doorway was 0.5 m wide, then the largest wavelength that would get
>through would be 1 m and its corresponding frequency would be off the
>charts - that is, not the type generated by a lightning strike (more in
>the microwave region).
Not quite accurate. The reason you hear static on an AM radio is that the
radio is AM, ie it is designed to respond to amplitude variations of the RF
signal. The reason you don't hear static on FM is because the receiver
is designed to *not* respond to amplitude variations (it is designed to
respond only to frequency variations). It does that with a hard limiter in
front of the discriminator.
The majority of the energy in a lightning pulse is indeed below 1 MHz,
but there is still significant energy all the way up into the UHF spectrum
(300-3000 MHz, or in wavelengths, 1 meter to 10 cm). Commercial FM
broadcasting is in the range of 88 to 108 MHz (VHF). Lightning is still
a problem for these stations. There is sufficient energy in this spectrum
which can pass through channel filters and get back into the transmitter
equipment that protective measures have to be taken to prevent arcing
in the cavities and transmission lines.
Openings in a Faraday shield or cage have to be significantly smaller
than a wavelength in order to offer much attenuation. The rule of thumb
is that the openings should be less than 1/10th of a wavelength at the
frequencies of interest. So to be really effective against lightning energy,
any opening should be less than 1 centimeter, *and no conductor can
be allowed to pass through that opening*. Penetrations of the shielding
will conduct the energy inside as if the cage weren't there. That means
any wiring in or out of a shielded area must be properly suppressed.
Now a car body has openings larger than this, so it isn't really proof
against lightning. There have been cases reported of bolts that have
passed window to window through a car. Being in a car is much safer
than being outside, but it isn't an absolute guarantee that you won't be
injured by a lightning discharge.
Gary
Gary Coffman
>> from the tip of the rod), they effectively form the tallest and best
>> conductive paths for the bolt to travel to Earth. Lightning has to follow
>> electrical laws just like any other electrical current flow, and the majority
>> of the current will flow along the lowest impedance path offered to it.
>
> So, standing next to a tree is not a good idea... blood conducts
>better than sap?
Sort of. Lets take as example a southern pine tree, 1 foot or greater in
diameter. It has a resistance on the order of 5,000 ohms per lineal foot
to the initial lightning discharge (that decreases greatly later in the discharge
as the wood is charred and the sap is ionized). With a 20 kiloampere pulse,
that means a point 6 feet above the ground will express a potential difference
of up to
E = 20,000 * 5,000 * 6 = 600,000,000 volts (600 megavolts)
Since air has a breakdown potential of about 5,000 volts per inch,
anything that is grounded (like you standing on the wet Earth) which
is within 10,000 feet is a potential target for a *sideflash*. Of course
the Earth itself is only 6 feet away, so this appears somewhat moot.
But what this is telling us is that any target that's closer to a tree than
the tree is tall is a potential target for a sideflash.
So you don't want to stand under a *lone* tree during a lightning storm.
Now a real lightning rod is copper, and has a very low resistance per foot.
So the potential above ground at a point along the rod or its downlead is
small, and the danger of a sideflash is minimal. It effectively produces a
"cone of protection" with a base diameter similar to the height of the rod.
In other words, anything within that cone is very unlikely to be the target
of the strike because the rod will take it instead.
(Note, things change for very tall structures like towers or skyscrapers.
The cone of protection becomes a sphere of protection with a maximum
diameter on the order of about 100 feet. Lightning doesn't always hit the
*top* of a tall structure.)
The energy dump into the copper, (E * I * t) is also small, so it barely warms
when taking a strike. This isn't the case with the tree. The voltage expressed
across the tree's resistance is high, the current is high, and the energy dump
is sufficient to cause the tree to suffer a violent steam explosion. (For those
who want to calculate joules, t is on the order of 10 microseconds.)
A pine tree is a pretty good (one use) lightning rod. The many pointed
needles act as discharge points, generating ion streamers that reach
up toward the cloud (a sharp point begins to stream when the discharge
current exceeds about 20 microamps). When a step leader from the cloud
meets a streamer coming up from below, the electrical path is completed
and a stroke occurs.
So, standing (or lying) near, but not too near (I'd suggest staying at least
twice the height of the tree away from it), a lone tree can be protection
against the lightning stroke in the same way that a real lightning rod can
protect a house, ie by being the preferred target of the bolt. But do watch
out for flying splinters.
Being in a forest can actually be safer than standing on open ground.
The odds that the trees immediately adjacent to you will be struck are
relatively low. OTOH if you are the highest point in the open, or are
standing immediately next to that point, the odds of your being struck
directly or by a sideflash are pretty good.
Note that all this is pretty crude and qualitative. Exact modeling of the
lightning event is very complicated. As Mark noted, we don't really
understand every factor involved well enough to give the sort of precise
engineering answers we'd like.
(Note that the average charge exchange in a lightning strike is on the
order of 20 coulombs. If that happened over a 1 second period, the
current would only be 20 amps. But it happens over a few microseconds,
so peak amperage is very large, and extreme magnetic fields are generated.
This is the realm of magnetohydrodynamics, a plasma physics discipline
that we haven't quite mastered yet.)
Gary
Jerry M. Wright
wrote:
> rec.backcountry added back.
>
>> > Ed Huesers wrote:
>> >>Would an igloo provide an adequate faraday
>> >>shield? How large can the door/opening be?
>> >> Would one strike melt a hole through an 8' thick snow wall?
>
>Gary Coffman wrote:
>> Ice and snow are very poor conductors. Ion mobility is nearly zip.
>
> An article in Scientific American made reference to a quazi liquid
>layer existing on the surface of ice "until tens of degrees below
>freezing".
The problem here is that conductivity is proprotional to the
cross-sectional area of the conductor. A layer a few molecules thick
is not a good conductor. A well known problem in microchip design
when trying to reduce the size of the conductive pathways.
>
[snip]
Eric J. Anderson
>
> Now a real lightning rod is copper, and has a very low resistance per foot.
> So the potential above ground at a point along the rod or its downlead is
> small, and the danger of a sideflash is minimal. It effectively produces a
> "cone of protection" with a base diameter similar to the height of the rod.
> In other words, anything within that cone is very unlikely to be the target
> of the strike because the rod will take it instead.
I've a question to clarify. The previous owner of our home, a
lineman for the local electric company claimed (I think) that the
top-most wire on many power poles is nothing but a ground wire, and
that it acts just as the copper rod you describe above...in that
it actually protects the main power lines directly below it from
a direct strike. He implied that one of the safest places to stand
during a lightning stom would, in fact, be directly under a power
line (and assumably equidistant between two utility poles) since
there is a 'cone' of protection afforded by the ground wire up top.
So, is that true? (Not that you'll find a lot of power lines to
cower under in the backcountry...usually.)
> a lone tree can be protection against the lightning stroke in
> the same way that a real lightning rod can protect a house, ie
> by being the preferred target of the bolt. But do watch out for
> flying splinters.
We found a tall tree on a forested hill last summer that had
been struck. One of the "splinters" was about 12 feet long and
6" thick and was quite solidly impaled into the ground about
25 feet from the tree. It's one of those occurances that I file
under "Glad I Wasn't There When it Happened!"
Eric
Gunner ©
<er...@as.arizona.edu> wrote:
>Gary Coffman wrote:
>>
>> Now a real lightning rod is copper, and has a very low resistance per foot.
>> So the potential above ground at a point along the rod or its downlead is
>> small, and the danger of a sideflash is minimal. It effectively produces a
>> "cone of protection" with a base diameter similar to the height of the rod.
>> In other words, anything within that cone is very unlikely to be the target
>> of the strike because the rod will take it instead.
>
>I've a question to clarify. The previous owner of our home, a
>lineman for the local electric company claimed (I think) that the
>top-most wire on many power poles is nothing but a ground wire, and
>that it acts just as the copper rod you describe above...in that
>it actually protects the main power lines directly below it from
>a direct strike. He implied that one of the safest places to stand
>during a lightning stom would, in fact, be directly under a power
>line (and assumably equidistant between two utility poles) since
>there is a 'cone' of protection afforded by the ground wire up top.
>
>So, is that true? (Not that you'll find a lot of power lines to
>cower under in the backcountry...usually.)
>
Im not sure thats a particularly good idea, considering many lightning
bolts originate in the ground, and travel upwards
Gunner
>> a lone tree can be protection against the lightning stroke in
>> the same way that a real lightning rod can protect a house, ie
>> by being the preferred target of the bolt. But do watch out for
>> flying splinters.
>
>We found a tall tree on a forested hill last summer that had
>been struck. One of the "splinters" was about 12 feet long and
>6" thick and was quite solidly impaled into the ground about
>25 feet from the tree. It's one of those occurances that I file
>under "Glad I Wasn't There When it Happened!"
>
>Eric
"Of all tyrannies, a tyranny sincerely exercised for the
good of its victims may be the most oppressive. It may be
better to live under robber barons than under omnipotent
moral busybodies. The robber baron's cruelty may sometimes
sleep, his cupidity may at some point be satiated: but those
who torment us for our own good will torment us without end,
for they do so with the approval of their own conscience."
C.S. Lewis
James Wilkins
>
> >I've a question to clarify. The previous owner of our home, a
> >lineman for the local electric company claimed (I think) that the
> >top-most wire on many power poles is nothing but a ground wire, and
> >So, is that true? (Not that you'll find a lot of power lines to
> >cower under in the backcountry...usually.)
> >
> Im not sure thats a particularly good idea, considering many lightning
> bolts originate in the ground, and travel upwards
> Gunner
The direction doesn't matter. The electrons will climb past you up the
steel towers and jump upward from that ground wire on top.
Look carefully at a transmission line. The three phases hang from
insulators while the top wire is tied directly to the towers.
jw
Gary Coffman
>>
>> Now a real lightning rod is copper, and has a very low resistance per foot.
>> So the potential above ground at a point along the rod or its downlead is
>> small, and the danger of a sideflash is minimal. It effectively produces a
>> "cone of protection" with a base diameter similar to the height of the rod.
>> In other words, anything within that cone is very unlikely to be the target
>> of the strike because the rod will take it instead.
>
>I've a question to clarify. The previous owner of our home, a
>lineman for the local electric company claimed (I think) that the
>top-most wire on many power poles is nothing but a ground wire, and
>that it acts just as the copper rod you describe above...in that
>it actually protects the main power lines directly below it from
>a direct strike. He implied that one of the safest places to stand
>during a lightning stom would, in fact, be directly under a power
>line (and assumably equidistant between two utility poles) since
>there is a 'cone' of protection afforded by the ground wire up top.
>
>So, is that true? (Not that you'll find a lot of power lines to
>cower under in the backcountry...usually.)
The top wire in some distribution systems is grounded, though
usually not at every pole. This does tend to protect the wires
closely below it by being the preferred target of the strike. But
unlike a lightning rod, the wire is not designed to be an early
streamer.
The sharp point on the lightning rod is an important part of
its function. The streamers it generates effectively make the
rod a *much* more attractive target than other items in the
lightning rod's surroundings.
The wire doesn't have this property, so its protective function
covers a much smaller volume. I wouldn't expect it to offer any
substantial protection to anyone standing on the ground near
the line. In fact I wouldn't advise standing anywhere near power
lines during a lightning storm.
The danger of sideflash is substantial because even though the
line is grounded, it is not by the most direct path, so there will be
considerable inductance, and a tendency for the bolt to sideflash
to a more direct path to Earth. The sideflash won't be full strength,
but it can still be plenty strong enough to injure or kill.
Gary
Tim
> to alt.binaries.pictures.wallpaper. Most are only so-so, but number 25
> is rather impressive if you scroll all the way to the bottom of it.
And to the right... That's a good pic (as someone responded there).
#3 isn't bad either, I think.
Too bad it takes sooo long to unload (and then load again) the 21609
messages you guy's have posted (umm, RCM).
Tim
--
"When I can't stops me fiddlin', I just takes me Ritalin;
I'm poppin' an' sailin' man! (toot toot!)"
- Bart Simpson
Ron Wong
<er...@as.arizona.edu> wrote:
> I've a question to clarify. The previous owner of our home, a
> lineman for the local electric company claimed (I think) that the
> top-most wire on many power poles is nothing but a ground wire, and
> that it acts just as the copper rod you describe above...in that
> it actually protects the main power lines directly below it from
> a direct strike. He implied that one of the safest places to stand
> during a lightning stom would, in fact, be directly under a power
> line (and assumably equidistant between two utility poles) since
> there is a 'cone' of protection afforded by the ground wire up top.
>
> So, is that true? (Not that you'll find a lot of power lines to
> cower under in the backcountry...usually.)
When most people comment about lightning rods, it's in terms of what
happens when the rods are struck by lightning. This occurs when the
electric field strength in the area of the rod matches/exceeds the
ionization potential of the air (the value depends on the humidity among
other things but 10,000 V/cm is a common figure).
What many people don't understand is that the PRIMARY function of a
lightning rod is to REDUCE the chance of a lightning strike in the
immediate area of the rod. In other words, to try to prevent lightning
from happening in the first place.
Of course if the electrical conditions are such that the electric field
intensity is sufficient to cause lightning to occur then, because the rods
are the highest point in the region and have the highest field intensity
at their very tips, the strike WILL OCCUR, striking the rod and causing
energy to be transferred between the clouds and the ground (assuming the
lightning rod is properly grounded). But this is only a secondary
function. An important one, but secondary.
It is the primary function of a lightning rod that your previous owner was
referring to when he spoke of the top-most wire that was connecting the
power poles.
The metal transmission towers aren't the most perfect of lightning rods
but their construction involved a large number of metal rods that had
sharp edges that came to a point. In addition, the whole thing was very
nicely grounded.
The clouds above and the ground below build up charges of opposite sign
during an electrical storm. As a result, there is an electric field
between these two regions and it gets stronger and stronger as the charge
buildup continues.
If you connect a lightning rod to ground, the charges in the ground will
migrate towards the tip and create an intense electric field around the
tip. This will cause the electrically neutral molecules in the surrounding
air to move towards the tip. When the molecules come in contact with the
tip (actually, they frequently only have to come very close), the charge
will be transferred to the molecules. Since the molecules now have the
same charge as the rod, they are driven away to the clouds above (which
have the opposite charge of the ground and would tend attract these
molecules).
The net effect of this stream of ionized particles moving between the
lightning rod and the clouds is that the overall charge is reduced in the
region and with it the electric field intensity. This reduces the
likelihood of a lightning strike.
So, as you can see, the electric field above the line of connected power
poles is less in an electrical storm than the surrounding region.
But this doesn't mean that you are home free. If the storm's intensity is
such that the charge builds up at a rate greater than it can be dissipated
in the area of the power poles than the electric field in the area of the
towers may reach the point will lightning will occur. The well grounded
and connected power poles will convey the energy into the ground. Just
stay away from the poles (but don't get too far away either).
The question of lightning has come up in the sailing newsgroups. A couple
of messages came up from people who were live-aboards and they made
similar observations. One of them in Florida mentioned that during the
electrical storms in their region, you could see lightning strikes all
around the marina but none of the boats in the marina got struck. They
were amazed that with all the towering masts with their pointy
instruments on the top that none of them were hit by lightning.
Basically, all these towering lightning rods were doing a very good job
reducing the electric field strength of the marina to a point where
lightning couldn't take place in the marina. Personally, I think it may be
just a matter of time before a strike occurs in the marina. All you need
is a sufficiently intense storm and fewer boats in the marina.
Let's hope it never happens - ron
Ron Wong
<ke...@bellsouth.net> wrote:
>...
> Not quite accurate. The reason you hear static on an AM radio is that the
> radio is AM, ie it is designed to respond to amplitude variations of the RF
> signal. The reason you don't hear static on FM is because the receiver
> is designed to *not* respond to amplitude variations (it is designed to
> respond only to frequency variations). It does that with a hard limiter in
> front of the discriminator.
Actually, you can hear static on FM any time you want if you have an FM
receiver that allows you to digitally tune to a particular frequency.
In this day and age where, in the urban areas, the FM stations are stacked
up one against the other, its a little more challenging getting a
frequency that doesn't have a station assigned to it. In addition, many FM
radios are designed nowadays to only respond to frequencies where
frequency modulation is going on (thus allowing you to tune from one
station to another without all that white noise - i.e. static - in
between). This is what Gary is probably alluding to. But if you do have an
FM receiver that doesn't electronically skip over frequencies that aren't
frequency modulated, you'll hear plenty of static. My current Sony
portable does this.
The static is due to the same random electromagnetic radiation that
appears on our TV screens in the form of snow when we tune to a channel
that is not carrying a broadcast signal (the FM band lies inside the range
of frequencies assigned to TV broadcasting).
Tuning from one station to another in FM sounds the same as tuning in AM -
plenty of noise between the stations and a nice strong signal when you
locked on to a station. Anyone who remembers the 40's and 50's when FM
broadcasting was in its infancy will recall this experience.
What you'll discover in an electrical storm is that there is no additional
noise introduced in any of the FM frequencies when a lightning strike
occurs (you probably have noticed that the TV program continued unabated
as well - unless the TV station took a direct hit).
This makes sense since most of the electromagnetic radiation produced in a
lightning strike is in the kilocycle range (centering around 300 kHz I
believe) with the highest frequencies maxing out at around 1 to 2 MHz
(where the wavelength is 150 m - still too large to get through the door
of that Faraday cage). The TV/FM spectrum is well above this frequency.
ron
Gary Coffman
What you hear between stations with an FM broadcast receiver is *thermal noise*.
That's mostly generated *in the receiver itself*. You can prove that to yourself by
disconnecting the antenna. The noise won't change perceptably. That's not the
case with AM receivers (or TV receivers, which are also AM for the video portion
of the signal). For them, most of the noise, and all of the lightning static crashes,
are the result of externally generated noise.
The reason FM receivers have this large amount of thermal noise when not
tuned to a station is that they run open loop without benefit of AGC. The hard
limiter at the end of the IF chain removes any RF amplitude variations in any
received signal, so amplitude fidelity is not a requirement. This very high gain
coupled with limiting is part of what gives us the FM threshold effect.
With an AM receiver, amplitude fidelity is a requirement because the intelligence
in the signal is conveyed in the amplitude modulation. Therefore AGC is used to
control the gain of the IF stages and maintain fidelity. That's not the case with FM,
where the intelligence is conveyed only by frequency variations.
What you're actually hearing from the FM receiver tuned between stations is a
random distribution of thermal noise *frequencies* which are demodulated by
the discriminator. What you hear on an AM receiver is *amplitude* variations of
signals and noise received by the radio, and diode detected. IF gain is much
lower in the AM set, and only a small hiss is heard with the antenna disconnected
(at normal volume settings). By contrast, FM receivers *roar* with thermal noise
unless a squelch system is used to mute the audio when an external signal is
not present. (FM capture effect suppresses the noise when a FM signal is present.)
>What you'll discover in an electrical storm is that there is no additional
>noise introduced in any of the FM frequencies when a lightning strike
>occurs (you probably have noticed that the TV program continued unabated
>as well - unless the TV station took a direct hit).
This isn't true. A TV will show lightning static by white flashes in the picture.
(The same sort of flashes you get by running an electric razor near the TV,
or when an older Ford drives down your street.) The video portion of the
TV signal is AM (actually VSB, a modified form of AM). It responds to external
noise just like any other AM receiver, except you see the noise rather than
hear it. (The sound portion of the TV signal is FM.)
In fact, tuning a TV to an unused low VHF channel (ch 2 or 3), and adjusting
the brightness until the "snow" is just extinguished, gives you a fairly good
thunderstorm detector. As a storm approaches, white flashes will be observed
on the screen, with the frequency and intensity of the flashes increasing as the
storm nears. If the screen turns nearly all white, it is likely that a tornado is
nearby, since strong and very frequent lightning accompanies tornados.
If you use a rotatable directional antenna, you can even get a crude bearing
on the approaching storm. (I suppose those of you raised on cable TV aren't
familiar with all this.)
This strong storm detection method was presented in an IEEE broadcasting
group paper in the mid-50s. Before TV stations had doppler radars, many had
a B&W TV receiver set up this way in their weather offices. It is surprisingly
effective.
An AM broadcast radio, or a shortwave radio, will hear thunderstorms that
are much further away, because the signals are bounced off the ionosphere.
The evening static crashes you hear in the summer on an AM radio while
listening to WHO, WLW, WBZ, or any of the other distant clear channel stations,
are usually the result of storms hundreds or even thousands of miles away.
The benefit of listening for lightning crashes at VHF/UHF is that propagation
is more or less line of sight, so you won't hear anything until the storm is within
about 50 miles of the receiver. That's when you need to get concerned and
start taking appropriate protective actions.
Gary
Gary Coffman
>When most people comment about lightning rods, it's in terms of what
>happens when the rods are struck by lightning. This occurs when the
>electric field strength in the area of the rod matches/exceeds the
>ionization potential of the air (the value depends on the humidity among
>other things but 10,000 V/cm is a common figure).
>
>What many people don't understand is that the PRIMARY function of a
>lightning rod is to REDUCE the chance of a lightning strike in the
>immediate area of the rod. In other words, to try to prevent lightning
>from happening in the first place.
*More* incorrect information. It is essentially impossible to *prevent*
lightning from occurring. The snake oil salesmen pushing so-called
*dissipation arrays* are counting on the fact that you haven't done the
math.
>The clouds above and the ground below build up charges of opposite sign
>during an electrical storm. As a result, there is an electric field
>between these two regions and it gets stronger and stronger as the charge
>buildup continues.
Actually, field strength *decreases* when a storm approaches. The highest
field strength occurs on dry sunny days (nearly 300 volts per meter). During
a storm, the average field decreases to around 100 volts per meter. That's
mainly because the air becomes more conductive during a storm. There's
more charge, but it is less well separated.
But streaming by objects associated with the Earth, and step leaders produced
by the cloud, can cause very *localized* line potentials to become very large
indeed, ie the lightning discharge.
(Note that a sharp point concentrates the field so that streaming can begin
at modest potentials. A very sharp point can begin to stream at potentials
in the low 10s of volts. It is *not* necessary for potentials to approach the
10 kV per centimeter figure in order for streaming, step leaders, or the
lightning discharge to occur.)
>If you connect a lightning rod to ground, the charges in the ground will
>migrate towards the tip and create an intense electric field around the
>tip. This will cause the electrically neutral molecules in the surrounding
>air to move towards the tip. When the molecules come in contact with the
>tip (actually, they frequently only have to come very close), the charge
>will be transferred to the molecules. Since the molecules now have the
>same charge as the rod, they are driven away to the clouds above (which
>have the opposite charge of the ground and would tend attract these
>molecules).
This model is physically inaccurate. For a correct description, see "Lightning"
by Unman (Dover Press), or if you can find a copy, the classic paper "The Lightning
Discharge" by Gardener.
>The net effect of this stream of ionized particles moving between the
>lightning rod and the clouds is that the overall charge is reduced in the
>region and with it the electric field intensity. This reduces the
>likelihood of a lightning strike.
Unfortunately, when you do the math you find out that you cannot dissipate
the charge rapidly enough without the discharger breaking into streaming
(approx 20 uA per discharge point), and streaming is what causes the
lightning rod to be the preferred target for a lightning discharge.
The storm winds are constantly bringing fresh charge over your site.
You have to discharge approximately 120 coulombs of charge per minute
to prevent lightning discharges in your immediate vicinity.
You need literally *millions* of discharge points, *all equalized in potential*
so that none exceeds the 20 uA streaming threshold, in order to discharge
the cloud to ground circuit sufficiently to prevent lightning during a thunderstorm.
You can't do it. Anyone trying to sell you something that he claims can do it
is pushing snake oil.
The sticking point is equalizing potential among all of those points to prevent
the array from becoming a net *cause* of lightning strikes. Consider a pine
forest. It has millions of sharp points (the pine needles). So if the dissipation
array salesmen's spiel were true, pine forests would never be struck by
lightning. But of course they are, lightning is a major cause of forest fires.
The FAA conducted definitive tests (large scale enough to be statistically
significant) of the so-called dissipation arrays (installed by their proponents)
at hundreds of tower sites in Florida (lightning alley). What they found over
a two year period was that sites using the dissipation arrays were struck
slightly more frequently, and sustained significantly more damage, than the
sites protected by conventional lightning rods.
The reason more damage was sustained was that these dissipation arrays
generally don't provide as low an impedance path for the strikes as a properly
installed lightning rod system does. (In fact some of these systems tout as a
feature that they are high impedance, in order to prevent them from breaking
into streaming. Think of what that means, consider the high impedance tree
as your model.)
There was a very good series of articles on this in _Mobile Radio Technology_
magazine a few years ago. This brought lawsuits by some of the companies
peddling the dissipation arrays, but they were soundly thrashed in court by
real experts, and they've been strangely silent in public forums about the physics
of how their equipment is supposed to work since (though some seem to be
slithering back out from under their rocks recently).
Gary
w_tom
take the best path available. It is why lightning is statisically
predictable. The overhead ground wire is a better channel to ground than
through 'cone of protection' air. So a lightning strike takes the ground
wire.
BTW, the ground wire does not discharge the air. This is a theory,
unsupported by evidence, and wildly advocated by Early Streamer Emission
proponents. ESE promoters cannot even field a published (responsible)
professional paper to support the idea that these air terminals discharge the
air - make lightning not happen.
Dr. Abdul Mousa has many papers on why air terminals don't discharge the air
as well as multiple direct confrontations with salesman who promote the
concept. There is no supporting evidence that air terminals discharge the air
to make a lightning strike less possible. There is strong evidence that ESE
air terminals only to shunt a lightning strike to earth ground. If air
terminals operated by discharging the air, then we would not have to connect
them with such large wires. Those large wires are there only because air
terminals are about shunt lightning to earth ground by a safer path.
Also curious is recent research that blunt end air terminals (lightning
rods) are more effective than sharp, pointed rods. That is reported in
www.lightningsafety.com .
Air terminals (lightning rods) create a cone of protection which is
statistically predictable. Of course its effectiveness is only as good as its
earth ground. Earth ground is critical to the effectiveness of all
lightning protection. Air terminals are how humans offer up to the lightning
gods a better path to earth ground - so that lightning will not take a path
through humans or electronics.
Those same concepts of air terminals are how effective surge protection
works. Surge protectors are effective if they provide lightning a shorter
path to better earth ground before the surge can enter the building. Like
ESE, plug-in surge protectors are also promoted on urban myths. Surge
protectors only shunt a surge from one wire to all others. Without a short,
direct connection to earth ground, where does the surge go next? Into the
adjacent appliance?
When located at the service entrance and connected less than 10 feet to
earth ground, then lightning has a shorter, better connection to earth.
Therefore lightning does not enter the building to destructively find earth
ground through appliances. That is called effective surge protection. Both
surge protection and air terminals work on the same principal that Franklin
demonstrated in 1752 and that was routinely, effectively installed since
1930s. Lightning protection is about providing lightning with a safer,
better, alternative earth ground path. No path to earth ground means no
lightning protection.
Mark Kinsler
>mid-1800's it was trendy in women's fashions to have a chain attached to the back of
>your hat that dragged on the ground behind you. This was to protect you from all those
>lightning strikes that were taking people down all over the streets of Paris at the
>time.
Ah, yes. Those were "lightning rod hats," and were acknowledged to be
silly even by the women who wore them. It was just a fashion gag, not
meant to be at all functional. Hell, women wore stuffed birds in
their hats.
I believe that the period was somewhat earlier, probably around the
1760's, when lightning rods were being introduced all over the world
and Benjamin Franklin was wildly popular in Paris.
What hikers should do about lightning is essentially nothing except to
stay out of the rain. I'm afraid that none of those lightning safety
rules that you read have much, if any, scientific basis.
M Kinsler
512 E Mulberry St Lancaster Ohio USA 740.687.6368
http://www.frognet.net/~kinsler
Mark Kinsler
>I've a question to clarify. The previous owner of our home, a
>lineman for the local electric company claimed (I think) that the
>top-most wire on many power poles is nothing but a ground wire, and
>that it acts just as the copper rod you describe above...in that
>it actually protects the main power lines directly below it from
>a direct strike. He implied that one of the safest places to stand
>during a lightning stom would, in fact, be directly under a power
>line (and assumably equidistant between two utility poles) since
>there is a 'cone' of protection afforded by the ground wire up top.
>
>So, is that true? (Not that you'll find a lot of power lines to
>cower under in the backcountry...usually.)
I'd say so, but it's a matter of some controversy. The reason is that
a lightning strike to a utility line often results in a current path
being established from the struck wire and down the pole, especially
if the pole has a ground wire running from that top wire to its base.
This won't hurt the pole, but there's the question of the path that
the current takes after it leaves the base of the pole. This _could_
be up one leg and down the other if you're standing in the wrong
place.
>> a lone tree can be protection against the lightning stroke in
>> the same way that a real lightning rod can protect a house, ie
>> by being the preferred target of the bolt. But do watch out for
>> flying splinters.
>
>We found a tall tree on a forested hill last summer that had
>been struck. One of the "splinters" was about 12 feet long and
>6" thick and was quite solidly impaled into the ground about
>25 feet from the tree. It's one of those occurances that I file
>under "Glad I Wasn't There When it Happened!"
Well, noted scientists will tell you that when your number is up, it's
up.
M Kinsler
it's all written in the stars...
Mark Kinsler
>Look carefully at a transmission line. The three phases hang from
>insulators while the top wire is tied directly to the towers.
Actually, you'll find that the top wire is often separated from the
tower by a small insulator. This is a very sturdy little insulator
that has a low voltage rating--perhaps 200v or so. If lightning hits
the top wire, the insulator will arc over without being damaged and it
won't notice the insulator. But when it's not being hit, that wire is
insulated well enough to be used for all kinds of unlikely purposes:
carrying current for tower lights (needed near airports) or
communications.
The top wires they're using nowadays have optical fibers inside them.
These carry relaying and power-company communication signals. The
excess capacity is then sold on the common-carrier market to outfits
like Sprint and anyone else who needs that particular communication
channel.
M Kinsler
Chuck Simmons
>
> On Fri, 15 Jun 2001 01:12:52 -0700, ron...@purgethis.inreach.com (Ron Wong) wrote:
> >When most people comment about lightning rods, it's in terms of what
> >happens when the rods are struck by lightning. This occurs when the
> >electric field strength in the area of the rod matches/exceeds the
> >ionization potential of the air (the value depends on the humidity among
> >other things but 10,000 V/cm is a common figure).
> >
> >What many people don't understand is that the PRIMARY function of a
> >lightning rod is to REDUCE the chance of a lightning strike in the
> >immediate area of the rod. In other words, to try to prevent lightning
> >from happening in the first place.
>
> *More* incorrect information. It is essentially impossible to *prevent*
> lightning from occurring. The snake oil salesmen pushing so-called
> *dissipation arrays* are counting on the fact that you haven't done the
> math.
Interesting. Your post reminded me of an experience related to it. I
once worked at the transmitter building of an AM station that had two
283 foot sticks (directional and daytime only to protect KCBS). The two
sticks were bottom fed and insulated from ground. Each had a lightning
arrestor consisting of two steel balls, one grounded and the other
attached to the stick. Our habit was to adjust them so that there was
little static discharge on windy days - 1/8 to 3/16 inch. One windy day,
I had just checked the tower tuners and examined the arrestors and
returned to the building when there was a loud bang. The transmitter and
antenna tuner appeared normal so I did not think more about it until I
received a call from a neighboring farm. The farmer was wondering if I
was alright since he knew I was alone. It seems the east tower had taken
a direct lightning strike. That was the bang I heard. The two towers
were the tallest things within five miles at least and made a pretty
good target. Naturally, the tower tuners and the antenna tuner had been
designed with lightning in mind so the transmitter only saw a momentary
change in load impeadance and recovered before I could turn around and
see the meters.
I should point out that both sticks had lightning rods appearently to
protect the FAA required lights. There was no way to protect the towers
as tall as they were (this may explain why shunt feeding is sometimes
preferred to bottom feeding).
Chuck
--
... The times have been,
That, when the brains were out,
the man would die. ... Macbeth
Chuck Simmons chr...@webaccess.net
Mark Kinsler
>When most people comment about lightning rods, it's in terms of what
>happens when the rods are struck by lightning. This occurs when the
>electric field strength in the area of the rod matches/exceeds the
>ionization potential of the air (the value depends on the humidity among
>other things but 10,000 V/cm is a common figure).
The humidity, oddly enough, _increases_ the ionization potential.
>What many people don't understand is that the PRIMARY function of a
>lightning rod is to REDUCE the chance of a lightning strike in the
>immediate area of the rod. In other words, to try to prevent lightning
>from happening in the first place.
This is the famous and never-ending "diversion vs. dissipation"
argument. It goes straight back to Ben Franklin and still rages.
Endlessly.
>The net effect of this stream of ionized particles moving between the
>lightning rod and the clouds is that the overall charge is reduced in the
>region and with it the electric field intensity. This reduces the
>likelihood of a lightning strike.
>
>So, as you can see, the electric field above the line of connected power
>poles is less in an electrical storm than the surrounding region.
We've never found the slightest evidence of this. While it is true
that you can detect a fairly substantial current in a lightning rod,
and perhaps even a greater one in one of the many lightning dissipator
devices sold through the years, the earth is a good conductor and thus
cannot be "discharged." Every bit of charge that migrates through the
rod up to the cloud will be replaced from the surrounding earth.
>The question of lightning has come up in the sailing newsgroups. A couple
>of messages came up from people who were live-aboards and they made
>similar observations. One of them in Florida mentioned that during the
>electrical storms in their region, you could see lightning strikes all
>around the marina but none of the boats in the marina got struck. They
>were amazed that with all the towering masts with their pointy
>instruments on the top that none of them were hit by lightning.
Most masts have lightning rods built into them. These are connected
to a copper plate that's bonded to the hull near the keel. Sailing
enthusiasts are the last great believers in lightning dissipators,
which for marine applications tend to take the form of a
bronze-bristled brush.
>Basically, all these towering lightning rods were doing a very good job
>reducing the electric field strength of the marina to a point where
>lightning couldn't take place in the marina. Personally, I think it may be
>just a matter of time before a strike occurs in the marina. All you need
>is a sufficiently intense storm and fewer boats in the marina.
There might never be one. That's the problem with random events:
they're random. You might paint a boat blue and it'll get hit three
times in a row, thus starting a ban on blue paint for marine uses.
People in general and gamblers in particular look for patterns and try
to work the system. But in this case, there is no system: lightning
is thoroughly random.
Mark Kinsler
>The top wire in some distribution systems is grounded, though
>usually not at every pole. This does tend to protect the wires
>closely below it by being the preferred target of the strike. But
>unlike a lightning rod, the wire is not designed to be an early
>streamer.
>
>The sharp point on the lightning rod is an important part of
>its function. The streamers it generates effectively make the
>rod a *much* more attractive target than other items in the
>lightning rod's surroundings.
Controversy alert. I'm uncomfortable responding to any of Mr
Coffman's posts with anything but adulation, but this particular point
has been a matter of an ongoing controversy for three hundred years,
and it's not close to being settled yet. There doesn't seem to be any
obvious way to determine how much of a streamer you get out of a
pointy lightning rod vs a blunt one, and there doesn't seem to be any
way to determine whether that streamer helps to snag an incoming
stepped leader.
The argument is that any wire sticking up, pointy or not, looks pretty
pointy to a downgoing leader. I tend to favor the pointy side of the
argument, though.
Some idea of the difficulty of this research might be gained from the
results of an experiment at the New Mexico School of Mines to try to
settle this controversy as scientifically as possible. The study
covered seven years of strikes on a mountaintop known in legend and
general experience as the site of frequent strikes. So they
instrumented several towers and equipped them with pointed and blunt
rods. But in the entire seven-year period, they recorded a total of
twelve strikes. It's enough to drive you nuts.
>The wire doesn't have this property, so its protective function
>covers a much smaller volume. I wouldn't expect it to offer any
>substantial protection to anyone standing on the ground near
>the line. In fact I wouldn't advise standing anywhere near power
>lines during a lightning storm.
Dunno. The lines do seem to snag strikes out of the air pretty well.
The national lightning antenna network (http://www.lightningstorm.com,
highly recommended) is finally giving up some data, and
lightning-chasing services are working with power companies to
coordinate strike data from the antennas with strikes on power lines
detected by substation equipment.
In the early days of power lines, barbed wire was used as a top wire,
ostensibly to snag lightning better. It was "determined to be
ineffective" and thus eliminated in favor of plain wire. (This was
from a casual reference in a speech quoted in an IEE journal. I've
found no other discussion of it, and it's driven me nuts for years.)
>The danger of sideflash is substantial because even though the
>line is grounded, it is not by the most direct path, so there will be
>considerable inductance, and a tendency for the bolt to sideflash
>to a more direct path to Earth. The sideflash won't be full strength,
>but it can still be plenty strong enough to injure or kill.
I generally agree. Besides which, if you're under a power line in a
storm, you're probably getting wet, and who needs that.
Mark Kinsler
>*More* incorrect information. It is essentially impossible to *prevent*
>lightning from occurring. The snake oil salesmen pushing so-called
>*dissipation arrays* are counting on the fact that you haven't done the
>math.
To be fair, the math doesn't quite exist for either side of the
controversy. The problem I have with the dissipation people is less
with their theory than with the character of many of their principal
players. But neither side has anything resembling a good argument, or
even particularly reasonable field data. It's nobody's fault:
lightning is a miserable thing to study. The fact that it seems to
inflame controversy makes things even more difficult for objective
researchers, and thus most scientists stay the hell away from the
whole thing.
>
>Actually, field strength *decreases* when a storm approaches. The highest
>field strength occurs on dry sunny days (nearly 300 volts per meter). During
>a storm, the average field decreases to around 100 volts per meter. That's
>mainly because the air becomes more conductive during a storm. There's
>more charge, but it is less well separated.
What about Franklin's nifty device to detect thunderclouds? A bell
was connected to an air terminal, and another to ground. An
exceedingly light clapper was mounted between them: when the
air-terminal bell became charged, it would attract the clapper, and
then repel it to the grounded bell. Thus the bells would ring when
the field strength became greater. I've been planning to build one of
these things for years.
>But streaming by objects associated with the Earth, and step leaders produced
>by the cloud, can cause very *localized* line potentials to become very large
>indeed, ie the lightning discharge.
>
>(Note that a sharp point concentrates the field so that streaming can begin
>at modest potentials. A very sharp point can begin to stream at potentials
>in the low 10s of volts. It is *not* necessary for potentials to approach the
>10 kV per centimeter figure in order for streaming, step leaders, or the
>lightning discharge to occur.)
Well, the field strength must be high, but the geometry allows it to
be created by fairly low voltages. Which is saying the same thing.
>by Unman (Dover Press), or if you can find a copy, the classic paper "The Lightning
>Discharge" by Gardener.
That's Martin Uman, if you're doing a Web search.
>Unfortunately, when you do the math you find out that you cannot dissipate
>the charge rapidly enough without the discharger breaking into streaming
>(approx 20 uA per discharge point), and streaming is what causes the
>lightning rod to be the preferred target for a lightning discharge.
I should have known this. Lemme look it up in my plasma book.
>The storm winds are constantly bringing fresh charge over your site.
>You have to discharge approximately 120 coulombs of charge per minute
>to prevent lightning discharges in your immediate vicinity.
>
>You need literally *millions* of discharge points, *all equalized in potential*
>so that none exceeds the 20 uA streaming threshold, in order to discharge
>the cloud to ground circuit sufficiently to prevent lightning during a thunderstorm.
>You can't do it. Anyone trying to sell you something that he claims can do it
>is pushing snake oil.
>
>The sticking point is equalizing potential among all of those points to prevent
>the array from becoming a net *cause* of lightning strikes. Consider a pine
>forest. It has millions of sharp points (the pine needles). So if the dissipation
>array salesmen's spiel were true, pine forests would never be struck by
>lightning. But of course they are, lightning is a major cause of forest fires.
>
>The FAA conducted definitive tests (large scale enough to be statistically
>significant) of the so-called dissipation arrays (installed by their proponents)
>at hundreds of tower sites in Florida (lightning alley). What they found over
>a two year period was that sites using the dissipation arrays were struck
>slightly more frequently, and sustained significantly more damage, than the
>sites protected by conventional lightning rods.
Two years is awfully short, though.
> The reason more damage was sustained was that these dissipation arrays
>generally don't provide as low an impedance path for the strikes as a properly
>installed lightning rod system does. (In fact some of these systems tout as a
>feature that they are high impedance, in order to prevent them from breaking
>into streaming. Think of what that means, consider the high impedance tree
>as your model.)
>
>There was a very good series of articles on this in _Mobile Radio Technology_
>magazine a few years ago. This brought lawsuits by some of the companies
>peddling the dissipation arrays, but they were soundly thrashed in court by
>real experts, and they've been strangely silent in public forums about the physics
>of how their equipment is supposed to work since (though some seem to be
>slithering back out from under their rocks recently).
If you have a reference for that study by the FAA, I'd like to read
it. My impression that nobody had much field data and that most of
the conclusions relied on some anecdotal reports. It's almost
impossible to conduct a controlled experiment in this business, within
or without Lightning Alley.
I hadn't heard about the article or the lawsuit. This is typical of
the lightning protection business, however. Great buncha guys. The
"real experts" have gotten sued jointly and severally at various
times. There's some controversy as to who is a lightning expert and
who isn't.
M Kinsler
who, if they're suing people, isn't one.
w_tom
Gary Coffman wrote:
> ... For a correct description, see "Lightning"
> by Unman (Dover Press), or if you can find a copy, the classic paper "The Lightning
> Discharge" by Gardener. ....
w_tom
and ESE / dissipation arrays are based only upon theory - with no supporting
facts or evidence. As properly noted, since they cannot provide valid
scientific evidence that dissipation arrays work, then instead they resort to
lawsuits.
Dr. Mousa's paper entitled Applicability of Lighting Elimination Devices to
Substations and Power Lines was published after (what he describes as) extra
peer review in the IEEE Transactions on power Delivery 4 Oct 1998.
No one says dissipation arrays don't work. They simply work just like
Franklin Air Terminals, but CLAIM to be superior. As a result, some people now
install dissipation arrays containing radioactive material. IOW the array
becomes a greater threat to human life than the lightning - all for an idea
that has yet to be demonstrated effective.
JJVW
Dave Martel wrote:
>
> It's very simple. Don't stand out in an open field, or next to a rock
> or tree, or down in a gully or up on a ridge. :o]
Gary Coffman
>Actually, you'll find that the top wire is often separated from the
>tower by a small insulator. This is a very sturdy little insulator
>that has a low voltage rating--perhaps 200v or so. If lightning hits
>the top wire, the insulator will arc over without being damaged and it
>won't notice the insulator. But when it's not being hit, that wire is
>insulated well enough to be used for all kinds of unlikely purposes:
>carrying current for tower lights (needed near airports) or
>communications.
>
>The top wires they're using nowadays have optical fibers inside them.
>These carry relaying and power-company communication signals. The
>excess capacity is then sold on the common-carrier market to outfits
>like Sprint and anyone else who needs that particular communication
>channel.
Yes. That's how I got a garage full of duplex data radios. Georgia Power
changed over to fiber to control all of their switchgear and discarded their
"old" radio control system. (Most of the radios aren't really old, and use
modern radio technology, a real coup for me as a radio amateur involved
in amateur radio data communications.)
Apparently the power companies are planning to take this a step further
and replace the steel core of their "hot" lines with fiber too. According to
an article I read, the fiber is stronger for a given diameter than the steel
strand, allowing them to add more aluminum (ampacity) to the cable
while keeping it the same outer diameter (same wind load). They say
this will allow them to have better line ampacity and the ability to resell
data capacity too.
Here's the link to the article:
http://www.wired.com/news/technology/0,1282,44213,00.html
Gary
Mark Kinsler
>and ESE / dissipation arrays are based only upon theory - with no supporting
>facts or evidence. As properly noted, since they cannot provide valid
>scientific evidence that dissipation arrays work, then instead they resort to
>lawsuits.
What happened in Dr Mousa's case is that he was appointed to a
committee of the National Fire Protection Association, the outfit that
writes the National Electrical Code. (I'd lobbied to be on that
committee myself, but fortunately didn't make the cut.) The purpose
of the committee was to decide whether a "standard" for
lightning-prevention devices should be written and added to the
National Electrical Code. A "standard" for a device specifies how the
device shall be manufactured and how it shall be applied. The
existence of a standard legitimizes its existence and effectiveness,
so the lightning-dissipator people had been furiously lobbying for a
standard.
The committee said no, so the lightning disspator people sued the
committee and the horse they rode in on jointly and severally. I
personally don't know how such a lawsuit would have affected Dr Mousa
from a legal standpoint since he's a Canadian and the suit was
presumably filed in the US. But he was thorougly outraged at the idea
of a volunteer scientific working group being subject to legal action
for its activities.
Most everyone else, including me, tended to agree, and I believe that
this incident was the impetus for Dr Mousa's latest journal article.
Dr Mousa writes about interesting stuff, e.g., how do you fight a fire
that involves an energized electric power line?
> Dr. Mousa's paper entitled Applicability of Lighting Elimination Devices to
>Substations and Power Lines was published after (what he describes as) extra
>peer review in the IEEE Transactions on power Delivery 4 Oct 1998.
>
> No one says dissipation arrays don't work. They simply work just like
>Franklin Air Terminals, but CLAIM to be superior. As a result, some people now
>install dissipation arrays containing radioactive material. IOW the array
>becomes a greater threat to human life than the lightning - all for an idea
>that has yet to be demonstrated effective.
For all that, I've chosen to be officially neutral in the dissipation
vs. diversion wars. There are two reasons for this. One is that
there's little science to support the arguments of either side. The
second is that despite this lack of reasonable scientific evidence,
the dispute has raged unabated for about 250 years and I've been doing
research for a book or article on the subject.
The controversy itself is far more interesting than the arguments of
either side. You'll find a lightning disspator atop the US Capitol,
for example. And someone in the 1920's tried to make a huge disspator
out of the Eiffel Tower. Looked pretty strange, I must say.
I'm sorry to say that my less-than-reverent attitude (plus my
sparkling personality, no doubt) has made Dr Mousa thoroughly
infuriated at me, too. In general, lightning protection is a field
that's well worth staying away from.
Ed Huesers
Mark Kinsler wrote:
> I'm afraid that none of those lightning safety
> rules that you read have much, if any, scientific basis.
Well, the normal considerations discussed in rec.backcountry mention
lighting traveling distances of up to 500 ft. on the ground. It's
considered best to stay at least 100 ft. from objects that stand a
chance to be struck.
Also, sitting on your pack to insulate yourself from the ground is
considered or elevated a bit by sitting on a rock giving the lighting a
chance to just travel past and underneath you.
A mountain pass is considered safer than being higher on the ridge of
the mountain itself.
We have stayed up on the ridge of a mountain but in a low area
surrounded by rock that is higher. This picture for instance is looking
down the ridge at a rock outcropping that is about 100 yd. distance:
http://www.grandshelters.com/images/winter-survival-lt-s2.jpg
This is a picture looking the opposite direction and up the ridge of
the mountain:
http://www.grandshelters.com/images/igloo-building-ap-1-s2.jpg
And this one shows some rock crags that are also fairly close and
might have offered some protection:
http://www.grandshelters.com/images/igloo-ap-2-s2.jpg
Sooo.. from what I gather, unless the shape of an igloo creates some
mystical or divine phenomena, the quazi liquid layer on ice would not
conduct enough energy to offer protection from a direct hit.
Great thread guys, thanks.
Our group needed it.
Ed Huesers
http://www.grandshelters.com
Ed Huesers
> if you're under a power line in a
> storm, you're probably getting wet, and who needs that.
Hey! Some of us like getting wet. Gotta test out the gortex someway.
Ed [who after reading the pine needle theory will never react the
same when someone says, "Ed, your hair is sticking strait up"] Huesers
http://www.grandshelters.com
Ed Huesers
Chuck Simmons wrote:
> (this may explain why shunt feeding is sometimes
> preferred to bottom feeding).
Damn bottom feeders anyway.
Ed Huesers
http://www.grandshelters.com
w_tom
a river. He was told to put fiber inside the cable, just not for strength. On
a whim, he did it. It became part of the utility's large optic network used
both for control and extra capacity sold to others. He said he wished he was
around now to take credit for a whim that saved the company so much money. Most
interesting concept, the fiber inside overhead transmission wires.
They mention 'sag' in the article. What do they mean by 'sag'?
Ed Huesers
Lou wrote:
> After reading this I could not help but think of Galen and his
> "Bear-Pruf" tent line.
> Maybe he could incorporate the waste from tent manufacturing
> into a new line of hats.
Hey! Don't give him any ideas. The competion is already stiff enough
the way the tent serves multipule uses.
I recently heard a guy saying "This thing gives me a better workout
than my Stepmaster".
I just hope he doesn't wise up and start suppling a ground rod with
his tents.
Ed Huesers
http://www.grandshelters.com
Ed Huesers
Greg Rose wrote:
> As it is I just ran in panic.
Was that you?
We were taking shelter just below a pass one day and commenting on
just how stupid a hiker was that had just summitted the peak next to us
when lighting stuck. It looked to have struck the mountain on the other
side just past him.
He seemed to have very light feet coming down all that rock.
> Hey, it worked.
Hey, the conversation wouldn't be what it is if it hadn't.
Ed Huesers
Mark Kinsler
>
>Mark Kinsler wrote:
>> I'm afraid that none of those lightning safety
>> rules that you read have much, if any, scientific basis.
>
> Well, the normal considerations discussed in rec.backcountry mention
>lighting traveling distances of up to 500 ft. on the ground. It's
>considered best to stay at least 100 ft. from objects that stand a
>chance to be struck.
This is what we call anecdotal evidence. That 100 foot rule could be
100 meters or 100 inches for all anyone knows. It hasn't been been
tested in any lab and is essentially conjecture that merely sounds
like a good idea.
> Also, sitting on your pack to insulate yourself from the ground is
>considered or elevated a bit by sitting on a rock giving the lighting a
>chance to just travel past and underneath you.
Again, this is something that someone made up to make hikers feel like
they have some control over the situation. I doubt that your pack
would make a bit of difference, but there's no way to test this. It's
certain that whoever wrote this particular rule didn't test it.
> A mountain pass is considered safer than being higher on the ridge of
>the mountain itself.
If people are shooting at you, this is correct. If you're dealing
with lightning, have a look at where lightning strikes, and why. A
thundercloud that's ten miles up and ten miles long isn't gonna care
if you're on a ridge or not.
Does that rulebook tell you about "the lightning crouch?" That's one
of my favorites, especially when they tell you to hop like a bunny.
Mark Kinsler
> They mention 'sag' in the article. What do they mean by 'sag'?
If you try to run a wire between two supports without much sag, you'll
place so much tension on that wire that it'll break. The greater the
sag, the lower the tension: hence the very great sag that you see on
suspension bridge cables. We use this as a physics problem: cold
weather can reduce the sag enough to break the wire.
In any case, overhead wire people are always worried about sag.
You might be interested to note that in Germany they ran copper
telephone cables in the middle of 20kV power conductors. Yes, a lot
of terminal equipment was necessary to keep the power out of the
telephone equipment, but the most expensive part of any wired utility
is the right-of-way and its maintenance. Thus the high cost of
complex schemes to make the right-of-way more useful is often
justified.
Mark Kinsler
I got into trouble with the lightning protection newsgroup when I
asked for journal references on lightning safety rules (knowing
perfectly well that there weren't any.) When it became clear that
nobody in the bunch had a sense of humor, I quoted an old maxim that I
made up for the occasion:
"Every cowboy knows that if you sleep under a barbed-wire fence,
you're safe from lightning."
Boy, did they get sore at me.
M Kinsler
if everyone knows how to be safe from lightning, how come nobody's
ever killed themselves by violating all the safety rules?
Lou
"Ed Huesers" <e...@grandshelters.com> wrote in message
news:3B2BB62C...@grandshelters.com...
The new guy gets to carry the 8' ground rod and the sledge to drive
it with.Come to think of it the ground rod "could" do double duty as an
avalanche probe..........
Greg Rose
"Ed Huesers" wrote:
>
>
> Greg Rose wrote:
> > As it is I just ran in panic.
>
> Was that you?
> We were taking shelter just below a pass one day and commenting on
> just how stupid a hiker was that had just summitted the peak next to us
> when lighting stuck. It looked to have struck the mountain on the other
> side just past him.
> He seemed to have very light feet coming down all that rock.
Nah, not me. Mt. Watana in the Talkeetna Mts. of AK. Nobody else around
and it was an exposed ridge with no readily apparent shelter spots, plus I
had on a metal frame pack.
Running down the slope seemed the only plausible option, yet that did
nothing to stem the feeling of total panic.
OK, I was scared shitless, I admit it.
>
> > Hey, it worked.
>
> Hey, the conversation wouldn't be what it is if it hadn't.
That's a fact.
Greg Rose
Gary Coffman
>There might never be one. That's the problem with random events:
>they're random. You might paint a boat blue and it'll get hit three
>times in a row, thus starting a ban on blue paint for marine uses.
>People in general and gamblers in particular look for patterns and try
>to work the system. But in this case, there is no system: lightning
>is thoroughly random.
I strongly disagree with that statement. Lightning discharges always
follow electrical laws. If we had complete information about the state
of the environment prior to the discharge, we could predict exactly
the path a particular bolt would take.
But of course we do *not* have complete foreknowledge of the
locations of the stray ions and dust particles in the atmosphere
at the time of the strike. We don't have precise knowledge of the
magnitude and distribution of charges in the cloud. Etc. So in
practice we can't predict in advance the exact path of a particular
discharge.
But that's due to the lack of foreknowledge on our part, and is
not due to randomness in the behavior of the discharge. There's
a big difference between a truly random event and an event for
which we lack sufficient foreknowledge to calculate behavior.
Gary
Gary Coffman
>>Mark Kinsler wrote:
>>> I'm afraid that none of those lightning safety
>>> rules that you read have much, if any, scientific basis.
>>
>> Well, the normal considerations discussed in rec.backcountry mention
>>lighting traveling distances of up to 500 ft. on the ground. It's
>>considered best to stay at least 100 ft. from objects that stand a
>>chance to be struck.
>
>This is what we call anecdotal evidence. That 100 foot rule could be
>100 meters or 100 inches for all anyone knows. It hasn't been been
>tested in any lab and is essentially conjecture that merely sounds
>like a good idea.
Many of the rules that have been learned from astronomy can't be
tested in the lab either, nor can many from deep strata geology, or
more on point here, from meteorology, but that doesn't mean that
we simply dismiss them as "anecdotal".
There are a number of fields where we can't depend on a lab to
replicate field conditions. We have to depend on in situ observations
and inferences drawn from basic physical principles.
We do know that the Earth's surface layer can be modeled as a
sheet resistance, which if fairly uniform, will disperse currents so
that the potential between any two nearly adjacent points rapidly
approaches a low constant value just a short radial distance from
the current injection point. For typical soils, it is possible to fairly
closely approximate the radial distance at which the potential falls
to what are considered harmless levels.
Real soils aren't actually uniform in many cases. But the errors along
different radials due to this are typically less than an order of magnitude.
So if the usual engineering safety factor margins are observed, good
rules of thumb are possible for particular types of terrain.
Now this is discussing hazards from Earth currents due to a near strike.
There is also the possibility of sideflash. It is typical for sideflash distances
to be shorter than the height of the object being struck, for the obvious reason.
But over hard rock, there often isn't a good Earth termination at the base of
the object struck. In this situation, the safe radial distance to avoid a sideflash
can be significantly greater than the height of the object.
The lesson to be drawn from this is to be situationally aware of the sort
of terrain in which you find yourself when deciding what is and isn't a
reasonably safe location.
>> Also, sitting on your pack to insulate yourself from the ground is
>>considered or elevated a bit by sitting on a rock giving the lighting a
>>chance to just travel past and underneath you.
>
>Again, this is something that someone made up to make hikers feel like
>they have some control over the situation. I doubt that your pack
>would make a bit of difference, but there's no way to test this. It's
>certain that whoever wrote this particular rule didn't test it.
There are ways to model this. Again knowing that the Earth behaves
as a sheet resistance, and knowing that a potential can only be expressed
across two separated points in contact with that sheet resistance, it can
be concluded that a single point of contact minimizes the potential that
can be expressed across objects attached to that point due to Earth
currents induced by the nearby strike.
>> A mountain pass is considered safer than being higher on the ridge of
>>the mountain itself.
>
>If people are shooting at you, this is correct. If you're dealing
>with lightning, have a look at where lightning strikes, and why. A
>thundercloud that's ten miles up and ten miles long isn't gonna care
>if you're on a ridge or not.
It doesn't care from a distance of 10 miles, but lightning doesn't form
in a single 10 mile leap. It moves in a series of step leaders, and each
short move is subject to the influence of its immediate environment.
The behavior of lightning isn't just random whimsy. The discharge
has to follow the rules of electrical behavior. We generally can't know
a priori all of the environmental factors which shape the lightning path,
but it is foolish to dismiss the ones we do know or can influence.
Gary
Gary Coffman
>> You saw Dr Mousa roundly destroy the ESE arguments since he provided facts,
>>and ESE / dissipation arrays are based only upon theory - with no supporting
>>facts or evidence. As properly noted, since they cannot provide valid
>>scientific evidence that dissipation arrays work, then instead they resort to
>>lawsuits.
>
>What happened in Dr Mousa's case is that he was appointed to a
>committee of the National Fire Protection Association, the outfit that
>writes the National Electrical Code. (I'd lobbied to be on that
>committee myself, but fortunately didn't make the cut.) The purpose
>of the committee was to decide whether a "standard" for
>lightning-prevention devices should be written and added to the
>National Electrical Code. A "standard" for a device specifies how the
>device shall be manufactured and how it shall be applied. The
>existence of a standard legitimizes its existence and effectiveness,
>so the lightning-dissipator people had been furiously lobbying for a
>standard.
To clarify a bit, they were lobbying for a *new* standard (781) which
would specifically bless their "dissipator" devices. A number of standards
already exist in the Code for more soundly based lightning protective
measures.
GAry
Gary Coffman
>>So, as you can see, the electric field above the line of connected power
>>poles is less in an electrical storm than the surrounding region.
>
>We've never found the slightest evidence of this. While it is true
>that you can detect a fairly substantial current in a lightning rod,
>and perhaps even a greater one in one of the many lightning dissipator
>devices sold through the years, the earth is a good conductor and thus
>cannot be "discharged." Every bit of charge that migrates through the
>rod up to the cloud will be replaced from the surrounding earth.
Well, that's not exactly what the dissipation array folks are claiming.
They basically make two claims.
1) Their arrays discharge the *cloud* base over the site. This is different
from claiming to discharge the Earth. The latter is impossible even in theory,
as you note. The former is impossible *in practice* as I've noted.
Or 2) Their arrays create a "space charge" around the site which repels strikes
in the same way that the space charge in a triode vacuum tube reduces its
transconductance by "shielding" the plate from electrons. This claims sounds
good at first glance, until you realize that storms have wind, and that any space
charge created is blown away as rapidly as it can be formed.
Gary
Galen Hekhuis
during a thunderstorm and they did not all have masts stepped through to
the keel, let alone grounded for lightning "protection") and being a
retired electrical engineer I've been a little concerned about lightning
strikes and actually have a little knowledge about the ways of electrons.
To make a long story short, it's pretty much of a crap shoot. There isn't
enough mass (in my opinion, and that of others) that you can carry (tent,
metal framed backpack, etc.) to make much of a difference. One thing that
is often overlooked is that the same conditions that produce lightning
often produce high winds and heavy rain. The advice about crawling into a
ditch sounds OK in your armchair, but when the ditch floods from the rain
you're just another wet object (you may be the tallest) running around
looking for a place to hide. Wheeled vehicles may provide some Faraday
cage like protection, but during high winds or rising water conditions a
car or trailer is not exactly the best place to be. Also commonly
overlooked is the vast amount of heat and explosiveness often produced by a
lightning strike. Even religion isn't much of a guarantee, IIRC there were
a group of people who took refuge in a church, and well, you guessed it.
Maybe it was the wrong church, and Zeus got really upset about it. A guy
here in Florida got nailed by lightning (he lived) while inside his house!
As for that lone tree, don't go stand under it, but you also have to
realize that same lone tree didn't spring up overnight and this is not the
first thunderstorm it has had to go through, and it isn't just a pile of
match sticks.
There isn't much you can do once the storm is near, aside from avoiding
summits, cave entrances and the like. Just try to be aware and notice
where lightning has struck before (lightning does strike twice...even
thrice...and more) and realize that an electrical storm does not confer
some kind of immunity to other hazards.
I could put in a plug for Bear-Pruf Tents but I figure the kinds of folks
who would buy lightning "protection" will seek them out anyway.
Galen Hekhuis NpD, JFR, GWA ghek...@earthlink.net
They killed his mother, they burned his forest, he's back, and he's pissed
Bambo!
Mark Kinsler
Any experiment with random results follows physical laws. Flip a
coin. The results of the experiment could be accurately predicted
_if_ we had access to information about everything that affects that
coin's trajectory and spin. We could even predict the precise final
position of the coin as well as whether it lands heads up or tails up.
All of which is to say that both lightning and a flipped coin strictly
follow all the known rules of physics according to the prevalent
conditions, but we don't know the prevelant conditions. Ergo, the
phenomena are random for our purposes. In the design of gambling
devices, the object is to make the mechanism of randomness both
obvious to the observer but thoroughly unpredictable. A more complex
event like lightning is as random as a roulette wheel, but people try
to find a system in both.
Jerry M. Wright
<ghek...@earthlink.net> wrote:
[snip]
> Wheeled vehicles may provide some Faraday
>cage like protection, but during high winds or rising water conditions a
>car or trailer is not exactly the best place to be. Also commonly
>overlooked is the vast amount of heat and explosiveness often produced by a
>lightning strike.
Not to mention that car gas tanks contain highly volitile liquids know
to produce spectacular fireballs when ignited at once. From time to
time there are reports of parked cars being struck by lightening and
set on fire.
[snip]
Mark Kinsler
>
>Many of the rules that have been learned from astronomy can't be
>tested in the lab either, nor can many from deep strata geology, or
>more on point here, from meteorology, but that doesn't mean that
>we simply dismiss them as "anecdotal".
Astronomy and geology have two advantages. One is that the
experiments and observations are reasonbly repeatable, and the other
is that the two fields aren't burdened with a corps of rainmakers and
professional expert witnesses.
>There are a number of fields where we can't depend on a lab to
>replicate field conditions. We have to depend on in situ observations
>and inferences drawn from basic physical principles.
And if anyone had done even this much in lightning work, it would have
been great. But for the most part, they haven't.
>We do know that the Earth's surface layer can be modeled as a
>sheet resistance, which if fairly uniform, will disperse currents so
>that the potential between any two nearly adjacent points rapidly
>approaches a low constant value just a short radial distance from
>the current injection point. For typical soils, it is possible to fairly
>closely approximate the radial distance at which the potential falls
>to what are considered harmless levels.
Not with lightning. It ionizes discrete paths through the soil that
looks very much like the paths that it takes through the air. There's
not much voltage drop along any of these paths. Your model, which is
commonly accepted without much question, was first proposed by
E.D.Sunde of Bell Labs. Sunde said that any current injected into the
ground will spread in a fairly uniform sphere, and that the voltage at
any point will be determined by the resistivity of the soil. Sunde's
a marvelous researcher and pretty much the industry standard when it
comes to soil and electricity. But one of the reasons I got out of
graduate school alive is that I devised an experiment that proved this
particular model to be incorrect. Others have, as well.
>Real soils aren't actually uniform in many cases. But the errors along
>different radials due to this are typically less than an order of magnitude.
>So if the usual engineering safety factor margins are observed, good
>rules of thumb are possible for particular types of terrain.
These are suitable for determining earth potentials due to ground
faults on power lines and at substations. But those are low voltages
and low frequencies. They're easy to model and the models can be
verified through field and lab experiments. The lightning safety
community has appropriated these results, which were never meant to be
applied to lightning, and made up lightning safety rules that sound
lots better than they are.
>Now this is discussing hazards from Earth currents due to a near strike.
>There is also the possibility of sideflash. It is typical for sideflash distances
>to be shorter than the height of the object being struck, for the obvious reason.
>But over hard rock, there often isn't a good Earth termination at the base of
>the object struck. In this situation, the safe radial distance to avoid a sideflash
>can be significantly greater than the height of the object.
Yup.
>The lesson to be drawn from this is to be situationally aware of the sort
>of terrain in which you find yourself when deciding what is and isn't a
>reasonably safe location.
Which is essentially what I've always said: the best lightning safety
rules are your own instincts. If you don't feel safe, take such cover
as you feel is necessary.
>>> Also, sitting on your pack to insulate yourself from the ground is
>>>considered or elevated a bit by sitting on a rock giving the lighting a
>>>chance to just travel past and underneath you.
>>
>>Again, this is something that someone made up to make hikers feel like
>>they have some control over the situation. I doubt that your pack
>>would make a bit of difference, but there's no way to test this. It's
>>certain that whoever wrote this particular rule didn't test it.
>
>There are ways to model this. Again knowing that the Earth behaves
>as a sheet resistance, and knowing that a potential can only be expressed
>across two separated points in contact with that sheet resistance, it can
>be concluded that a single point of contact minimizes the potential that
>can be expressed across objects attached to that point due to Earth
>currents induced by the nearby strike.
Lightning current zig-zags through the earth as it does through the
heavens. We don't understand the mechanism, though some fairly
impressive experiments were mounted to try to find out. The work is
mostly found, for some reason, in the IEEE Transactions on Nuclear
Science from the 1970's and '80's.
>>> A mountain pass is considered safer than being higher on the ridge of
>>>the mountain itself.
>>
>>If people are shooting at you, this is correct. If you're dealing
>>with lightning, have a look at where lightning strikes, and why. A
>>thundercloud that's ten miles up and ten miles long isn't gonna care
>>if you're on a ridge or not.
>
>It doesn't care from a distance of 10 miles, but lightning doesn't form
>in a single 10 mile leap. It moves in a series of step leaders, and each
>short move is subject to the influence of its immediate environment.
That is true. Right now, we're trying to learn a bit about just what
causes those steps and to perhaps compare their behavior to other,
more observable phenomena in nature. We're in a strange position when
it comes to arc phenomena: we can make electric arcs do just about
anything we want, but the reasons that they do what they do isn't as
obvious as we wish.
However, we do know that mountains don't get hit at their tops much
more than on their sides. One difficulty is that it's almost
impossible for an observer to see precisely where the arc attaches to
a mountainside, and a strike that appears to have hit a ridge could
well be striking the slope on the other side of that ridge.
>The behavior of lightning isn't just random whimsy. The discharge
>has to follow the rules of electrical behavior. We generally can't know
>a priori all of the environmental factors which shape the lightning path,
>but it is foolish to dismiss the ones we do know or can influence.
That would be correct if we knew what those influences were and had
control over them. But at this stage in technology, we just don't.
Lawyers for people suing the local golf course don't want to hear
this. Nobody wants to hear this. It's a bit like saying that we
don't really understand all the causes of disease, which we don't,
either. Hence asbestos manufacturers get sued for causing health
problems in people who once lived near an insulation plant, and
country clubs get sued because someone got hit by lightning even when
he was in a lightning shelter.
And _that_ is what drives the lightning safety business, it turns out.
Mark Kinsler
>>We've never found the slightest evidence of this. While it is true
>>that you can detect a fairly substantial current in a lightning rod,
>>and perhaps even a greater one in one of the many lightning dissipator
>>devices sold through the years, the earth is a good conductor and thus
>>cannot be "discharged." Every bit of charge that migrates through the
>>rod up to the cloud will be replaced from the surrounding earth.
>
>Well, that's not exactly what the dissipation array folks are claiming.
>They basically make two claims.
>
>1) Their arrays discharge the *cloud* base over the site. This is different
>from claiming to discharge the Earth. The latter is impossible even in theory,
>as you note. The former is impossible *in practice* as I've noted.
>
>Or 2) Their arrays create a "space charge" around the site which repels strikes
>in the same way that the space charge in a triode vacuum tube reduces its
>transconductance by "shielding" the plate from electrons. This claims sounds
>good at first glance, until you realize that storms have wind, and that any space
>charge created is blown away as rapidly as it can be formed.
These are their latest claims. They proposed the earth discharge
theory in one of their papers from, I think, 1995 or thereabouts. The
claims change periodically.
Lately, the big deal has been the early streamer emission device or
ESE. These are supposed to be better than regular lightning rods
because they promote the emission of a streamer that no stepped leader
can possibly resist. I suppose there are lightning pheremones in it.
Anyway, the ESE has been the subject of a lot of trouble lately as
well. A lot of them look the same as the old disspators: lotsa points
on a pole. No way to find out how well they work or don't work.
There are, oddly enough, several firms who make ESE's who can't really
be faulted for their activities. They're making what their customers
have asked for, but they don't market the devices aggressively.
If anyone is still reading, I applaud you for your persistence in
trying to divine something out of this strange old controversy. A
reference was made to lightning dissipators that contain radioactive
material. There's a bit of real science to all this: radioactivity
will make the air conductive and cause a static charge to dissipate.
So in about 1910, some experiments were done with lightning rods
containing pitchblende and other radioactive substances. It would
have been better if they hadn't chosen the Pope's residence at the
Vatican to try one experiment with these: the first thunderstorm that
came along produced a lightning stroke that blew the Papal Seal right
off the building.
In lightning, the science is always weak but the stories are always
great.
Marvin W. Klotz
<snip>
>It would
>have been better if they hadn't chosen the Pope's residence at the
>Vatican to try one experiment with these: the first thunderstorm that
>came along produced a lightning stroke that blew the Papal Seal right
>off the building.
No doubt accompanied by a thundering voice from above, "He was right. And yet
it *does* move."
Marv
--
HSM Freeware Programs at
http://www.geocities.com/mklotz.geo
Louis Boyd
enclose the item(s) to be protected in an all metal closed building with
no external conductors or apertures. In some cases that's practical and
cost effective. If that's too expensive you can work backward from
there, accepting more risk with less cost.
Adding one lightning rod well above a building may reduce the costs
associated with lightning damage by 90% with under 10% of the cost of
full shielding. Just putting a metal roof on the building when it's
built and bonding it to a ring ground may be even better. The job of
someone engineering a lightning protection system is to find a good
economic compromise.
--
Lou Boyd
Fairborn Observatory
Ed Huesers
> If anyone is still reading, I applaud you for your persistence in
> trying to divine something out of this strange old controversy.
It's always been a concern amongst backcountry users and is a much
better read than the political posturing that has persisted on r.b. for
nearly the last year due to the past election. Whining, I know, sorry.
> A reference was made to lightning dissipators that contain
> radioactive material.
> There's a bit of real science to all this: radioactivity
> will make the air conductive and cause a static charge to dissipate.
> So in about 1910, some experiments were done with lightning rods
> containing pitchblende and other radioactive substances. It would
> have been better if they hadn't chosen the Pope's residence at the
> Vatican to try one experiment with these: the first thunderstorm that
> came along produced a lightning stroke that blew the Papal Seal right
> off the building.
Well, I attended a catholic school for my first 9 years and....
Laying in bed [some 20 years ago] with an approaching storm sounding
off, I decided to ask a question.
Testing the waters, so to speak, I asked a few questions with obvious
answers. The lower/ quieter [probably the back of the storm] reports I
took as a positive answer and the louder [probably the front of the
storm] reports were taken as a negative response.
After testing the waters, I asked the moral question that I was
interested in. There was an immediate and very close report with two
explosions.
The next day while driving around, I saw that a tree in front of the
nuns house had been struck and both trunks of the forked tree had
exploded.
You mentioned, divine....
> In lightning, the science is always weak but the stories are always
> great.
Uh huh.
Thanks again for playing.
Ed Huesers
w_tom
currently predictable by man. Even though lightning is only somewhat
statistically predicted by man does not mean the event is totally random.
Lightning is statistically predictable and man is expected to learn the task in
the future. To say that lightning is not statistically predictable is to say
that all lightning is totally random - which it is not.
When we say something is statistically predictable, we say so from the
perspective of natural laws - mathematics, science, etc. We do not declare
something totally random only because man has not yet learned how to make those
predictions.
w_tom
to pass through less conductive soil. Such ionization is not permitted around earth
ground rods, since that would make those rods immediately, no longer earth grounds. If
soil ionizes around the earth ground, then the earth ground rod was too insufficient.
When lightning is properly dissipated into earth by larger area grounds, then lightning
performs more like a sheet of electrical current passing through a block of conductive
material - without the zig-zag ionization.
Part of designing a good earth ground is to make it large enough that lightning treats
earth as a conductive sheet - so that lightning absolutely never creates ionized zig-zag
paths in the earth. As example of earth not monolithic: two adjacent towns - but only
one gets repeated CG strikes and damage, while the other town rarely has such strikes.
They have significantly different geologies. Not only is the material different, but
the town with more damage also has less monolithic soil. Where damage is most common,
there is less manmade grounding to make the soil appear more monolithic.
Mark Kinsler wrote:
> ...
> Not with lightning. It ionizes discrete paths through the soil that
> looks very much like the paths that it takes through the air. There's
> not much voltage drop along any of these paths. Your model, which is
> commonly accepted without much question, was first proposed by
> E.D.Sunde of Bell Labs. Sunde said that any current injected into the
> ground will spread in a fairly uniform sphere, and that the voltage at
> any point will be determined by the resistivity of the soil. Sunde's
> a marvelous researcher and pretty much the industry standard when it
> comes to soil and electricity. But one of the reasons I got out of
> graduate school alive is that I devised an experiment that proved this
> particular model to be incorrect. Others have, as well. ...
Mark Kinsler
>to pass through less conductive soil. Such ionization is not permitted around earth
>ground rods, since that would make those rods immediately, no longer earth grounds. If
>soil ionizes around the earth ground, then the earth ground rod was too insufficient.
>When lightning is properly dissipated into earth by larger area grounds, then lightning
>performs more like a sheet of electrical current passing through a block of conductive
>material - without the zig-zag ionization.
Insofar as anyone can tell, there is _always_ ionization around a
lightning ground terminal. I don't know how you'd prevent it. Do
you?
> Part of designing a good earth ground is to make it large enough that lightning treats
>earth as a conductive sheet - so that lightning absolutely never creates ionized zig-zag
>paths in the earth. As example of earth not monolithic: two adjacent towns - but only
>one gets repeated CG strikes and damage, while the other town rarely has such strikes.
>They have significantly different geologies. Not only is the material different, but
>the town with more damage also has less monolithic soil. Where damage is most common,
>there is less manmade grounding to make the soil appear more monolithic.
If we wait long enough, we'd find that the number of strikes in an
area over a long time is roughly proportional to the area being
considered. Now, there does seem to be some correlation between
lightning damage and earth conductivity, but these are very tricky
relationships because of the regional nature of lightning. Yup.
South African lightning seems to be different from Japanese lightning,
and both are different from that in the US. At least that's what
we've discovered thus far. More observations from better lightning
antenna networks may improve our knowledge, but these networks are new
and data is slow in coming.
Mark Kinsler
>Complete lighting protection is pretty easy to achieve. You just
>enclose the item(s) to be protected in an all metal closed building with
>no external conductors or apertures. In some cases that's practical and
>cost effective. If that's too expensive you can work backward from
>there, accepting more risk with less cost.
Consider the problem of armor-plating a battleship. If you provide a
completely bulletproof covering of armor, you cannot shoot back and
the ship won't float, either. So there are always compromises. And
you don't know when you've done a good job--you only know when the
armor _hasn't_ done its job. Quite similar are the joys of lightning
protection and surge protection.
>Adding one lightning rod well above a building may reduce the costs
>associated with lightning damage by 90% with under 10% of the cost of
>full shielding. Just putting a metal roof on the building when it's
>built and bonding it to a ring ground may be even better. The job of
>someone engineering a lightning protection system is to find a good
>economic compromise.
All of these help, of course, but hungry teams of lawyers are waiting
for a lightning stroke to come in along a power line, telephone line,
or up a water pipe. A really good bolt of lightning can cause a dandy
sideflash (see Mr Coffman's analysis of this: it's very good) that'll
light off the protected building's powder magazine or electrocute the
congregation at prayer. I'm afraid that people in general and courts
in particular are convinced that it's possible to have guaranteed
lightning protection, and that anything less constitutes gross and
actionable negligence.
Mark Kinsler
>currently predictable by man. Even though lightning is only somewhat
>statistically predicted by man does not mean the event is totally random.
>Lightning is statistically predictable and man is expected to learn the task in
>the future. To say that lightning is not statistically predictable is to say
>that all lightning is totally random - which it is not.
>
> When we say something is statistically predictable, we say so from the
>perspective of natural laws - mathematics, science, etc. We do not declare
>something totally random only because man has not yet learned how to make those
>predictions.
Well, it's about as good as we can do right now. It may be that we
will eventually be able to model the earth's atmosphere such that we
can predict the weather accurately every time, but right now it seems
pretty random to me and the National Weather Service, for that matter.
In the meantime, we have statistics to divine the unknowable. It does
a pretty good job of dealing with these stochastic processes and, in
fact, is our best guide to learning just what physical phenomena lurk
behind these processes.
Many physical phenomena are random. This doesn't mean that we're
declaring them independent of physical laws.
mor...@niuhep.physics.niu.edu
>lightning strike. Even religion isn't much of a guarantee, IIRC there were
>a group of people who took refuge in a church, and well, you guessed it.
>Maybe it was the wrong church, and Zeus got really upset about it. A guy
>here in Florida got nailed by lightning (he lived) while inside his house!
A fellow in a medieval recreation group had just gotten all his camping
gear authentic for the period of time his "character" existed (except
for the c-pap machine that gets him through the night (it helps him breath))
when his tent got hit by lightning, blew his bed and two wooden boxes, gave
him, his wife and daughter burns, and charged up the batteries that ran his
c-pap machine. (The batteries and the melted sword I am inclined to
put down to the story telling the gentleman is sometimes accused of, but
the rest of the details have witnesses.)
Robert
Dave Martindale
>>It would
>>have been better if they hadn't chosen the Pope's residence at the
>>Vatican to try one experiment with these: the first thunderstorm that
>>came along produced a lightning stroke that blew the Papal Seal right
>>off the building.
>No doubt accompanied by a thundering voice from above, "He was right. And yet
>it *does* move."
What year did this happen? (It was only a few years ago that the
Roman Catholic Church commissioned a multi-year study which eventually
concluded that Galileo was right after all. 500 years after the fact).
Dave
mor...@niuhep.physics.niu.edu
>
>What year did this happen? (It was only a few years ago that the
>Roman Catholic Church commissioned a multi-year study which eventually
>concluded that Galileo was right after all. 500 years after the fact).
No, the Pope asked for a reexamination of the investigation to determine
... more or less what went wrong. Depending on what you count the RCC
accepted the heliocentric view between 100 and 200 years after it had
forbid Galileo to teach it.
Not to excuse what they did, but there was no proof of the heliocentric
view until the mid 1800s, when in short order the parallax measurments and
the Foucault's pendulum experiment demonstrated that revolved around the Sun
and that it the earth rotated about its axis.
> Dave
Marvin W. Klotz
>da...@cs.ubc.ca (Dave Martindale) writes:
>>
>>What year did this happen? (It was only a few years ago that the
>>Roman Catholic Church commissioned a multi-year study which eventually
>>concluded that Galileo was right after all. 500 years after the fact).
The fact that they had to *study* the issue at all is sufficiently hilarious.
"A few years ago", what was there to "study"? Not to mention the ugly,
editorial cartoon caricature suggested by a bunch of superstitious old men in
dresses "studying" scientific journals.
>
>No, the Pope asked for a reexamination of the investigation to determine
>... more or less what went wrong. Depending on what you count the RCC
>accepted the heliocentric view between 100 and 200 years after it had
>forbid Galileo to teach it.
So, does this mean they were annoyed, not by what he asserted, but rather by the
fact that he didn't ask their permission before saying it?
>
>Not to excuse what they did, but there was no proof of the heliocentric
>view until the mid 1800s, when in short order the parallax measurments and
>the Foucault's pendulum experiment demonstrated that revolved around the Sun
>and that it the earth rotated about its axis.
>
There was no *proof* of the geocentric theory either. And since when did an
organized religion feel the need for proof of anything before forming an
immutable opinion/belief?
The whole episode just reinforces the general principle that the church(es)
should stay clear of making pronouncements it can't support about any aspect of
the physical world. One disproven belief casts doubt on all beliefs.
Ed Huesers
> Does that rulebook tell you about "the lightning crouch?" That's one
> of my favorites, especially when they tell you to hop like a bunny.
I've always thought that position was required to kiss your axe good
bye.
Bunny hop! ROTFLMAO!
Ed Huesers
Mark Kinsler
I'm not making it up. If you find yourself in an open field during a
lightning storm, you should lie down so that you're lower than the
surrounding territory. But according to the sheet resistance theory
of lightning current flow, two points on the ground surface will be at
vastly differing potentials, so if you bridge them with your body
you'll get electrocuted. Ergo, you cannot lie down, and you must keep
your feet together. Hence the lightning crouch, a term I didn't make
up, either.
Then, of course, comes the problem of getting out of the thunderstorm.
You can't walk, because you might be bridging a high voltage
difference during the few microseconds that lightning hits the field.
So you must keep your feet together, but still move. That's where
they came up with the recommendation to hop.
I think that the American Red Cross may have figured out that people
were laughing at them and deleted the hopping recommendation, but it
shows up in various sets of lightning safety rules that get published
every summer by one organization or another.
The people on the lightning protection newsgroup became at once vague
and very angry at me when I asked for the scientific basis of these
and the other lightning safety rules. Did you know that you're
supposed to stay at least ten feet away from all plumbing, all
windows, all electrical appliances, and all windows for at least two
hours after the storm has passed? How about not using the telephone
when a storm threatens? Don't watch TV, either.
M Kinsler
not making these up. Really.
Bob Gross
do unless there is an emergency call to be made. I don't know about the
current statistics, but a couple of decades ago, the GTE telephone companies
served about 10% of all of the telephone subscribers in the United States.
At that time, they figured to have about two subscribers per year killed due
to lightning surge coming in on the phone line and hitting the large object
next to the telephone handset (the human skull). This is especially common
when the subscriber has the handset to one ear and is resting his hand or
foot on a cold water pipe or similar plumbing tied to ground. The lightning
surge protector present on the interface box at the edge of your house
should protect the user from some or most of this if it is working properly.
Hmmm. When was the last time that you had your lightning protector tested?
I'd rather be in a tent out in the middle of the woods.
---Bob Gross---
"Mark Kinsler" <kin...@frognet.net> wrote in message
news:3b2ee0a3...@news.earthlink.net...
Mark Kinsler
>do unless there is an emergency call to be made. I don't know about the
>current statistics, but a couple of decades ago, the GTE telephone companies
>served about 10% of all of the telephone subscribers in the United States.
The grain of truth at the bottom of "don't use the phone during
thunderstorms" is a directive issued by one or another phone company
about using the telephone during _any_ sort of storm or other
emergency situation. A conventionally-switched telephone system can
only accomodate about 14% of its subscribers. If everyone picks up
the phone at the same time, some true emergency calls might be
blocked. It's still good advice.
>At that time, they figured to have about two subscribers per year killed due
>to lightning surge coming in on the phone line and hitting the large object
>next to the telephone handset (the human skull).
Two per year? That seems awfully high.
>This is especially common
>when the subscriber has the handset to one ear and is resting his hand or
>foot on a cold water pipe or similar plumbing tied to ground.
Who the heck does that? It sounds like someone was working out the
theory of how a telephone electrocution might happen and the
speculation found its way into a safety article.
>The lightning
>surge protector present on the interface box at the edge of your house
>should protect the user from some or most of this if it is working properly.
Yup. Only it's rated to clamp at around 700V--at least the old carbon
ones were. The newest devices use MOV's or the occasional
gas-discharge tube. They last about forever because the working
voltage of the telephone line is so much lower than any possible surge
voltage.
>Hmmm. When was the last time that you had your lightning protector tested?
They generally fail by short-circuit, as I recall. We used to clean
the crud out of the carbon ones at WTSO radio in Madison, WI on a
pretty regular basis.
>I'd rather be in a tent out in the middle of the woods.
Probably wouldn't make a lot of difference. If a tree nearby
sustained a lightning stroke and a sideflash jumped to your tent pole
and then through you, some bored writer for an outdoor magazine would
soon be recommending that everyone sleep at least fifteen feet away
from a tent pole. Someone would then start selling lightning-proof
fiberglass tent poles.
M Kinsler
mor...@niuhep.physics.niu.edu
>>No, the Pope asked for a reexamination of the investigation to determine
>>... more or less what went wrong. Depending on what you count the RCC
>>accepted the heliocentric view between 100 and 200 years after it had
>>forbid Galileo to teach it.
If I recall the timing, about 70 years after he died they published his
book. Somewhat less than 200 years after his death they removed his
book from the censored list. I take this to mean that the earlier
publication was of a somewhat modified manuscript emphasizing the
"theoretical" nature of the heliocentric model.
>So, does this mean they were annoyed, not by what he asserted, but
>rather by the fact that he didn't ask their permission before saying it?
They, i.e. the RCC in the 1600s, were most annoyed that he started
interpreting scripture and that he insisted that anybody who disagreed with
him was a moron.
Well, reallly the second part was simply what lost him his allies,
including the Pope. If he had been content to say "I don't know what
Joshua meant, but this is the best way to interpret the observational
evidence." he likely would never have been brought up on any charges.
The fact that he was wrong on other issues of science, notably
the tides, meant his arrogance was even more grating.
Again this does not excuse what the Church did, which was to prevent him
from publishing even purely secular explanations of the heliocentric
model.
>>Not to excuse what they did, but there was no proof of the heliocentric
>>view until the mid 1800s, when in short order the parallax measurments and
>>the Foucault's pendulum experiment demonstrated that revolved around the Sun
>>and that it the earth rotated about its axis.
>There was no *proof* of the geocentric theory either.
In the realm of physics, the lack of parallax is evidence in favor of the
geocentric theory, as is the fact that we don't feel any motion.
Both scripture and theology offer some support to the idea of a
geocentric universe as well.
>And since when did an organized religion feel the need for proof of
>anything before forming an immutable opinion/belief?
IMHO one of the Church's failings is that it has tended to be overly
dogmatic on details. It usually carefully studies a matter before
forming an opinion, but then it tends to ossify around that opinion.
OTOH Cardinal Bellarmine, who argued for the literal understanding of
Joshua, wrote the following concerning his disagreement with Galileo:
I say that if a real proof be found that the sun is fixed and
does not revolve round the earth, but the earth round the sun,
then it will be necessary, very carefully, to proceed to the
explanation of the passages of Scripture which appear to be
contrary, and we should rather say that we have misunderstood
these than pronounce that to be false which is demonstrated.
(this is a favorite quote of mine for dealing with scientific creationists)
>The whole episode just reinforces the general principle that the church(es)
>should stay clear of making pronouncements it can't support about any aspect
>of the physical world. One disproven belief casts doubt on all beliefs.
I quite agree.
Robert
Galen Hekhuis
>>Using the telephone during a lightning storm is a reasonably stupid thing to
>>do unless there is an emergency call to be made. I don't know about the
>>current statistics, but a couple of decades ago, the GTE telephone companies
>>served about 10% of all of the telephone subscribers in the United States.
>
>The grain of truth at the bottom of "don't use the phone during
>thunderstorms" is a directive issued by one or another phone company
>about using the telephone during _any_ sort of storm or other
>emergency situation. A conventionally-switched telephone system can
>only accomodate about 14% of its subscribers. If everyone picks up
>the phone at the same time, some true emergency calls might be
>blocked. It's still good advice.
As a useless data point, I was online with a laptop with an internal modem
during a thunderstorm in Burgaw, NC. I used to sort of like to do that
because often times lightning would put out the AC power, yet my laptop
would switch to battery power and the phone connection would remain intact.
I could surf the web when my brother (who used a desktop) was shut down.
Then one day the power didn't shut down, but the phone line was struck by
lightning. Aside from knocking out all the phones in my brother's house,
my laptop (which was on my lap) literally blew up. I was relatively
unharmed, though discolored in several places, but the laptop (and its
internal modem) was a total loss. Now I live in Florida, and the phone
line goes to a "gateway," which allows me a wireless connection to the net
from my laptop. Now I can have no wires at all when I run my laptop during
a thunderstorm. You can use a "cordless" phone quite safely during
lightning, and even a battery and inverter (to power the base) if the AC
goes out. In my experience using a conventional (wired) phone during a
thunderstorm is a tad risky.
SPeacock
burn on lip. While talking to my girl friend, I was gazing out the window
toward the line leading up and over the hill. It seems I felt the shock and
spark, THEN saw the flash on the highest pole (about 1/2 mile away), then heard
the 'ding' of the bell as it fused...all in short order. The next thing I knew,
my parents were looking down at me with the dumbest expression on their faces.
The joke was that it was the 'electricity' of true love - a kiss, that caused
the damage.
Jerry M. Wright
wrote:
[snip]
>Then, of course, comes the problem of getting out of the thunderstorm.
>You can't walk, because you might be bridging a high voltage
>difference during the few microseconds that lightning hits the field.
>So you must keep your feet together, but still move. That's where
>they came up with the recommendation to hop.
A bounding run will have the same effect on contact, get you out of
the area much faster and is very likely what most people would be
inclined do anyway.
[snip]
>
>The people on the lightning protection newsgroup became at once vague
>and very angry at me when I asked for the scientific basis of these
>and the other lightning safety rules. Did you know that you're
>supposed to stay at least ten feet away from all plumbing, all
>windows, all electrical appliances, and all windows for at least two
>hours after the storm has passed? How about not using the telephone
>when a storm threatens? Don't watch TV, either.
When I was in NY there was one death by lightening in Syracuse after a
storm when the woman was talking on the phone. Relatives were suing
the phone company. Phone company claims the lines were OK and that
the fatal charge came through the refrigerator she was leaning on
while talking on the phone. I don't know how it turned out. 2 hours
seems excessive but I know discharge can happen after the storm has
apparently passed.
Jim Stewart
Bob Gross wrote:
>
> Using the telephone during a lightning storm is a reasonably stupid thing to
> do unless there is an emergency call to be made.
(snip)
> The lightning
> surge protector present on the interface box at the edge of your house
> should protect the user from some or most of this if it is working properly.
>
Not really. The protector found on a phone line (we call 'em carbons,
although it's been quite a while since they were made of carbon) is a
gas-discharge tube. It's designed to arc over, protecting the user, at
300 volts.
But the safety training material that telephone guys get says that the
protector will be destroyed at XX kilovolts - memory fails me, but XX is
around 5 or 10KV IIRC. It provides no protection at higher voltages,
like any kind of a lightning strike nearby. Nor from a nearby ground
fault from a 230KV power line...
-js
Bob Gross
and fail short. The newer gas tube protectors fail open. The MOV's and other
semiconductor protectors can go either way.
If a protector fails short, then you find out about it in a hurry (the
damned phone does not work). But if a protector fails open, typically there
is nothing that will detect that except for a special test device. Virtually
nobody uses those at either the central office or at the subscriber drop.
Unless, of course, some poor soul got zapped. Then there is a full-scale
investigation and the protector tester shows up. That does not help the poor
soul. Sometimes it enriches the surviving family.
---Bob Gross---
"Mark Kinsler" <kin...@frognet.net> wrote in message
news:3b2f61a9...@news.earthlink.net...
cy...@tiny.net
some grains of salt with it for the quality of a long long ago memory
and some more for the possible inaccuracy of my then sources):
There was also the problem that some particular heresy was running
through the known world at that time that had the heliocentric stuff
as part of it and re-inforcing it. I don't believe that Gallileo had
even heard of the heresy when he promulgated his theory, but the
Church had. And may even have thought that he was in on it. If they
allowed his theory to be publicized, they'd practically be endorsing
the heresy (no, I don't recall what it was. It was one of those that
went away when all its followers recanted or were killed.)
On 19 Jun 2001 14:48:02 GMT, mor...@niuhep.physics.niu.edu wrote:
(heaps of stuff snipped)
>
>They, i.e. the RCC in the 1600s, were most annoyed that he started
>interpreting scripture and that he insisted that anybody who disagreed with
>him was a moron.
>
>Well, reallly the second part was simply what lost him his allies,
>including the Pope. If he had been content to say "I don't know what
>Joshua meant, but this is the best way to interpret the observational
>evidence." he likely would never have been brought up on any charges.
>
>The fact that he was wrong on other issues of science, notably
>the tides, meant his arrogance was even more grating.
>
>Again this does not excuse what the Church did, which was to prevent him
>from publishing even purely secular explanations of the heliocentric
>model.
---
rbc: vixen Fairly harmless
http://www.visi.com/~cyli
mor...@niuhep.physics.niu.edu
>According to what I read when doing a high school paper (Meaning take
>some grains of salt with it for the quality of a long long ago memory
>and some more for the possible inaccuracy of my then sources):
>There was also the problem that some particular heresy was running
>through the known world at that time that had the heliocentric stuff
>as part of it and re-inforcing it. I don't believe that Gallileo had
>even heard of the heresy when he promulgated his theory, but the
>Church had. And may even have thought that he was in on it. If they
>allowed his theory to be publicized, they'd practically be endorsing
>the heresy (no, I don't recall what it was. It was one of those that
>went away when all its followers recanted or were killed.)
I believe you are half right.
If you read up on Bruno you will find that he was burned at the stake
for heresy, he believed in heliocentrism as part of that heresy. I
don't *think* this had anything to do with Galilleo.
Robert
Gary Coffman
>>
>>Many of the rules that have been learned from astronomy can't be
>>tested in the lab either, nor can many from deep strata geology, or
>>more on point here, from meteorology, but that doesn't mean that
>>we simply dismiss them as "anecdotal".
>
>Astronomy and geology have two advantages. One is that the
>experiments and observations are reasonbly repeatable, and the other
>is that the two fields aren't burdened with a corps of rainmakers and
>professional expert witnesses.
Well, I won't disagree with the latter statement. However, just to name
two well known things from astronomy and geology that can't be replicated
in the lab, nor exactly repeated in the field, how about supernovas and
earthquakes? Yet neither of those events is considered random. They
only occur under specific conditions in specific places. The same can
be said of lightning.
If lightning were as random as a coin toss, as you maintained in another
post, then over time it would show a uniform distribution over the Earth,
and would as frequently strike a one story building as it strikes the 1000
foot broadcast tower adjacent to the building. But that's not the case.
I"ve got strike counter data from enough broadcast sites (12 TV and
22 radio stations owned by my company) to be very confident that the
towers are struck very much more often than the buildings. In fact,
I have no examples of a transmitter building being struck directly
(though of course strike currents can come in on power, telco, or
transmission lines from the actual strike point).
Any randomness associated with lightning strikes is not like an honest
coin toss. It is like playing with heavily loaded dice. The size and distribution
of charge separations, the shape of terrain (natural or artificial), the presence
or absence of heavily streaming objects, etc serve to load the dice.
>>We do know that the Earth's surface layer can be modeled as a
>>sheet resistance, which if fairly uniform, will disperse currents so
>>that the potential between any two nearly adjacent points rapidly
>>approaches a low constant value just a short radial distance from
>>the current injection point. For typical soils, it is possible to fairly
>>closely approximate the radial distance at which the potential falls
>>to what are considered harmless levels.
>
>Not with lightning. It ionizes discrete paths through the soil that
>looks very much like the paths that it takes through the air. There's
>not much voltage drop along any of these paths. Your model, which is
>commonly accepted without much question, was first proposed by
>E.D.Sunde of Bell Labs. Sunde said that any current injected into the
>ground will spread in a fairly uniform sphere, and that the voltage at
>any point will be determined by the resistivity of the soil. Sunde's
>a marvelous researcher and pretty much the industry standard when it
>comes to soil and electricity. But one of the reasons I got out of
>graduate school alive is that I devised an experiment that proved this
>particular model to be incorrect. Others have, as well.
I haven't seen a copy of your dissertation (though I'd like to), but I have
read other papers on this subject. As best I can determine, the sort of
behavior you describe only happens if the Earth termination is relatively
high impedance. It certainly doesn't happen in a broadcast ground field.
I've dug up and replaced enough broadcast ground fields to be confident
of that. The charge is adequately distributed over a large enough volume
that ordinary sheet resistance models work for the resulting Earth currents.
Currents do arc from soil grain to soil grain (and also capacitively couple
according to the models I respect), but with an adequate Earth termination,
current density in any given volume of soil is held to a low enough value
that worm tracks aren't produced. In my experience, adequate Earth
terminations are formed by broadcast radial systems, or by large Ufer
grounds. A handful of ground rods aren't an adequate Earth termination
in most soils. So what you're saying may be perfectly correct in a back
country situation. I wouldn't be so rash as to suggest that a tree root
network is an adequate substitute for a field of buried copper, though
I am rash enough to suggest that it would probably have some effect.
I also have a comment on the notion that mountain peaks aren't
the preferred target for lightning strikes. At least in cases where
the peaks are mostly hard rock, they're lousy conductors. A spot
lower on the mountain with a more conductive soil layer may
be a more tempting target.
An example is Georgia's Stone Mountain. It is a large exposed
granite peak. It is virtually impossible to achieve a good ground
anywhere on its top. OTOH, most other "mountains" here in
Georgia have soil and trees all the way to their peaks, and are
better targets than the valleys in between. An example is Sweat
Mountain, which has one of our microwave relay sites at its peak.
That gets hit regularly, but houses further down the mountain
are rarely struck.
Another example, I'm responsible for a 350 foot tower about
5 miles from my house. According to the strike counter, it has
been struck 17 times in the past 2 years. OTOH, my tower
here at the house, a 70 footer in similar terrain, has only been
struck twice in the last 2 years. (There's a storm front moving
in as I type this, so those numbers are subject to change.)
I am willing to assert that taller objects, which are good
conductors and are well grounded, do get struck much
more often than shorter ones. Much more often than mere
random chance would imply. Our taller broadcast towers
(typically 1040 feet AGL) often get multiple strikes from
the same storm cell.
I was a bit taken aback by your statement that it is hard
to find sites that are frequently struck. I'd think that any
reasonably tall broadcast tower in the Southeast would
provide abundant strike events. Most strikes would pass
unnoticed without strike counters, though, because the
systems are engineered to handle most strikes without
damage.
Gary
John Paul Minda
1:26 AM:
> Using the telephone during a lightning storm is a reasonably stupid thing to
> do unless there is an emergency call to be made.
So what happens if you (as is the case in many homes) are talking on a
cordless phone? I assume that is safe? As long as you are not sitting in the
tub.
JPM
------------------------------------
John Paul Minda
Beckman Institute
The University of Illinois
405 N. Mathews Ave.
Urbana, IL 61801
(217) 333-2012
mi...@beckman.uiuc.edu
http://www.staff.uiuc.edu/~minda
------------------------------------
Ron Wong
<ke...@bellsouth.net> wrote:
> On Fri, 15 Jun 2001 01:12:52 -0700, ron...@purgethis.inreach.com (Ron
Wong) wrote:
> >When most people comment about lightning rods, it's in terms of what
> >happens when the rods are struck by lightning. This occurs when the
> >electric field strength in the area of the rod matches/exceeds the
> >ionization potential of the air (the value depends on the humidity among
> >other things but 10,000 V/cm is a common figure).
> >
> >What many people don't understand is that the PRIMARY function of a
> >lightning rod is to REDUCE the chance of a lightning strike in the
> >immediate area of the rod. In other words, to try to prevent lightning
> >from happening in the first place.
>
> *More* incorrect information. It is essentially impossible to *prevent*
> lightning from occurring. The snake oil salesmen pushing so-called
> *dissipation arrays* are counting on the fact that you haven't done the
> math.
It seems Gary overlooked the word "REDUCE" despite the fact that it was
in BOLD print.
Saying the "function of a lightning rod is to REDUCE the chance of a
lightning strike in the immediate area of the rod. In other words, to try
to prevent lightning from happening in the first place." is like saying
the "function of a stop light at a busy intersection is to REDUCE the
chance of an auto accident in the immediate area of the stop light (the
intersection). In other words, to try to prevent an auto accident from
happening."
The stop light won't prevent auto accidents from happening, it will simply
reduce the number by reducing the chance of them happening (in this case,
by regulating the flow of traffic). It would be ridiculous to respond to
this remark by saying it's impossible to *prevent* an accident from
happening. Accidents happen. That's why the phrase "to try to prevent"
appears in the second sentence of my statement.
As far as the subject of lightning is concerned, I went on to bring up the
fact that lightning rods get struck by lightning. I wouldn't have brought
this up if my position was that it was impossible for lightning to strike
when using a lightning rod as Gary implies.
Apparently, Gary has a beef with those who put their faith in "dissipation
arrays" and has taken the opportunity to climb on his soapbox to make his
point. His arguments regarding their claims may be valid but I made no
such claims.
Regarding my remark that
> ...
> >If you connect a lightning rod to ground, the charges in the ground will
> >migrate towards the tip and create an intense electric field around the
> >tip. This will cause the electrically neutral molecules in the surrounding
> >air to move towards the tip. When the molecules come in contact with the
> >tip (actually, they frequently only have to come very close), the charge
> >will be transferred to the molecules. Since the molecules now have the
> >same charge as the rod, they are driven away to the clouds above (which
> >have the opposite charge of the ground and would tend attract these
> >molecules).
He responds:
> This model is physically inaccurate. For a correct description, see
> "Lightning" by Unman (Dover Press), or if you can find a copy, the classic
> paper "The Lightning Discharge" by Gardener.
Well, Martin A. Uman (that's his correct last name) has written a couple
of books that I am in fact familiar with: "All About Lightning" and "The
Lightning Discharge" (I'm not familiar with Gardener's book which,
amazingly enough, seems to have the same title). Mr. Uman has mentioned in
his work the dubious claim that charged air molecules created by a
lightning rod neutralize the charge of the clouds during an electrical
storm. He discounts this claim and I can't think of any reason to support
it either. Apparently, this is also a claim made by those who market
"dissipation arrays". I made no such claim. I only said that the charged
molecules, having the opposite charge to that of the clouds above, would
be attracted to them.
> Unfortunately, when you do the math you find out that you cannot dissipate
> the charge rapidly enough without the discharger breaking into streaming
> (approx 20 uA per discharge point), and streaming is what causes the
> lightning rod to be the preferred target for a lightning discharge.
>...
This may all be true but, once again, we have here another example of the
fact that when people discuss lightning rods it always tends to be in
terms of what happens when a lightning discharge takes place.
> The FAA conducted definitive tests (large scale enough to be statistically
> significant) of the so-called dissipation arrays (installed by their
proponents)
> at hundreds of tower sites in Florida (lightning alley). What they found over
> a two year period was that sites using the dissipation arrays were struck
> slightly more frequently, and sustained significantly more damage, than the
> sites protected by conventional lightning rods.
Well, the fact that both the dissipation arrays and the conventional
lightning rods where struck by lightning seems to say that they both
performed quite well in their secondary function - diverting the lightning
to themselves.
The fact that the dissipation arrays didn't always protect the structures
to which they were attached is another story - and not a good one at that.
Chalk one up in favor of the conventional lightning rod.
Assuming the comparisons are based on everything being equal, the fact
that "sites using the dissipation arrays were struck slightly more
frequently...than the sites protected by conventional lightning rods"
would seem to suggest that the conventional lightning rod was doing a
slightly better job than the array in fulfilling its primary role of
preventing lightning from occurring. There were less strikes where the
rods were as opposed to the arrays.
Seems the conventional lightning rod wins hands down when it comes to lightning.
------------------------------------
I use to give a demonstration to people who supposedly were knowledgeable
about the basic laws of electricity and magnetism. It involved a device
called a Van de Graaf generator.
For those of you who are not familiar with the device, it basically
consists of a large, hollow metal sphere perched on top of a tube made of
plastic (i.e. an insulator). A belt is used to continuously carry charges
up through the center of the tube to the metal sphere where they are
deposited. A very strong electrical field around the sphere is produced as
a result of this action. According to the manufacturer one could, under
the right conditions, create a difference of potential between the sphere
and ground of around 300,000 volts.
Using a grounded wand, I could regularly produce electrical discharges of
20-25 cm in length - branching paths and all. It was enough to hold
everyone's attention.
At some point in the demonstration, I would take out an ordinary thumb
tack and ask, "which has the greater curvature? The large metal sphere or
the pointed end of this thumb tack?" and they would respond with their
answers.
I would then take a small piece of tape, push the point of the tack
through the tape from the sticky side, and then attach the tack to the
sphere by means of the tape. I would then ask the group, "Where will the
charges being stored on the sphere tend to be concentrated the most? On
the region with the largest radius of curvature, the sphere, or the least,
the end of the tack?" and they would reply.
I would then go on to ask them, "As a result, where is the electric field
going to be the most intense? The sphere with it's large radius of
curvature or the end of the tack?" and finally, "So where will we see the
most electrical discharges? ...".
To show that the presence of the tack did not affect the ability of the
generator to develop very high voltages, I would take the grounded wand
and hold it 20-25 cm from the opposite side of the sphere from the tack so
that they could see that regular discharges were still possible due to the
very strong electric field there. Then, while continuing to hold the wand
at this same distance, I would slowly bring it around to where the tack
was.
It was at this point that they discovered that they had all come up with
the wrong answer.
------------------------------
The next time you go to one of those interactive science museums, take
along a tack and some tape. There's usually a Van de Graaf generator being
used somewhere in the museum. You can ask the "explainer" to do this
demonstration for you or, if it is really hands on, you can do it
yourself.
Then you'll have a better idea why, although a single sailboat out at sea
during an electrical storm is a prime candidate for a lightning strike -
it happens quite frequently - a whole bunch of them tied up in a marina
will often find that lightning seems to strike every place but where all
the "lightning rods" are.
Of course, the rods don't stop lightning from happening in the marina.
Someone in a boating newsgroup brought up the fact that during one intense
electrical storm, a sailing boat just a few berths removed from the one he
was tied up to got hit and sank (He was pretty rattled. He was living
aboard his own boat at the time and was there when the other one took the
hit).
Like they say, when you time is up...
Speaking of which...
Gary Coffman
>In article <d9vkit8ich8kmi32s...@4ax.com>, Gary Coffman
><ke...@bellsouth.net> wrote:
>> On Fri, 15 Jun 2001 01:12:52 -0700, ron...@purgethis.inreach.com (Ron
>Wong) wrote:
>> >What many people don't understand is that the PRIMARY function of a
>> >lightning rod is to REDUCE the chance of a lightning strike in the
>> >immediate area of the rod. In other words, to try to prevent lightning
>> >from happening in the first place.
>>
>> *More* incorrect information. It is essentially impossible to *prevent*
>> lightning from occurring. The snake oil salesmen pushing so-called
>> *dissipation arrays* are counting on the fact that you haven't done the
>> math.
>
>It seems Gary overlooked the word "REDUCE" despite the fact that it was
>in BOLD print.
No I didn't. To reduce the occurrence of strikes means you have to
PREVENT some strikes from occurring. There's no practical way to
do that.
>Saying the "function of a lightning rod is to REDUCE the chance of a
>lightning strike in the immediate area of the rod. In other words, to try
>to prevent lightning from happening in the first place." is like saying
>the "function of a stop light at a busy intersection is to REDUCE the
>chance of an auto accident in the immediate area of the stop light (the
>intersection). In other words, to try to prevent an auto accident from
>happening."
>
>The stop light won't prevent auto accidents from happening, it will simply
>reduce the number by reducing the chance of them happening (in this case,
>by regulating the flow of traffic). It would be ridiculous to respond to
>this remark by saying it's impossible to *prevent* an accident from
>happening. Accidents happen. That's why the phrase "to try to prevent"
>appears in the second sentence of my statement.
Non sequitur. Lightning does not obey traffic signals. Nor does a lightning
rod function like a traffic signal. It provides a "cone of protection" but it doesn't
do that by *reducing* the number of strikes. It does that by safely taking the
strikes that would otherwise hit nearby vulnerable structures.
>Apparently, Gary has a beef with those who put their faith in "dissipation
>arrays" and has taken the opportunity to climb on his soapbox to make his
>point. His arguments regarding their claims may be valid but I made no
>such claims.
I do have a problem with con artists, and the purveyors of these "dissipation
arrays" are essentially trying to run a scam based on false premises. If they
were unaware that their premises are false at one point, there have been
adequate public refutations to make them aware of it by now.
>Regarding my remark that
>> ...
>> >If you connect a lightning rod to ground, the charges in the ground will
>> >migrate towards the tip and create an intense electric field around the
>> >tip. This will cause the electrically neutral molecules in the surrounding
>> >air to move towards the tip. When the molecules come in contact with the
>> >tip (actually, they frequently only have to come very close), the charge
>> >will be transferred to the molecules. Since the molecules now have the
>> >same charge as the rod, they are driven away to the clouds above (which
>> >have the opposite charge of the ground and would tend attract these
>> >molecules).
>
>He responds:
>
>> This model is physically inaccurate. For a correct description, see
>> "Lightning" by Unman (Dover Press), or if you can find a copy, the classic
>> paper "The Lightning Discharge" by Gardener.
>
>Well, Martin A. Uman (that's his correct last name) has written a couple
>of books that I am in fact familiar with: "All About Lightning" and "The
>Lightning Discharge" (I'm not familiar with Gardener's book which,
>amazingly enough, seems to have the same title).
That's the right spelling of Uman's name. My books are at work so
I was posting from memory. Gardener didn't write a book on lightning
as far as I know. He did write what is considered a classic paper that
appeared in IRE Transactions.
>Mr. Uman has mentioned in
>his work the dubious claim that charged air molecules created by a
>lightning rod neutralize the charge of the clouds during an electrical
>storm. He discounts this claim and I can't think of any reason to support
>it either. Apparently, this is also a claim made by those who market
>"dissipation arrays". I made no such claim. I only said that the charged
>molecules, having the opposite charge to that of the clouds above, would
>be attracted to them.
Yes, that's why streamers rise. However, it is impossible in practice to
force enough charge to the cloud deck rapidly enough to actually neutralize
the cloud. All the streamers do is provide a more conductive path for the
step leaders coming down from the cloud to find their way to the point
of emission. In other words, it increases the probability that the strike
will lead back to the streaming object, ideally our lightning rod.
>> Unfortunately, when you do the math you find out that you cannot dissipate
>> the charge rapidly enough without the discharger breaking into streaming
>> (approx 20 uA per discharge point), and streaming is what causes the
>> lightning rod to be the preferred target for a lightning discharge.
>>...
>This may all be true but, once again, we have here another example of the
>fact that when people discuss lightning rods it always tends to be in
>terms of what happens when a lightning discharge takes place.
Yeah, well that's what they do. They accept strikes.
>> The FAA conducted definitive tests (large scale enough to be statistically
>> significant) of the so-called dissipation arrays (installed by their proponents)
>> at hundreds of tower sites in Florida (lightning alley). What they found over
>> a two year period was that sites using the dissipation arrays were struck
>> slightly more frequently, and sustained significantly more damage, than the
>> sites protected by conventional lightning rods.
>
>Well, the fact that both the dissipation arrays and the conventional
>lightning rods where struck by lightning seems to say that they both
>performed quite well in their secondary function - diverting the lightning
>to themselves.
Their *only* function.
>The fact that the dissipation arrays didn't always protect the structures
>to which they were attached is another story - and not a good one at that.
>
>
>Chalk one up in favor of the conventional lightning rod.
Correct. The dissipation arrays typically don't offer as low an impedance
to the strike as the conventional rod. That's pretty much inherent in their
designs. So when they are struck, they aren't as capable of preventing
sideflash to the structures they are supposed to protect.
>Assuming the comparisons are based on everything being equal, the fact
>that "sites using the dissipation arrays were struck slightly more
>frequently...than the sites protected by conventional lightning rods"
>would seem to suggest that the conventional lightning rod was doing a
>slightly better job than the array in fulfilling its primary role of
>preventing lightning from occurring. There were less strikes where the
>rods were as opposed to the arrays.
No. The difference wasn't statistically significant. More importantly, neither
reduced the number of strikes to their towers compared to similar unprotected
towers (within the bounds of statistical significance, of course). In other words,
the study showed absolutely no evidence that either a conventional Franklin rod
or a "dissipation array" significantly changed the number of strikes suffered by
a site. What the Franklin rod did do is receive the bulk of the strikes itself, and
safely conduct them to Earth, instead of letting the strikes take non-engineered
paths to Earth.
The "dissipation array" folks were hoping to show a significant reduction in
the number of strikes to a site compared to an unprotected site, or to a site
protected by Franklin rods. But the study failed to show any such thing.
>Seems the conventional lightning rod wins hands down when it comes to lightning.
It does its intended job well, and costs significantly less than the hyped dissipation
arrays, which often do not protect their sites as well from the strike currents. Neither
reduces the probability of a strike.
>------------------------------------
>
>I use to give a demonstration to people who supposedly were knowledgeable
>about the basic laws of electricity and magnetism. It involved a device
>called a Van de Graaf generator.
<description of experiment snipped for brevity, see original post>
The problem with this experiment is that it doesn't scale to what happens
in actual thunderstorms. The potentials are far too low, the amount of charge
is far too small, and much too localized, there's no wind, the atmospheric
pressure is too high in proportion to the scale of the apparatus, etc to properly
mimic what happens in an actual storm. Mark has been saying that we can't
properly reproduce lightning in the laboratory, and he is correct. Things just
don't scale properly.
Gary
Gary Coffman
>Lately, the big deal has been the early streamer emission device or
>ESE. These are supposed to be better than regular lightning rods
>because they promote the emission of a streamer that no stepped leader
>can possibly resist. I suppose there are lightning pheremones in it.
Well, the notion that streamers effectively increase the height of the
conductor which is streaming does appear to be sound. Being closer
to the cloud, and further from other lightning targets, it is logical to
conclude that these conductive paths will make the streaming object
a more probable target for the strike. In other words, the increased
effective height makes it more likely to be intersected by a downward
questing step leader, completing the cloud to ground circuit. It is
difficult to craft a satisfactory explanation of why lightning rods work
if we don't invoke streamers of some sort.
But there's considerable debate whether strong streamers from
a single point or many weaker streamers from many points are the
more effective choice. I tend to fall into the camp that thinks the
stronger streamers from a single point are more effective, because
they are less likely to be completely dispersed by the wind. But that's
mostly just opinion. The data, such as it is, can't yet provide us with
a good answer. All the data can tell us with good confidence is that
streaming doesn't *reduce* the chance of a lightning strike.
Gary
Gary Coffman
>>Using the telephone during a lightning storm is a reasonably stupid thing to
>>do unless there is an emergency call to be made. I don't know about the
>>current statistics, but a couple of decades ago, the GTE telephone companies
>>served about 10% of all of the telephone subscribers in the United States.
>
>The grain of truth at the bottom of "don't use the phone during
>thunderstorms" is a directive issued by one or another phone company
>about using the telephone during _any_ sort of storm or other
>emergency situation. A conventionally-switched telephone system can
>only accomodate about 14% of its subscribers. If everyone picks up
>the phone at the same time, some true emergency calls might be
>blocked. It's still good advice.
There's good reason to avoid contact with long overhead wires during
a thunderstorm. Ben Franklin noted this when flying a kite in a storm.
(Some of his contemporaries were killed attempting to repeat his
experiment.)
Either a direct strike to the miles long overhead wire, or induction from
a nearby strike, is enough to produce a large voltage on the line. The
gas tube protectors used by the phone company at the line entrance
to the house have a limited ability to clamp the surge. They usually can
handle induction surge on the line, but often can't handle direct strike
currents. So the phone instrument can become electrified. You will be
too if you are holding it at the time of the strike.
There are better surge protectors than the ones used by the phone
companies, and the ones used by the phone companies often aren't
properly grounded. It is possible to protect the line so that you and
your equipment aren't damaged by a strike to the line, but it often isn't
done in ordinary phone installations. So it is a good practice to avoid
using the phone during a storm.
Gary
Jerry Rhodes
>a thunderstorm.
You don't work on a barbed wire fence when lightning is about either.
People have been killed by a strike that occured over the horizon when
working on long line fences in the west.
If you see a black cloud you call it a day.
Jerry
Gary Coffman
Yeah, same thing with long wire antennas. Years ago I had a 1000 foot
rhombic antenna that gave me a nasty shock while there was a thunderstorm
going on over the horizon. I could see the reflected flashes in the clouds at
the horizon, but radar showed the storm was over 60 miles away, so I didn't
stop work on the antenna. The jolt I felt was like grabbing a sparkplug on
a running auto. I've also observed arcing across ladder line, a 6 inch gap,
during nearby storms.
The suppressors on phone lines, if they're installed properly and in
good condition, can handle this sort of induction. But the induction
from a much nearer strike can be a problem. I've seen modems and
fax machines literally blown apart due to lightning surge coming in on
the phone line.
I went out with a buddy from Georgia Power to examine a house that
had suffered a lightning hit on the power lines. Every outlet was blown
out of the wall, and there were black lines along the walls where the
wiring ran. It was an amazing sight, and a wonder the house didn't
burn down.
A neighbor had the gate of his chain link fence welded shut by induction
from a strike to a large oak tree in the middle of his yard. (Most of the
appliances in his house were DOA too.) You don't have to take a direct
hit to suffer damage, or possibly death. Induction can get you too.
Now I have followed good practices with my current residence. I use
a single point entry panel and excellent suppressors (Polyphaser).
My tower has an excellent ground field as well. When I take strikes,
normally I suffer no damage. However, recently I've lost the front end
transistors in the radios of both my Jeeps, which are normally parked
fairly near my tower (about 50 feet away). I assume that to be due to
induction into their antennas from the nearby strike on the tower.
Gary
Ed Huesers
> > > As it is I just ran in panic.
> Ed Huesers wrote:
> > Was that you?
> > It looked to have struck the mountain on the other
> > side just past him.
> > seemed to have very light feet coming down all that rock.
Greg Rose wrote:
> Nah, not me.
> OK, I was scared shitless, I admit it.
> > > Hey, it worked.
> > Hey, the conversation wouldn't be what it is if it hadn't.
> That's a fact.
Damn campfire stories, oops, that log fell in of it's own accord.
Ed Huesers
Mark Kinsler
>
>Well, I won't disagree with the latter statement. However, just to name
>two well known things from astronomy and geology that can't be replicated
>in the lab, nor exactly repeated in the field, how about supernovas and
>earthquakes? Yet neither of those events is considered random. They
>only occur under specific conditions in specific places. The same can
>be said of lightning.
Well, here we run into the definition of randomness. However, you're
certainly correct about earthquakes. There's always been a great
demand for accurate earthquake prediction, and, as in the case of
lightning prediction, there are a good many journalists and lawyers
who won't accept the fact that it can't be done.
>
>If lightning were as random as a coin toss, as you maintained in another
>post, then over time it would show a uniform distribution over the Earth,
>and would as frequently strike a one story building as it strikes the 1000
>foot broadcast tower adjacent to the building. But that's not the case.
>
>I"ve got strike counter data from enough broadcast sites (12 TV and
>22 radio stations owned by my company) to be very confident that the
>towers are struck very much more often than the buildings. In fact,
>I have no examples of a transmitter building being struck directly
>(though of course strike currents can come in on power, telco, or
>transmission lines from the actual strike point).
You're right. (The big problem with all of this on the lightning
protection newsgroup is that nobody seems to be able to discuss these
matters as calmly and authoritatively as you can.)
One of the latest weird and wonderful discoveries in lightning-land is
that tall structures, aircraft, and wires shot up by rockets seem to
generate their own special brand of lightning. If these structures
weren't there, there wouldn't be a lightning strike in the area at
all. I'm not certain how anyone figured this out, but I tend to trust
the people who wrote the articles.
>Any randomness associated with lightning strikes is not like an honest
>coin toss. It is like playing with heavily loaded dice. The size and distribution
>of charge separations, the shape of terrain (natural or artificial), the presence
>or absence of heavily streaming objects, etc serve to load the dice.
True. There's clearly a mechanism involved, but nobody knows what the
heck it is. That makes it random as far as we're concerned at
present. Presumably we'll figure it out someday.
Well, we used wet sand, which is pretty conductive. The conductivity
didn't affect our results a bit. A broadcast-antenna ground plane
would be a rather different environment, I should think.
>Currents do arc from soil grain to soil grain (and also capacitively couple
>according to the models I respect), but with an adequate Earth termination,
>current density in any given volume of soil is held to a low enough value
>that worm tracks aren't produced. In my experience, adequate Earth
>terminations are formed by broadcast radial systems, or by large Ufer
>grounds. A handful of ground rods aren't an adequate Earth termination
>in most soils. So what you're saying may be perfectly correct in a back
>country situation. I wouldn't be so rash as to suggest that a tree root
>network is an adequate substitute for a field of buried copper, though
>I am rash enough to suggest that it would probably have some effect.
Most electric power system grounds consist of just a ground wire stuck
into the earth, typically wound around the buried end of a utility
pole. Most lightning rod systems have the same sort of arrangement.
Substations, which have to be concerned with fault currents, use far
more elaborate arrangements. But these aren't really designed with
lightning in mind. For that matter, neither are your tower grounds.
>I also have a comment on the notion that mountain peaks aren't
>the preferred target for lightning strikes. At least in cases where
>the peaks are mostly hard rock, they're lousy conductors. A spot
>lower on the mountain with a more conductive soil layer may
>be a more tempting target.
This doesn't seem to be a factor. Dry rock seems to get hit quite a
bit, and it's enough to drive you nuts if you're trying to ground
things. When tunnels were being drilled through the Alps, the
explosive guys found out to their delight that electric blasting caps
could be set off by lightning strikes atop the mountain, three miles
above the excavation.
>An example is Georgia's Stone Mountain. It is a large exposed
>granite peak. It is virtually impossible to achieve a good ground
>anywhere on its top. OTOH, most other "mountains" here in
>Georgia have soil and trees all the way to their peaks, and are
>better targets than the valleys in between. An example is Sweat
>Mountain, which has one of our microwave relay sites at its peak.
>That gets hit regularly, but houses further down the mountain
>are rarely struck.
That's a great name for a mountain. But I'll bet that Stone Mountain
has sustained a few strikes in its time.
>Another example, I'm responsible for a 350 foot tower about
>5 miles from my house. According to the strike counter, it has
>been struck 17 times in the past 2 years. OTOH, my tower
>here at the house, a 70 footer in similar terrain, has only been
>struck twice in the last 2 years. (There's a storm front moving
>in as I type this, so those numbers are subject to change.)
Do you have a strike counter on the mast at your home?
>I am willing to assert that taller objects, which are good
>conductors and are well grounded, do get struck much
>more often than shorter ones. Much more often than mere
>random chance would imply. Our taller broadcast towers
>(typically 1040 feet AGL) often get multiple strikes from
>the same storm cell.
I agree with you. But if you erect a tall tower and then wait for it
to get struck, you'll likely be waiting a very long time.
>I was a bit taken aback by your statement that it is hard
>to find sites that are frequently struck. I'd think that any
>reasonably tall broadcast tower in the Southeast would
>provide abundant strike events. Most strikes would pass
>unnoticed without strike counters, though, because the
>systems are engineered to handle most strikes without
>damage.
What I meant was that experience has shown that it's very difficult to
erect a lightning rod that'll consistently be hit with enough strikes
to do research with. This has been the experience of almost everyone
who's tried it. One fellow in Austria, www.aldis.com, I think, has
had more luck than most in this regard. He's been able to take
high-speed movies of a strike, in fact. I just discovered his site
yesterday.
Presumably we're making some progress with lightning, but it's
notoriously slow. Real science is like that, sometimes.
I'd like to develop a really cheap transient recorder and place a
million or so of the things on electrical outlets, utility poles,
buildings, fences, telephone lines, and everywhere else I could think
of. If these were networked together and could give us the time and
waveform of any electrical transient that appeared across their
terminals, we could start to get some real data to aid in the
development of good lightning and transient protection.
Mark Kinsler
Gary Coffman
>>Well, I won't disagree with the latter statement. However, just to name
>>two well known things from astronomy and geology that can't be replicated
>>in the lab, nor exactly repeated in the field, how about supernovas and
>>earthquakes? Yet neither of those events is considered random. They
>>only occur under specific conditions in specific places. The same can
>>be said of lightning.
>
>Well, here we run into the definition of randomness. However, you're
>certainly correct about earthquakes. There's always been a great
>demand for accurate earthquake prediction, and, as in the case of
>lightning prediction, there are a good many journalists and lawyers
>who won't accept the fact that it can't be done.
We may not yet be able to name the day and hour, but we do know
with a high confidence that there will be an earthquake along the
San Andreas or New Madrid faults within this century. We know with
equally high confidence that there won't be a major earthquake
in the area of the southern extent of the Canadian shield in the next
30 years. This isn't a matter of randomness, it is inherent in the
different geologies of the regions.
Similarly, I know with high confidence that none of the towers in this
area are going to be struck by lightning today. The meteorological
conditions aren't right, and won't be right for several more days, to
allow that to occur. If we were dealing with purely random events,
I couldn't make those assertions with any confidence.
>>If lightning were as random as a coin toss, as you maintained in another
>>post, then over time it would show a uniform distribution over the Earth,
>>and would as frequently strike a one story building as it strikes the 1000
>>foot broadcast tower adjacent to the building. But that's not the case.
>>
>>I"ve got strike counter data from enough broadcast sites (12 TV and
>>22 radio stations owned by my company) to be very confident that the
>>towers are struck very much more often than the buildings. In fact,
>>I have no examples of a transmitter building being struck directly
>>(though of course strike currents can come in on power, telco, or
>>transmission lines from the actual strike point).
>
>You're right. (The big problem with all of this on the lightning
>protection newsgroup is that nobody seems to be able to discuss these
>matters as calmly and authoritatively as you can.)
>
>One of the latest weird and wonderful discoveries in lightning-land is
>that tall structures, aircraft, and wires shot up by rockets seem to
>generate their own special brand of lightning. If these structures
>weren't there, there wouldn't be a lightning strike in the area at
>all. I'm not certain how anyone figured this out, but I tend to trust
>the people who wrote the articles.
I'd be equally confident in saying that if there is no thunderstorm
activity in the area, they wouldn't be struck either. But that should
be obvious. When you change one of the major factors leading to
an event, then you must inherently change the nature of the event.
The key fact to recognize here is that you *are* changing one of
the major factors. Again I must stress that it isn't just a matter of
random chance when and where lightning strikes.
I would take issue with the notion that no lightning event would
occur in the area absent the presence of a tall structure, aircraft,
or rocket. As long as the cloud to cloud or cloud to ground potentials
are sufficient to allow a discharge to occur, one will occur. It might
not be at the same precise point, in fact we can assert that by definition
changing the topography will change the strike point, but it will occur
in the same general area and time.
The lightning prevention shysters would have us believe otherwise.
They claim that their systems *prevent* strikes from occurring, but I
assert that isn't possible. Their systems can't change electrical law
or the physics of the thunderstorm. They cannot dissipate enough
charge quickly enough to eliminate the potentials that make lightning
strikes inevitable.
>>Any randomness associated with lightning strikes is not like an honest
>>coin toss. It is like playing with heavily loaded dice. The size and distribution
>>of charge separations, the shape of terrain (natural or artificial), the presence
>>or absence of heavily streaming objects, etc serve to load the dice.
>
>True. There's clearly a mechanism involved, but nobody knows what the
>heck it is. That makes it random as far as we're concerned at
>present. Presumably we'll figure it out someday.
This is the point that's giving me heartburn. There are many factors involved
in precisely defining a particular lightning event. We know many of them, some
we don't. You seem to be denying that we have *any* knowledge of the factors
involved. I strongly disagree. If that were the case, then there would be no point
to lightning mitigation work. We might as well lay lightning rods on the ground
as mount them on tall spires. We might as well provide elaborate protection to
a one story building while ignoring the 1000 foot tower next door. I simply can't
agree with that attitude.
One of my professors was fond of saying that *all* events are probabilistic.
There is a finite probability that all the air molecules in this room would have
their Brownian movements coordinated for a moment in such a fashion that
all of the air would gather in one corner of the room. There is a finite probability
that all of the molecules in a hammer head might align with the spaces between
molecules in the anvil at some moment, and the hammer would pass harmlessly
through the anvil. Etc. But that doesn't mean we should give up breathing or
that we should not bother to provide an anvil for our blacksmithing.
Wet sand (fresh water) should have a conductivity on the order of
1 to 2 mS/m, and a dielectric constant approaching 80. A good
broadcast ground field has a conductivity on the order of 5 to 20
mS/m and a dielectric constant on the order of 13. But more
importantly it has buried copper radials extending out in all directions
to about 1000 feet from the center along 120 radials. That means
that instead of the current being concentrated in a cross sectional
area of about 0.2 sqft as it enters the soil, it is distributed over an area
of 785,000 sqft before having to enter the soil. Obviously, current
density is a *lot* lower in the latter case, and total capacitance to Earth
is much greater.
Large Ufer grounds offer similar benefits. For example, Lucent's cable
plant here has a Ufer covering an area of about 1,000,000 square feet.
A typical slab home's Ufer only covers about 1400 square feet, but that's
still about 5,000 times better than a stake in the ground.
>>Currents do arc from soil grain to soil grain (and also capacitively couple
>>according to the models I respect), but with an adequate Earth termination,
>>current density in any given volume of soil is held to a low enough value
>>that worm tracks aren't produced. In my experience, adequate Earth
>>terminations are formed by broadcast radial systems, or by large Ufer
>>grounds. A handful of ground rods aren't an adequate Earth termination
>>in most soils. So what you're saying may be perfectly correct in a back
>>country situation. I wouldn't be so rash as to suggest that a tree root
>>network is an adequate substitute for a field of buried copper, though
>>I am rash enough to suggest that it would probably have some effect.
>
>Most electric power system grounds consist of just a ground wire stuck
>into the earth, typically wound around the buried end of a utility
>pole. Most lightning rod systems have the same sort of arrangement.
>Substations, which have to be concerned with fault currents, use far
>more elaborate arrangements. But these aren't really designed with
>lightning in mind. For that matter, neither are your tower grounds.
Actually, our tower grounds are designed with lightning in mind.
For TV and FM broadcasting, an elaborate ground field is not
necessary for transmission efficiency (it is for AM broadcast),
but we install them anyway because we know that they give us
a good Earth termination for lightning strikes, we know that
our towers are prime targets for lightning, and we can't afford
the downtime that unmitigated lightning damage would produce.
>>Another example, I'm responsible for a 350 foot tower about
>>5 miles from my house. According to the strike counter, it has
>>been struck 17 times in the past 2 years. OTOH, my tower
>>here at the house, a 70 footer in similar terrain, has only been
>>struck twice in the last 2 years. (There's a storm front moving
>>in as I type this, so those numbers are subject to change.)
>
>Do you have a strike counter on the mast at your home?
Yes. I've made it a policy to install strike counters on any
tower for which I'm responsible. Having one on my little
tower isn't strictly necessary, but like having a home weather
station, it is interesting to have the data.
>>I am willing to assert that taller objects, which are good
>>conductors and are well grounded, do get struck much
>>more often than shorter ones. Much more often than mere
>>random chance would imply. Our taller broadcast towers
>>(typically 1040 feet AGL) often get multiple strikes from
>>the same storm cell.
>
>I agree with you. But if you erect a tall tower and then wait for it
>to get struck, you'll likely be waiting a very long time.
That certainly hasn't been my experience. The first time an intense
cell passes over, there's a very good probability a 1000 foot tower
will be struck, often multiple times. We have a lot of intense cells here
during thunderstorm season. (Of course an intense cell may miss
the tower's location in any given storm, so we can't count on a strike
every time there's a thunderstorm. But as I noted, even my short tower
has been hit twice in the last two years. Our taller towers are struck
much more often.)
>>I was a bit taken aback by your statement that it is hard
>>to find sites that are frequently struck. I'd think that any
>>reasonably tall broadcast tower in the Southeast would
>>provide abundant strike events. Most strikes would pass
>>unnoticed without strike counters, though, because the
>>systems are engineered to handle most strikes without
>>damage.
>
>What I meant was that experience has shown that it's very difficult to
>erect a lightning rod that'll consistently be hit with enough strikes
>to do research with. This has been the experience of almost everyone
>who's tried it. One fellow in Austria, www.aldis.com, I think, has
>had more luck than most in this regard. He's been able to take
>high-speed movies of a strike, in fact. I just discovered his site
>yesterday.
One thing you have to learn when dealing with natural events like
lightning is patience. The same can be said about astronomy or
geology. Natural events don't happen to order. But there are places
and times where and when events happen much more frequently than
other places and times. Broadcast towers in the southeast are an
example.
>I'd like to develop a really cheap transient recorder and place a
>million or so of the things on electrical outlets, utility poles,
>buildings, fences, telephone lines, and everywhere else I could think
>of. If these were networked together and could give us the time and
>waveform of any electrical transient that appeared across their
>terminals, we could start to get some real data to aid in the
>development of good lightning and transient protection.
Cheap would be difficult. A strike counter is just a totalizer. It doesn't
tell us precisely when a strike occurred, or its intensity or waveform.
Yet they cost over $100 each. To do what you want to do, the recorders
would need to be more complex, yet cost on the order of a dollar or
two.
Several years ago, we deployed a lightning tracker system for our
local station's weather department. It involved 8 sensors dispersed
about a 22 county area. They were linked back to a central computer
by leased lines (to obtain consistent delay figures) and used differential
time of arrival methods to let us pinpoint when and where a lightning
strike in our primary coverage area occurred. It worked, but the sensors
cost over $2,000 each and the leased line charges mounted up. Now
we pay a service to give us a cruder, but larger scale, picture of lightning
activity.
Neither strike counters nor the more elaborate DTOA system I described
would give the sort of data needed to understand all of the factors involved
in the lightning discharge. But the concept used by the latter system of a
relatively small number of fixed sites containing broad bandwidth receivers
is a valid one.
What's needed is a more complete recording of the event, and subsequent
analysis. That's possible. If we can forego realtime, such a network is
feasible to install today over a large enough area to give a good data volume,
and yet not cost too many millions of dollars to install and operate. (Sounds
like time for a government grant application.)
Gary
Gary Coffman
>>Well, I won't disagree with the latter statement. However, just to name
>>two well known things from astronomy and geology that can't be replicated
>>in the lab, nor exactly repeated in the field, how about supernovas and
>>earthquakes? Yet neither of those events is considered random. They
>>only occur under specific conditions in specific places. The same can
>>be said of lightning.
>
>Well, here we run into the definition of randomness. However, you're
>certainly correct about earthquakes. There's always been a great
>demand for accurate earthquake prediction, and, as in the case of
>lightning prediction, there are a good many journalists and lawyers
>who won't accept the fact that it can't be done.
We may not yet be able to name the day and hour, but we do know
with a high confidence that there will be an earthquake along the
San Andreas or New Madrid faults within this century. We know with
equally high confidence that there won't be a major earthquake
in the area of the southern extent of the Canadian shield in the next
30 years. This isn't a matter of randomness, it is inherent in the
different geologies of the regions.
Similarly, I know with high confidence that none of the towers in this
area are going to be struck by lightning today. The meteorological
conditions aren't right, and won't be right for several more days, to
allow that to occur. If we were dealing with purely random events,
I couldn't make those assertions with any confidence.
>>If lightning were as random as a coin toss, as you maintained in another
>>post, then over time it would show a uniform distribution over the Earth,
>>and would as frequently strike a one story building as it strikes the 1000
>>foot broadcast tower adjacent to the building. But that's not the case.
>>
>>I"ve got strike counter data from enough broadcast sites (12 TV and
>>22 radio stations owned by my company) to be very confident that the
>>towers are struck very much more often than the buildings. In fact,
>>I have no examples of a transmitter building being struck directly
>>(though of course strike currents can come in on power, telco, or
>>transmission lines from the actual strike point).
>
>You're right. (The big problem with all of this on the lightning
>protection newsgroup is that nobody seems to be able to discuss these
>matters as calmly and authoritatively as you can.)
>
>One of the latest weird and wonderful discoveries in lightning-land is
>that tall structures, aircraft, and wires shot up by rockets seem to
>generate their own special brand of lightning. If these structures
>weren't there, there wouldn't be a lightning strike in the area at
>all. I'm not certain how anyone figured this out, but I tend to trust
>the people who wrote the articles.
I'd be equally confident in saying that if there is no thunderstorm
activity in the area, they wouldn't be struck either. But that should
be obvious. When you change one of the major factors leading to
an event, then you must inherently change the nature of the event.
The key fact to recognize here is that you *are* changing one of
the major factors. Again I must stress that it isn't just a matter of
random chance when and where lightning strikes.
I would take issue with the notion that no lightning event would
occur in the area absent the presence of a tall structure, aircraft,
or rocket. As long as the cloud to cloud or cloud to ground potentials
are sufficient to allow a discharge to occur, one will occur. It might
not be at the same precise point, in fact we can assert that by definition
changing the topography will change the strike point, but it will occur
in the same general area and time.
The lightning prevention shysters would have us believe otherwise.
They claim that their systems *prevent* strikes from occurring, but I
assert that isn't possible. Their systems can't change electrical law
or the physics of the thunderstorm. They cannot dissipate enough
charge quickly enough to eliminate the potentials that make lightning
strikes inevitable.
>>Any randomness associated with lightning strikes is not like an honest
>>coin toss. It is like playing with heavily loaded dice. The size and distribution
>>of charge separations, the shape of terrain (natural or artificial), the presence
>>or absence of heavily streaming objects, etc serve to load the dice.
>
>True. There's clearly a mechanism involved, but nobody knows what the
>heck it is. That makes it random as far as we're concerned at
>present. Presumably we'll figure it out someday.
This is the point that's giving me heartburn. There are many factors involved
in precisely defining a particular lightning event. We know many of them, some
we don't. You seem to be denying that we have *any* knowledge of the factors
involved. I strongly disagree. If that were the case, then there would be no point
to lightning mitigation work. We might as well lay lightning rods on the ground
as mount them on tall spires. We might as well provide elaborate protection to
a one story building while ignoring the 1000 foot tower next door. I simply can't
agree with that attitude.
One of my professors was fond of saying that *all* events are probabilistic.
There is a finite probability that all the air molecules in this room would have
their Brownian movements coordinated for a moment in such a fashion that
all of the air would gather in one corner of the room. There is a finite probability
that all of the molecules in a hammer head might align with the spaces between
molecules in the anvil at some moment, and the hammer would pass harmlessly
through the anvil. Etc. But that doesn't mean we should give up breathing or
that we should not bother to provide an anvil for our blacksmithing.
>>>>We do know that the Earth's surface layer can be modeled as a
Wet sand (fresh water) should have a conductivity on the order of
1 to 2 mS/m, and a dielectric constant approaching 80. A good
broadcast ground field has a conductivity on the order of 5 to 20
mS/m and a dielectric constant on the order of 13. But more
importantly it has buried copper radials extending out in all directions
to about 1000 feet from the center along 120 radials. That means
that instead of the current being concentrated in a cross sectional
area of about 0.2 sqft as it enters the soil, it is distributed over an area
of 785,000 sqft before having to enter the soil. Obviously, current
density is a *lot* lower in the latter case, and total capacitance to Earth
is much greater.
Large Ufer grounds offer similar benefits. For example, Lucent's cable
plant here has a Ufer covering an area of about 1,000,000 square feet.
A typical slab home's Ufer only covers about 1400 square feet, but that's
still about 5,000 times better than a stake in the ground.
>>Currents do arc from soil grain to soil grain (and also capacitively couple
>>according to the models I respect), but with an adequate Earth termination,
>>current density in any given volume of soil is held to a low enough value
>>that worm tracks aren't produced. In my experience, adequate Earth
>>terminations are formed by broadcast radial systems, or by large Ufer
>>grounds. A handful of ground rods aren't an adequate Earth termination
>>in most soils. So what you're saying may be perfectly correct in a back
>>country situation. I wouldn't be so rash as to suggest that a tree root
>>network is an adequate substitute for a field of buried copper, though
>>I am rash enough to suggest that it would probably have some effect.
>
>Most electric power system grounds consist of just a ground wire stuck
>into the earth, typically wound around the buried end of a utility
>pole. Most lightning rod systems have the same sort of arrangement.
>Substations, which have to be concerned with fault currents, use far
>more elaborate arrangements. But these aren't really designed with
>lightning in mind. For that matter, neither are your tower grounds.
Actually, our tower grounds are designed with lightning in mind.
For TV and FM broadcasting, an elaborate ground field is not
necessary for transmission efficiency (it is for AM broadcast),
but we install them anyway because we know that they give us
a good Earth termination for lightning strikes, we know that
our towers are prime targets for lightning, and we can't afford
the downtime that unmitigated lightning damage would produce.
>>Another example, I'm responsible for a 350 foot tower about
>>5 miles from my house. According to the strike counter, it has
>>been struck 17 times in the past 2 years. OTOH, my tower
>>here at the house, a 70 footer in similar terrain, has only been
>>struck twice in the last 2 years. (There's a storm front moving
>>in as I type this, so those numbers are subject to change.)
>
>Do you have a strike counter on the mast at your home?
Yes. I've made it a policy to install strike counters on any
tower for which I'm responsible. Having one on my little
tower isn't strictly necessary, but like having a home weather
station, it is interesting to have the data.
>>I am willing to assert that taller objects, which are good
>>conductors and are well grounded, do get struck much
>>more often than shorter ones. Much more often than mere
>>random chance would imply. Our taller broadcast towers
>>(typically 1040 feet AGL) often get multiple strikes from
>>the same storm cell.
>
>I agree with you. But if you erect a tall tower and then wait for it
>to get struck, you'll likely be waiting a very long time.
That certainly hasn't been my experience. The first time an intense
cell passes over, there's a very good probability a 1000 foot tower
will be struck, often multiple times. We have a lot of intense cells here
during thunderstorm season. (Of course an intense cell may miss
the tower's location in any given storm, so we can't count on a strike
every time there's a thunderstorm. But as I noted, even my short tower
has been hit twice in the last two years. Our taller towers are struck
much more often.)
>>I was a bit taken aback by your statement that it is hard
>>to find sites that are frequently struck. I'd think that any
>>reasonably tall broadcast tower in the Southeast would
>>provide abundant strike events. Most strikes would pass
>>unnoticed without strike counters, though, because the
>>systems are engineered to handle most strikes without
>>damage.
>
>What I meant was that experience has shown that it's very difficult to
>erect a lightning rod that'll consistently be hit with enough strikes
>to do research with. This has been the experience of almost everyone
>who's tried it. One fellow in Austria, www.aldis.com, I think, has
>had more luck than most in this regard. He's been able to take
>high-speed movies of a strike, in fact. I just discovered his site
>yesterday.
One thing you have to learn when dealing with natural events like
lightning is patience. The same can be said about astronomy or
geology. Natural events don't happen to order. But there are places
and times where and when events happen much more frequently than
other places and times. Broadcast towers in the southeast are an
example.
>I'd like to develop a really cheap transient recorder and place a
>million or so of the things on electrical outlets, utility poles,
>buildings, fences, telephone lines, and everywhere else I could think
>of. If these were networked together and could give us the time and
>waveform of any electrical transient that appeared across their
>terminals, we could start to get some real data to aid in the
>development of good lightning and transient protection.
Cheap would be difficult. A strike counter is just a totalizer. It doesn't
Gary Coffman
>What's needed is a more complete recording of the event, and subsequent
>analysis. That's possible. If we can forego realtime, such a network is
>feasible to install today over a large enough area to give a good data volume,
>and yet not cost too many millions of dollars to install and operate. (Sounds
>like time for a government grant application.)
Following up on myself, seems NASA is funding just such an effort. It is being
run by Dr. Risen at New Mexico Tech.
Gary
Mark Kinsler
>We may not yet be able to name the day and hour, but we do know
>with a high confidence that there will be an earthquake along the
>San Andreas or New Madrid faults within this century. We know with
>equally high confidence that there won't be a major earthquake
>in the area of the southern extent of the Canadian shield in the next
>30 years. This isn't a matter of randomness, it is inherent in the
>different geologies of the regions.
>Similarly, I know with high confidence that none of the towers in this
>area are going to be struck by lightning today. The meteorological
>conditions aren't right, and won't be right for several more days, to
>allow that to occur. If we were dealing with purely random events,
>I couldn't make those assertions with any confidence.
Well, yes--though there are quite a few instances of lightning strikes
on tall structures when thunderstorms aren't anywhere in the vicinity.
This drove the GE guys crazy when they spent a summer monitoring the
Empire State Building in the 1930's.
I think the utter randomness I've asserted is more on the microscopic
level. The guys who spend a lot of time in court in golf-course
lightning-injury cases are concerned that the lightning hit the golfer
instead of the roof of the shelter that he was standing near. This is
a matter of a few feet, and I suspect that there's some real
randomness at this end of the scale. We run into the same
target-shooting mentality when we deal with power-line surge
protectors: why did it damage the modem instead of the toaster, or
vice-versa?
However, there's still quite a bit of randomness (is that a word?) in
any arcing through air even under laboratory conditions. Take two
spherical electrodes and increase the voltage between them until an
arc jumps across them. This voltage will certainly be a function of
the spacing, the temperature, the radius of the electrodes and the gas
pressure. But if you get an arc at voltage Vx in experiment 1, it's
quite unlikely that you'll get an arc at precisely voltage Vx in
experiment 2, even if you've been careful to keep conditions the same.
(We worry a lot about "electrode conditioning"--i.e., pits forming in
or crud forming on the electrodes.) Nor will the path of the arc be
the same in each case, and this is a critical factor in lightning
lawsuits.
Arc initiation voltage may or may not be related to the intensity of
visible light near the gap, the intensity of ultraviolet radiation
near the gap, the influence of cosmic rays in the gap, the waveform of
the voltage (is the voltage increased quickly or slowly?) and a host
of other factors that have been researched for quite a long time.
>I'd be equally confident in saying that if there is no thunderstorm
>activity in the area, they wouldn't be struck either. But that should
>be obvious. When you change one of the major factors leading to
>an event, then you must inherently change the nature of the event.
>The key fact to recognize here is that you *are* changing one of
>the major factors. Again I must stress that it isn't just a matter of
>random chance when and where lightning strikes.
>I would take issue with the notion that no lightning event would
>occur in the area absent the presence of a tall structure, aircraft,
>or rocket. As long as the cloud to cloud or cloud to ground potentials
>are sufficient to allow a discharge to occur, one will occur. It might
>not be at the same precise point, in fact we can assert that by definition
>changing the topography will change the strike point, but it will occur
>in the same general area and time.
Well, the rocket guys claim that their activities initiate lightning
where it otherwise wouldn't occur. Apparently you can do the same
trick with a laser. From what I've been able to find out, there
always exists a pretty healthy electric potential between the earth
and the sky. Thunderclouds bring the charged electrodes, so to speak,
closer together.
>The lightning prevention shysters would have us believe otherwise.
>They claim that their systems *prevent* strikes from occurring, but I
>assert that isn't possible. Their systems can't change electrical law
>or the physics of the thunderstorm. They cannot dissipate enough
>charge quickly enough to eliminate the potentials that make lightning
>strikes inevitable.
I agree, but I'm afraid that my feelings are based more on the
aggressive behavior of the people involved than on any demonstrated
science. As soon as any charge is dissipated by a pointy object, more
is conducted to its base through the earth. However, there is an
issue of space charge which is a bit sticky. Depending on who you
believe and what the actual conditions are, the ions formed at the
pointy ends of wires will either form globules around the ends of the
wires and turn them into blunt objects, thus making lightning likely
to hit somewhere else on the protected structure, or the ions sort of
shoot into space and form streamers that encourage the lightning
stroke to come hither, or they don't do much of anything and are
quickly blown away by the wind and/or falling rain anyway.
It seems to be just about impossible to do the most obvious
experiment, which is to test lightning rods and various patent
lightning dissipators under identical conditions. At least I've never
seen any coherent discussions on the topic.
>>True. There's clearly a mechanism involved, but nobody knows what the
>>heck it is. That makes it random as far as we're concerned at
>>present. Presumably we'll figure it out someday.
>
>This is the point that's giving me heartburn. There are many factors involved
>in precisely defining a particular lightning event. We know many of them, some
>we don't. You seem to be denying that we have *any* knowledge of the factors
>involved. I strongly disagree. If that were the case, then there would be no point
>to lightning mitigation work. We might as well lay lightning rods on the ground
>as mount them on tall spires. We might as well provide elaborate protection to
>a one story building while ignoring the 1000 foot tower next door. I simply can't
>agree with that attitude.
Sorry. I didn't mean to sound quite as severe as that. But
lightning is a far less scientific and, at this point, deterministic
field than most people inside and outside of the lightning protection
business seem to want to admit.
I think that a good deal of the problem is that there's been little
progress in the field since Ben Franklin. He seems to have gotten it
right the first time, and there really haven't been any improvements
since Poor Richard's Almanack. Some buildings and people still get
hit by lightning, but we don't seem to be able to do much about it
except to improve the transient-absorption capacity of electrical
equipment.
>>Well, we used wet sand, which is pretty conductive. The conductivity
>>didn't affect our results a bit. A broadcast-antenna ground plane
>>would be a rather different environment, I should think.
>
>Wet sand (fresh water) should have a conductivity on the order of
>1 to 2 mS/m, and a dielectric constant approaching 80. A good
>broadcast ground field has a conductivity on the order of 5 to 20
>mS/m and a dielectric constant on the order of 13. But more
>importantly it has buried copper radials extending out in all directions
>to about 1000 feet from the center along 120 radials. That means
>that instead of the current being concentrated in a cross sectional
>area of about 0.2 sqft as it enters the soil, it is distributed over an area
>of 785,000 sqft before having to enter the soil. Obviously, current
>density is a *lot* lower in the latter case, and total capacitance to Earth
>is much greater.
>
>Large Ufer grounds offer similar benefits. For example, Lucent's cable
>plant here has a Ufer covering an area of about 1,000,000 square feet.
>A typical slab home's Ufer only covers about 1400 square feet, but that's
>still about 5,000 times better than a stake in the ground.
This is very much the sort of thing that's done with substations and
electric power lines. Steel power pylons are often built with copper
radials to ensure good grounding.
From a theoretical standpoint, however,one of the problems with all
this is, uh, just where is Earth? The literature refers to "distant
ground," It all seems terribly diffuse, and I suppose it is. In
electrical work, we expect voltages and arcs to appear across two
discrete terminals. It seems that in lightning work we're not so
privileged: both hot and ground are regions, not points.
>Actually, our tower grounds are designed with lightning in mind.
>For TV and FM broadcasting, an elaborate ground field is not
>necessary for transmission efficiency (it is for AM broadcast),
>but we install them anyway because we know that they give us
>a good Earth termination for lightning strikes, we know that
>our towers are prime targets for lightning, and we can't afford
>the downtime that unmitigated lightning damage would produce.
I didn't know that. I sort of assumed that FM and TV towers were
grounded through the various support slabs and that the lights and
transmitter were protected through various sorts of lightning chokes,
arc gaps, and other measures.
>>>Another example, I'm responsible for a 350 foot tower about
>>>5 miles from my house. According to the strike counter, it has
>>>been struck 17 times in the past 2 years. OTOH, my tower
>>>here at the house, a 70 footer in similar terrain, has only been
>>>struck twice in the last 2 years. (There's a storm front moving
>>>in as I type this, so those numbers are subject to change.)
>>
>>Do you have a strike counter on the mast at your home?
>
>Yes. I've made it a policy to install strike counters on any
>tower for which I'm responsible. Having one on my little
>tower isn't strictly necessary, but like having a home weather
>station, it is interesting to have the data.
You've got better data there than a lot of the people who write papers
on lightning.
>>I agree with you. But if you erect a tall tower and then wait for it
>>to get struck, you'll likely be waiting a very long time.
>
>That certainly hasn't been my experience. The first time an intense
>cell passes over, there's a very good probability a 1000 foot tower
>will be struck, often multiple times. We have a lot of intense cells here
>during thunderstorm season. (Of course an intense cell may miss
>the tower's location in any given storm, so we can't count on a strike
>every time there's a thunderstorm. But as I noted, even my short tower
>has been hit twice in the last two years. Our taller towers are struck
>much more often.)
The last results I saw were from a New Mexico study of lightning rods
on a frequently-struck mountain. They yielded all of twelve strikes
in seven years, which isn't a useful sample in my opinion. I don't
think that the New Mexico people were too pleased, either, but they
published what they had. Apparently lightning struck everywhere but
on their equipment. There are many similar stories in lightning
research.
>>What I meant was that experience has shown that it's very difficult to
>>erect a lightning rod that'll consistently be hit with enough strikes
>>to do research with. This has been the experience of almost everyone
>>who's tried it. One fellow in Austria, www.aldis.com, I think, has
>>had more luck than most in this regard. He's been able to take
>>high-speed movies of a strike, in fact. I just discovered his site
>>yesterday.
>
>One thing you have to learn when dealing with natural events like
>lightning is patience. The same can be said about astronomy or
>geology. Natural events don't happen to order. But there are places
>and times where and when events happen much more frequently than
>other places and times. Broadcast towers in the southeast are an
>example.
I suppose that they oughta transfer operations down there. Maybe
they'd have more luck.
>>I'd like to develop a really cheap transient recorder and place a
>>million or so of the things on electrical outlets, utility poles,
>>buildings, fences, telephone lines, and everywhere else I could think
>>of. If these were networked together and could give us the time and
>>waveform of any electrical transient that appeared across their
>>terminals, we could start to get some real data to aid in the
>>development of good lightning and transient protection.
>
>Cheap would be difficult. A strike counter is just a totalizer. It doesn't
>tell us precisely when a strike occurred, or its intensity or waveform.
>Yet they cost over $100 each. To do what you want to do, the recorders
>would need to be more complex, yet cost on the order of a dollar or
>two.
I know. I'm hoping for great things in cheap microcontrollers with
recording capability. I think it'll be possible, though not right
now. If digital voice-recording keychains can be made for this price,
a transient recorder with a modem of some sort might not be too much
more difficult.
>Several years ago, we deployed a lightning tracker system for our
>local station's weather department. It involved 8 sensors dispersed
>about a 22 county area. They were linked back to a central computer
>by leased lines (to obtain consistent delay figures) and used differential
>time of arrival methods to let us pinpoint when and where a lightning
>strike in our primary coverage area occurred. It worked, but the sensors
>cost over $2,000 each and the leased line charges mounted up. Now
>we pay a service to give us a cruder, but larger scale, picture of lightning
>activity.
I was at a power conference a few years ago and there were a bunch of
lightning-tracking services giving spiels thereof. I'm not convinced
that the accuracy has reached the point where we can get data that's
helpful for lightning-protection evaluation, but it certainly seems to
be getting there. This will make a great deal of difference in the
field, perhaps enabling us to do the sort of experiments that we've
had no luck with for so long.
If anyone's still reading this, tune to http://www.lightningstorm.com
and click on the free lightning map. Way cool.
>Neither strike counters nor the more elaborate DTOA system I described
>would give the sort of data needed to understand all of the factors involved
>in the lightning discharge. But the concept used by the latter system of a
>relatively small number of fixed sites containing broad bandwidth receivers
>is a valid one.
It is. Old Man Sunde had a nifty method that he used when he was
researching lightning for Bell Labs many years ago. His strike
counters were connected to a network of wires and would turn on
several tape recorders. By determining the length of the tape between
the initial activation and the sound of the thunder, he could guess at
the distance between the recorder and the strike. A bit of
triangulation revealed the location of the strike.
>What's needed is a more complete recording of the event, and subsequent
>analysis. That's possible. If we can forego realtime, such a network is
>feasible to install today over a large enough area to give a good data volume,
>and yet not cost too many millions of dollars to install and operate. (Sounds
>like time for a government grant application.)
I don't know if there's a nickel available for this anymore. The only
gov't money that anyone was able to squeeze out for lightning research
wasn't for lightning, really, but for electromagnetic pulse research
to see what would happen to communication and power systems when the
Russkies dropped the big one. Those times have passed.
M Kinsler
Mark Kinsler
That's where the earlier research I mentioned was done, and good luck
to them. NASA seems to fund a lot of lightning research, but their
record is rather strange: apparently they bought into the idea of
lightning suppressors and similar witchcraft years ago.
I suppose that's what research is all about.
M Kinsler
house
from misuse of the technical term "random".
"Random" is not the same as "uniform random".
A *uniform* probability distribution assigns the same probability of an
event ocurring to every point in its domain, but this is only one of an
infinitely large set of probability distributions. (Ask any statistics
student, who has to learn what functions do or do not define a probability
distribution :( , and may be asked to memorise tweny or more common ones).
There is no problem about saying that some locations at certain times are
more likely to experience a lightning stike than others, but that this is a
random process. It is just that the probabilities of the random events are
different according to local conditions.
Frank
"Gary Coffman" <ke...@bellsouth.net> wrote in message
news:q851ltgvfgfr6c834...@4ax.com...
> On Fri, 13 Jul 2001 02:22:56 GMT, kin...@frognet.net (Mark Kinsler)
wrote:
> >
cy...@tiny.net
wrote:
(snipped)
>
>If anyone's still reading this,
With great fascination
tune to http://www.lightningstorm.com
>and click on the free lightning map. Way cool.
Indeed.
Galen Hekhuis
I hate "me too" posts but me too.
Gary Coffman
>>We may not yet be able to name the day and hour, but we do know
>>with a high confidence that there will be an earthquake along the
>>San Andreas or New Madrid faults within this century. We know with
>>equally high confidence that there won't be a major earthquake
>>in the area of the southern extent of the Canadian shield in the next
>>30 years. This isn't a matter of randomness, it is inherent in the
>>different geologies of the regions.
>
>>Similarly, I know with high confidence that none of the towers in this
>>area are going to be struck by lightning today. The meteorological
>>conditions aren't right, and won't be right for several more days, to
>>allow that to occur. If we were dealing with purely random events,
>>I couldn't make those assertions with any confidence.
>
>Well, yes--though there are quite a few instances of lightning strikes
>on tall structures when thunderstorms aren't anywhere in the vicinity.
>This drove the GE guys crazy when they spent a summer monitoring the
>Empire State Building in the 1930's.
Sure. But there has to be particulate matter in the air to carry charge,
and wind turbulence to separate it (both commonly found in cities near
skyscrapers). A thunderstorm and its ice particles isn't the only way to
build up a significant localized charge in the atmosphere (just the most
common way). Either one can form what is conceptually a Windhurst
machine in the atmosphere.
But it has been dead calm here for the past couple of days, and the
temperature and humidity are modest for this time of year. (A dry cold
front passed through, very unusual this far south at this time of year.)
So I can make the claim that conditions aren't right for lightning here
right now.
The normal fair weather field gradient (300 volts/meter) is a different thing.
It is due to the charge differential between the "electrosphere" (about 80
miles up) and the Earth (picture them as the two plates of a capacitor). At
that altitude, the mean free path of electrons and ions is large enough that
the electrosphere is a good conductor. So charge is fairly uniformly
distributed on it around the entire globe. Similarly, the Earth is a fairly
good conductor too, and charge tends to distribute uniformly on it
(when viewed on the proper scales, temporal as well as spatial).
When not shadowed by storms, this global capacitor provides the basic
electrical gradient in the atmosphere (shadowing is why the gradient falls
under a storm to about 100 volts/meter, though of course local gradients
can become very high, ie the points where lightning is going to occur).
The fair weather gradient isn't great enough to generate lightning, though
it is tropical thunderstorms that keep it charged up (via the various sorts of
meso-lightning "sprites" recently photographed by NASA).
>I think the utter randomness I've asserted is more on the microscopic
>level.
<snip>
>Arc initiation voltage may or may not be related to the intensity of
>visible light near the gap, the intensity of ultraviolet radiation
>near the gap, the influence of cosmic rays in the gap, the waveform of
>the voltage (is the voltage increased quickly or slowly?) and a host
>of other factors that have been researched for quite a long time.
I agree. The step leader in lightning assumes what has been likened to
a fractal shape due to the minute inhomogeneities of the atmosphere at
the microscopic level. The path approaches, but isn't quite the same as,
a drunkard's walk. Unlike a true drunkard's walk, there is structure at
every level, it isn't actually random when viewed at the proper scales.
>>I would take issue with the notion that no lightning event would
>>occur in the area absent the presence of a tall structure, aircraft,
>>or rocket. As long as the cloud to cloud or cloud to ground potentials
>>are sufficient to allow a discharge to occur, one will occur. It might
>>not be at the same precise point, in fact we can assert that by definition
>>changing the topography will change the strike point, but it will occur
>>in the same general area and time.
>
>Well, the rocket guys claim that their activities initiate lightning
>where it otherwise wouldn't occur. Apparently you can do the same
>trick with a laser.
Yes, but that's a different thing. The converse of a statement doesn't
prove the statement. It is certainly true that lightning can be induced
at points where it wouldn't normally appear. But that's different from
claiming that in places where lightning would normally occur, removing
a tall structure will prevent it from occurring.
>From what I've been able to find out, there
>always exists a pretty healthy electric potential between the earth
>and the sky. Thunderclouds bring the charged electrodes, so to speak,
>closer together.
Not quite, see what I said about the global circuit above. Storms actually
shadow the normal gradient. It is only their *locally generated* gradients
that lead to lightning discharges. (Again it is a matter of scale, you have
to look at it at the proper scale for what you're trying to observe.)
>>The lightning prevention shysters would have us believe otherwise.
>>They claim that their systems *prevent* strikes from occurring, but I
>>assert that isn't possible. Their systems can't change electrical law
>>or the physics of the thunderstorm. They cannot dissipate enough
>>charge quickly enough to eliminate the potentials that make lightning
>>strikes inevitable.
>
>I agree, but I'm afraid that my feelings are based more on the
>aggressive behavior of the people involved than on any demonstrated
>science. As soon as any charge is dissipated by a pointy object, more
>is conducted to its base through the earth. However, there is an
>issue of space charge which is a bit sticky. Depending on who you
>believe and what the actual conditions are, the ions formed at the
>pointy ends of wires will either form globules around the ends of the
>wires and turn them into blunt objects, thus making lightning likely
>to hit somewhere else on the protected structure, or the ions sort of
>shoot into space and form streamers that encourage the lightning
>stroke to come hither, or they don't do much of anything and are
>quickly blown away by the wind and/or falling rain anyway.
Wind certainly plays an important role in two different ways. It will quickly
blow away a static globular "space charge", and turn a streamer into a ribbon
(I found a great picture of that effect while browsing, check out
http://www.weather-photography.com/Lightning/HTML/w-087-07.html ).
But more importantly, it also quickly moves the cloud mass across the Earth.
So fresh charge is always being brought over any particular point on the Earth.
Thus not only do you have to discharge the cloud that *was* above you, but
you have to keep discharging the fresh cloud being brought over you. (You
obviously can't discharge the mirror charge in the Earth, it is essentially
infinite for all practical purposes because it is continuously replenished by
the entire globe.)
That's where the math I mentioned in an earlier post comes in. In effect,
about 20 coulombs of fresh charge comes over a site every 30 seconds
to 10 minutes (depending on the particular storm and its winds). That
charge has to be neutralized to prevent a lightning stroke. That requires
a current discharge to the atmosphere of between 2/3 amp and 1/30th
amp. But to prevent streaming, a point can't be allowed to discharge
at a rate exceeding about 20 microamperes. So the discharge has to
be throttled to about 10,000 times too small to do the job if it is to be a
net preventative to lightning strikes.
>>>True. There's clearly a mechanism involved, but nobody knows what the
>>>heck it is. That makes it random as far as we're concerned at
>>>present. Presumably we'll figure it out someday.
>>
>>This is the point that's giving me heartburn. There are many factors involved
>>in precisely defining a particular lightning event. We know many of them, some
>>we don't. You seem to be denying that we have *any* knowledge of the factors
>>involved. I strongly disagree. If that were the case, then there would be no point
>>to lightning mitigation work. We might as well lay lightning rods on the ground
>>as mount them on tall spires. We might as well provide elaborate protection to
>>a one story building while ignoring the 1000 foot tower next door. I simply can't
>>agree with that attitude.
>
>Sorry. I didn't mean to sound quite as severe as that. But
>lightning is a far less scientific and, at this point, deterministic
>field than most people inside and outside of the lightning protection
>business seem to want to admit.
I think most of your problem is that you're looking at lightning on the
wrong scales. I'd contend that at the macro level we understand lightning
very well, and we can do things at the macro level to influence it. At the
micro level, we understand it well too, but we don't have an a priori
knowledge of all the micro level details of a particular event. That means
we can't predict the exact microstructure of a particular lightning event,
but we can still manipulate it.
To put this in other words, we can't predict the exact behavior of individual
water molecules in a stream, though we understand the forces involved
quite well. But we can certainly build a dam that affects the gross behavior
of the stream. Lightning mitigation efforts are somewhat similar to this
analogy. We can do things that we know with a high degree of confidence
will influence the behavior of lightning in our vicinity, even though we can't
predict in detail what will occur at the micro level.
Exactly so. We're dealing with the plates of large scale capacitors that
have a non-uniform dielectric medium between them, and a surface
roughness that varies from point to point. The plates aren't uniformly
conductive either. Some regions are more conductive than others,
ie they have different skin depths.
The dielectric is going to fail when the potential on the plates gets high
enough, and the capacitor is going to arc over. There are factors we can't
know a priori about the dielectric, but there are other things we can know
and influence, in particular, the surface shape of the Earth terminal in our
immediate vicinity (ie a lightning rod or other structure), and the conductivity
of the local area (ie radials or a Ufer).
>>Actually, our tower grounds are designed with lightning in mind.
>>For TV and FM broadcasting, an elaborate ground field is not
>>necessary for transmission efficiency (it is for AM broadcast),
>>but we install them anyway because we know that they give us
>>a good Earth termination for lightning strikes, we know that
>>our towers are prime targets for lightning, and we can't afford
>>the downtime that unmitigated lightning damage would produce.
>
>I didn't know that. I sort of assumed that FM and TV towers were
>grounded through the various support slabs and that the lights and
>transmitter were protected through various sorts of lightning chokes,
>arc gaps, and other measures.
Those measures aren't much good if there isn't a good Earth termination,
and that requires a large scale connection to Earth. The discharge is going
to happen, and the current has to go somewhere. If we don't provide it a
good engineered path, it'll go where it wills, and our chokes and arc gaps
won't do us a bit of good. We're really juggling impedance ratios. The idea
is to provide high impedance paths were we don't want the current to go,
and low impedance paths where we do want it to go. The current will
divide in inverse ratio to the impedances offered to it in accord with
Kirchhoff's laws.
Then we'll use single point techniques and ground windows to equalize
the remaining potentials across equipment we need to protect. Equal potentials
equate to no damaging current flow. We don't really care if our equipment and
associated wiring becomes elevated in potential, as long as all of it elevates
together to the same potential. (Well we do care that potentials to other things
in the local environment don't get so high that we have sideflashes.)
Gary
Ed Huesers
Mark Kinsler wrote:
> I'd like to develop a really cheap transient recorder and place a
> million or so of the things on electrical outlets, utility poles,
> buildings, fences, telephone lines, and everywhere else I could think
> of. If these were networked together and could give us the time and
> waveform of any electrical transient that appeared across their
> terminals, we could start to get some real data to aid in the
> development of good lightning and transient protection.
Well, I know a good moldmaker that could use some work.
Ed Huesers
http://www.grandshelters.com
Mark Kinsler
Can he write a great grant proposal?
It'd be a truly massive undertaking to get anything like real data,
and there's no guarantee that the results would be at all useful.
Lightning causes a lot of damage, but it's not exactly the greatest
plague of our time. So research money is scarce.
M Kinsler