AFCI and UPS?
Nate Nagel
got a weird issue. In an upstairs bedroom, I have a computer that is
running off a UPS. After investigating the wiring in the house, I have
found a significant number of grounds "bootlegged." Little by little I
am picking away at them, but until spring, the upstairs will remain as
is because it's darn cold in the attic at the moment.
So here's the deal. I have determined that there is one properly
grounded outlet in the room where the computer is, because that outlet
is fed directly from the breaker panel, and that homerun is in BX not
the cloth covered Romex that the rest of the concealed wiring is. So as
a stopgap until I get around to fixing everything correctly I wanted to
make sure that the computer's UPS was plugged into that particular
outlet for proper surge protection. I bought a heavy extension cord,
unplugged the UPS from the outlet that it was plugged into, plugged the
extension cord into the correct outlet. Then I plugged the UPS into the
extension cord, and the lights went out. Reset the breaker, figuring
that I just made an arc, if I plugged it in faster it would be OK. Same
effect. Reset the breaker, plugged the UPS back into the outlet that it
was using before, and everything is fine. This is a less than year old
APC unit.
Is there some fundamental incompatibility between this UPS and the AFCI?
I'm wondering if the UPS tests for the presence of an earth ground
(there is a "building wiring fault" light on it) and if so does that
cause enough current to trip the AFCI? (I wouldn't have found this
before, because the outlet into which it's currently plugged is not
grounded but has the ground bootlegged to the neutral.) I've heard that
some AFCIs are also GFCIs is why I'm asking. The AFCI is a Siemens
Q115AF breaker in the breaker panel.
I'll replace the receptacle in the morning (once it's light out) and see
if that solves the problem, but I'm not sure what kind of fault could be
in a receptacle that would cause a breaker, AFCI or not, to trip only
when a load is connected to it but not when a plug is inserted.
Does this mean also that I run the risk of having the AFCI trip if the
surge protection of the UPS kicks in?
Should I just give up the idea of having everything "to code" and ditch
the AFCI and/or investigate the possibility of running a dedicated
circuit to that bedroom to feed an outlet solely for the computer, not
protected by the AFCI?
any thoughts greatly appreciated...
nate
--
replace "fly" with "com" to reply.
http://home.comcast.net/~njnagel
Charles Schuler
grounds. Did you mean GFCIs?
Nate Nagel
> AFCIs are rather new. Any installation that new should not have faked
> grounds. Did you mean GFCIs?
>
>
No, I put the AFCI in myself after purchasing the house. However, a lot
of the wiring is late-40's vintage. That was a good enough reason in my
mind to think that an AFCI would be if anything more important here than
in new construction.
John Grabowski
"Nate Nagel" <njn...@flycast.net> wrote in message
news:er81q...@news2.newsguy.com...
Is that circuit part of a three wire cable (Black, red, white) used for two
circuits or is it just a two wire bx with the armor as the grounding
conductor?
w_tom
> No, I put the AFCI in myself after purchasing the house. However, a lot
> of the wiring is late-40's vintage. That was a good enough reason in my
> mind to think that an AFCI would be if anything more important here than
> in new construction.
First, break the problem into parts; then diagnose those parts. For
example, do you have other GFCIs in the bathroom or kitchen? If so,
then good. Run that computer extension cord to that GFCI; see
computer or extension cord causes the other GFCI to trip.
Have you run anything significant on that tripping AGFI circuit?
For example, if the AFGI circuit is somehow wired so that its neutral
(white) wire is started with another circuit, then AGFI trip will
occur. For example, an iron on that newly AGFIed circuit would be a
good test.
If any newly modified safety ground has somehow shorted to a neutral
wire, then that also will trip the AGFI. But long before taking
anything apart, first verify which suspect (computer and extension
cord, or AGFI circuit) causes GFCI trip.
Second, that plug-in protector is too far from earth AND is hoping
to earth via a wire bundled with other wires. Just two in a long list
of reasons why a power strip protector does not even claim (in
numerical specifications) protection from surges that would cause
damage. The length of that earthing wire (a short distance to earth
and a significantly long distance between computer and surge
protector) has long been necessary for effective protection. That
also means the 1940s mains box must be upgraded so that earthing both
meets and exceeds post 1990 National Electrical Code requirements.
Nate Nagel
It's two conductor BX with the armor grounded.
John Grabowski
AFCI circuit breakers are not required here in New Jersey, so I don't have
much experience diagnosing problems with them. My thoughts are that your
AFCI does not like the BX armor ground, there is something else going on
with the BX cable, or it does not like your UPS. Have you tried plugging in
another three prong appliance into the outlet?
Nate Nagel
No, that outlet has been unused since we've moved in. It is however the
first one in the circuit so the only possibilities I can see are that
the AFCI is actually a GFCI as well (anyone know?) and there's enough
current flowing through the real ground due to the UPS's fault sensing
circuit, OR that there is a physical problem with that receptacle, which
I'll be attempting to rule out shortly, now that the sun is up. (I've
been on a program of replacing all the receptacles as soon as wiring
"issues" are cleared, because a lot of the old receptacles were loose as
well.)
RBM
ma of a GFCI
"Nate Nagel" <njn...@flycast.net> wrote in message
news:er9i6...@news3.newsguy.com...
Nate Nagel
Update:
I replaced the receptacle. when I turned the breaker back on it tripped
immediately. I popped the cover on the panel and ohmed everything out,
seemed OK although the resistance hot to ground was lower than I thought
(~1 meg; nothing plugged in) although granted my meter is very old and
possibly inaccurate. Everything seemed OK so I tried pulling the wires
*leaving* the receptacle I'd just replaced. Then reset the breaker.
Fine. Plugged in UPS. Fine. Reattached wires and replaced breaker
with regular 15A breaker. Fine. Tried another AFCI breaker (I just so
happened to have one laying around because I wanted to split the
upstairs into two circuits eventually.) Tripped immediately. Removed
the wires leaving the receptacle again, new AFCI did not trip.
So what I apparently have is something is causing the AFCI to
electronically trip but not because of overcurrent. It's got to be a
wiring fault because I have everything unplugged. and all I know is
that it is somewhere upstairs - one circuit serves the whole second
floor, except for one outlet in the hallway (apparently intended for an
air conditioner.)
I'm not sure why it tripped only when I plugged the UPS in before,
coincidence, or just reached some kind of threshold? who knows?
F'ing great. Of course it's about 10 degrees outside, and I assume most
of this wiring is in the attic. I'm a little too paranoid to just leave
the regular 15A breaker in... or are the Siemens AFCI breakers known to
be problematic?
Nate Nagel
Heh... all right, I feel dumb now.
When I started to replace the receptacle, I plugged the UPS and the
associated extension cord into a receptacle in the hallway which was on
a different circuit.
I just went upstairs to start pulling receptacles out of the wall to
inspect the wiring/try to track down the presumed ground fault. Despite
the fact that I thought I'd unplugged every appliance upstairs, what did
I see at the first receptacle I came to, but a printer plugged in to a
wall receptacle. It, of course, was connected (by a USB cable) to the
computer that was connected to the UPS. I feel obligated to point out
at this point that I did *not* connect it this way, and in fact, the
reason that I feel particularly stupid is I was just explaining
yesterday why it was important to connect all peripherals of a computer
to either the same power strip or the same UPS so there's no issues with
floating grounds etc. and voltage imbalances that end up going through
USB or other ports. (was actually on my list of things to rectify.)
I dug out another extension cord (the reason the printer was plugged
into a different receptacle is that it is on the other side of the room
from the computer desk) and plugged the printer into the back of the UPS
on one of the "surge only" receptacles. All appears to be good now.
I'm still not sure why the breaker tripped when I simply tried to move
the UPS' power feed from one receptacle to another, unless the fact that
the UPS now had a proper ground while the printer still had a bootlegged
ground was enough to cause a problem.
I ASSume that this little exercise does indeed confirm that the Siemens
AFCI is also a GFCI...
w_tom
> Heh... all right, I feel dumb now.
>
> When I started to replace the receptacle, I plugged the UPS and the
> associated extension cord into a receptacle in the hallway which was on
> a different circuit.
> Despite the fact that I thought I'd unplugged every appliance upstairs,
> connected (by a USB cable) to the computer that was connected to
> connect all peripherals of a computer to either the same power strip
> or the same UPS so there's no issues with floating grounds etc. and
>
> I dug out another extension cord ... and plugged the printer into the
> back of the UPS on one of the "surge only" receptacles. All appears
> to be good now.
>
> I'm still not sure why the breaker tripped when I simply tried to move
> the UPS' power feed from one receptacle to another, unless the fact that
> the UPS now had a proper ground while the printer still had a bootlegged
> ground was enough to cause a problem.
>
> I ASSume that this little exercise does indeed confirm that the Siemens
> AFCI is also a GFCI...
Yes the AFCI is a GFCI (plus). What the USB printer is plugged into
should make no difference. Implied is that something is leaking via
the printer. Printer on another circuit is not desireable only
because backup protection is missing. But if working properly,
powering from both circuits should not have contributed to AGFI
tripping.
Meanwhile, temporarily disconnect neutral wire and safety ground
wire of the AGFI circuit in mains box. Infinite ohms should exist
between those two wires. Also infinite ohms should exist between that
disconnected neutral and mains box. All tests best performed with AC
electric off so that leakage and other problems do not distort that
ohm meter reading nor harm meter.
Disconnected safety ground might or might not conduct to mains box -
that is acceptable. But disconnected neutral must not measure
conductivity to disconnected safety ground nor to mains box (test
obviously done with appliances unplugged).
One final test. Ohm meter test (use extension cord to measure this)
should measure conductivity from receptacle safety ground prong to
disconnected safety ground wire in mains box. If this conductivity
does not exist, then that test explains everything seen previously and
suggests problem still exists.
Testing appliances (computer and printer) on another GFCI circuit,
and tested with computer and printer split between GFCI and non-GFCI
circuits should still be performed as posted earlier. Information
from that test is necessary - and would provide more useful
information for many reasons including because GFCI trips at only 5
ma.
AGFIs have been required for years now in New Jersey on bedroom
circuits.
Nate Nagel
all I can say is that it apparently did, because moving the power source
to the UPS solved the problem. I printed a test page after hooking
everything back up, so there appears to be no damage (thank you AFCI)
>
> Meanwhile, temporarily disconnect neutral wire and safety ground
> wire of the AGFI circuit in mains box. Infinite ohms should exist
> between those two wires. Also infinite ohms should exist between that
> disconnected neutral and mains box. All tests best performed with AC
> electric off so that leakage and other problems do not distort that
> ohm meter reading nor harm meter.
>
> Disconnected safety ground might or might not conduct to mains box -
> that is acceptable. But disconnected neutral must not measure
> conductivity to disconnected safety ground nor to mains box (test
> obviously done with appliances unplugged).
>
> One final test. Ohm meter test (use extension cord to measure this)
> should measure conductivity from receptacle safety ground prong to
> disconnected safety ground wire in mains box. If this conductivity
> does not exist, then that test explains everything seen previously and
> suggests problem still exists.
I can't disconnect the "safety ground" because it grounds to the breaker
box through the cable clamp... there's no separate ground conductor in
the BX. (the wiring is cloth covered as well, and appears to be tinned,
to give you an idea of the age of the stuff I'm dealing with.) I will
test hot and neutral just to satisfy my curiosity however now that I
have located *all* the appliances that were plugged in and can
disconnect them all... When I get around to rewiring the second floor,
I may see if I can drop a new homerun to the box, but I'm guessing that
the BX has at least one hidden staple somewhere along the way. It'd be
nice if I could do that though, because then I'd just pull some 12/2WG
Romex and switch to a 20A breaker (assuming that everything else on that
circuit is accessable from the attic.)
> Testing appliances (computer and printer) on another GFCI circuit,
> and tested with computer and printer split between GFCI and non-GFCI
> circuits should still be performed as posted earlier. Information
> from that test is necessary - and would provide more useful
> information for many reasons including because GFCI trips at only 5
> ma.
I could try that... I'd have to actually wire in a GFCI receptacle
upstairs though, would be easier than moving all the equipment to the
basement or kitchen :) (I actually do have a couple laying around
though for future use in garage etc. so it's not a huge deal.)
> AGFIs have been required for years now in New Jersey on bedroom
> circuits.
They've probably been required here for years as well, I don't think
this house has had any electrical upgrades in the last 20 years however
(previous owners were not really DIY types, and were here 18 years -
while the owner before that was apparently a contractor, so I'm guessing
he is responsible for a lot of the work) and I'm guessing that the work
wasn't inspected then, ref: bootlegged grounds mentioned above.
John Gilmer
> Charles Schuler wrote:
> > AFCIs are rather new. Any installation that new should not have faked
> > grounds. Did you mean GFCIs?
> >
> >
>
> No, I put the AFCI in myself after purchasing the house.
Unless the authorities "know" about what you are doing, I suggest you
replace the AFCI with either a normal breaker or jus a GFCI breaker.
The AFCI is quickly developing a reputation among the "pros" as something
that will cause the homeowner to make a lot of trips between the bedroom and
the basement.
If the NEC really, really considered arc faults to be a special danger in
bedrooms, it should have considered making arc detection set off an alarm
rather than cutting off the power.
This scheme (AFCI for bedroom circuits) is the equivalent of having a smoke
detector turn off the lights rather than just make a loud noise.
Mark Lloyd
you're supposed to light a candle :-)
--
Mark Lloyd
http://notstupid.laughingsquid.com
"Unlike biological evolution. 'intelligent design' is
not a genuine scientific theory and, therefore, has
no place in the curriculum of our nation's public
school classes." -- Ted Kennedy
Bud--
>
> Second, that plug-in protector is too far from earth AND is hoping
> to earth via a wire bundled with other wires. Just two in a long list
> of reasons why a power strip protector does not even claim (in
> numerical specifications) protection from surges that would cause
> damage. The length of that earthing wire (a short distance to earth
> and a significantly long distance between computer and surge
> protector) has long been necessary for effective protection.
>
Complete bullcrap.
Manufacturer specs for protection are readily available for plug-in
suppressors.
Both the IEEE guide on surges and surge protection at:
http://www.mikeholt.com/files/PDF/LightningGuide_FINALpublishedversion_May051.pdf
and the NIST guide at:
http://www.nist.gov/public_affairs/practiceguides/surgesfnl.pdf
say that plug-in suppressors are effective.
Plug-in suppressors, as explained in the IEEE guide, work primarily by
clamping the voltage on all wires (power and signal) to the common
ground at the suppressor. They do not work primarily by earthing.
--
bud--
John Gilmer
> >This scheme (AFCI for bedroom circuits) is the equivalent of having a
smoke
> >detector turn off the lights rather than just make a loud noise.
> >
>
> you're supposed to light a candle :-)
I was a little too hard on the NEC. The AFCI will cut off a circuit well
before a loose connection gets hot enough to actually be a safety problem.
It does happen. I found a loose wire when I realized that a plug for a
lamp was WARM. Years ago the fire department responded to a small fire
from a "hot" outlet.
I hope that withing a few years the AFCI manufacturers/NEC will sort things
out. In the past the NEC as paid attention to the problems of false
tripping.
For the time being, I would suggest that folks don't volunteer to get them
installed.
Bud--
>>>This scheme (AFCI for bedroom circuits) is the equivalent of having a
>
> smoke
>
>>>detector turn off the lights rather than just make a loud noise.
>>>
>>
>>you're supposed to light a candle :-)
>
>
> I was a little too hard on the NEC. The AFCI will cut off a circuit well
> before a loose connection gets hot enough to actually be a safety problem.
>
The AFCIs out there now detect parallel arcs - line to neutral or ground
(a fault). They don't detect a loose connection - a series arc. Series
arc protection is required starting Jan 2008. Far as I know these
devices are not yet on the market.
Parallel arcs can draw high current and I have read AFCIs detect pulses
around 60A. Series arcs are limited by the load (will usually be lower
than the breaker amp rating) and should be much harder to detect and
separate from "normal" arcs.
> It does happen. I found a loose wire when I realized that a plug for a
> lamp was WARM. Years ago the fire department responded to a small fire
> from a "hot" outlet.
>
> I hope that withing a few years the AFCI manufacturers/NEC will sort things
> out. In the past the NEC as paid attention to the problems of false
> tripping.
>
> For the time being, I would suggest that folks don't volunteer to get them
> installed.
>
Starting in the 2008 code AFCIs will be required on all 15 & 20A 120V
residential branch circuits. Coincidentally these will be the new AFCIs
that include series arc detection that aren't yet on the market - large
numbers of devices with inadequate field experience will be installed.
In this instance perhaps you aren't hard enough on the NEC. It
will be interesting if all the panel manufacturers even have the new
AFCIs by 2008.
At least the AFCIs out there now have a track record. Advice about not
volunteering sounds much better for the new AFCIs.
--
bud--
John Gilmer
>
> Starting in the 2008 code AFCIs will be required on all 15 & 20A 120V
> residential branch circuits. Coincidentally these will be the new AFCIs
> that include series arc detection that aren't yet on the market - large
> numbers of devices with inadequate field experience will be installed.
> In this instance perhaps you aren't hard enough on the NEC. It
> will be interesting if all the panel manufacturers even have the new
> AFCIs by 2008.
>
> At least the AFCIs out there now have a track record. Advice about not
> volunteering sounds much better for the new AFCIs.
I guess I don't understand WTF is going on.
A "series" fault is, exactly, what?
My hope is that the gadgets detect a loose connection that might result in
something overheating.
What, exactly, DOES the gadget detect?
w_tom
> A "series" fault is, exactly, what?
>
> My hope is that the gadgets detect a loose connection that might result in
> something overheating.
>
> What, exactly, DOES the gadget detect?
Often bedrooms have devices plugged in using a two wire (zip cord)
extension wire. The bed continuously rolls over that wire until
eventually the wire arcs - causes a fire. AFCI will detect that short
and cut off power before an arc can cause a fire. This is why AFCIs
are required for all bedroom circuits.
However, if you use a real Christmas tree, then the outlet that
powers lights on that tree should also have an AFCI so that a short
and resulting arc in Christmas tree lights do not create a fire. A
Christmas tree fire created when his wife only turned on lights took
out the entire house - killed almost all pets - in but 5 minutes.
These are arcs that kill and that an AFCI quashes.
Bud--
You can view arcs as being of 2 kinds.
One is from a hot wire to a neutral or ground wire. This is called a
parallel arc. The possible arc current is the available fault current
which can be very high. w_tom describes a fire source from a parallel
arc. AFCIs now detect only parallel arcs.
The other kind of arc is from a loose connection, like a worn out
receptacle that loosly grips a plug. This is a series arc; it is in
series with the circuit. It is not a "fault" as it is not across the source.
AFCIs now look for high current pulses, maybe 60A. The current in a
series arc is limited to the load current, and that is far too low to be
detected by AFCIs now sold. As I said previously, detecting a series arc
at load current levels is probably not easy and differentiating from
"normal" arcs (switches, brush motors) is probably very difficult.
Waveform "signatures" are used now, and analysis should be much more
sophisticated in series AFCIs.
I was surprised when I learned AFCIs detect only parallel arcs, but
fault is the F in AFCI. Starting 1-1-08 the NEC requires AFCIs installed
(produced?sold?) to detect both series and parallel arcs. I heard
SquareD just announced them on its web site - don't know if they are
actually available.
And when the 2008 NEC comes to your turf, AFCIs (the new ones) will be
required on all new 15 & 20A 120V residential branch circuits (except
alarm panels).
The NEC (and UL) refer to the AFCIs used now that are parallel only as
"branch/feeder AFCIs" and the new series-parallel ones as "combination
type AFCIs". I'm sure it was obvious from the name which ones detected
parallel and which detected series arcs.
Far as I know, AFCIs sold now are also required to have 30mA
ground-fault detection. (This is not the same level as the 5mA detection
of a GFCI.) I think the logic is arcs may go from H-N to include ground
if a ground is present. There is an interesting paper at:
http://www.eatonelectrical.com/unsecure/cms1/AFCI_UL_SPECIAL_SERVICES_INVESTIGATION.PDF
on "glowing connections" (series arcs) at receptacle binding screws that
may (or may not) eventually cause a trip with a parallel AFCI on the
ground fault function.
Devices can obviously be made with both AFCI and 5mA GFCI detection. I
believe they will have 2 test buttons.
AFCIs came out of work done at UL sponsored by the Consumer Product
Safety Commission, which was interested in reducing the number of fires.
An interesting paper from the Consumer Product Safety Commission on
AFCIs is at:
http://www.cpsc.gov/volstd/afci/AFCIFireTechnology.pdf
It explains the rationalle for using AFCIs, and why normal circuit
breakers aren't adequate. It also describes how AFCIs work. It is
technical enough you may like it.
--
bud--
Pop`
More simply stated, they detect when something causes an arc, e.g. a spark
and they then open the ckt.
John Gilmer
> You can view arcs as being of 2 kinds.
>
> One is from a hot wire to a neutral or ground wire. This is called a
> parallel arc. The possible arc current is the available fault current
> which can be very high. w_tom describes a fire source from a parallel
> arc. AFCIs now detect only parallel arcs.
Why bother? That type of arc will also trip the breaker.
Seems to me that NEC has "outsmarted" itself.
Bud--
>>You can view arcs as being of 2 kinds.
>>
>>One is from a hot wire to a neutral or ground wire. This is called a
>>parallel arc. The possible arc current is the available fault current
>>which can be very high. w_tom describes a fire source from a parallel
>>arc. AFCIs now detect only parallel arcs.
>
>
> Why bother? That type of arc will also trip the breaker.
>
> Seems to me that NEC has "outsmarted" itself.
>
A short circuit will trip the breaker but this is an arc. Ideally the
breaker would trip "instantaneously", but that takes a lot of current.
The available fault current in an extension cord may not be high enough
to trip a breaker on "instantaneous". If it is high enough, the arc is
not necessarily continuous and the breaker still may not trip on
"instantaneous". And an arc is not a short circuit - the current will be
less than the available fault current. That often leaves the breaker in
its inverse-time mode. With a constant load of 30A on a 15A breaker, for
example, the breaker may not trip for 2 minutes. The time delay likely
with an arc leaves opportunity for the arc to start a fire. AFCIs were
developed to detect an arc and provide a fast trip.
The NEC requirement for AFCIs was based on research done by UL for the
CPSC. When the research was done AFCIs didn't exist. The CPSC, UL and
the NFPA (author of the NEC) think parallel AFCIs can prevent a
significant percentage of fires with electrical causes, estimated at
40,000 per year.
The information above is a part of that contained at
http://www.cpsc.gov/volstd/afci/AFCIFireTechnology.pdf
The link contains a more detailed explanation of why circuit breakers
aren't adequate.
--
bud--
w_tom
>> One is from a hot wire to a neutral or ground wire. This is called a
>> parallel arc. The possible arc current is the available fault current
>> which can be very high. w_tom describes a fire source from a parallel
>> arc. AFCIs now detect only parallel arcs.
>
> Why bother? That type of arc will also trip the breaker.
If that were true, then the Christmas tree fire would not have taken
out the entire house in only five minutes. A conventional circuit
breaker trips after the fire has started. This fire is why AFCIs are
now required for bedrooms - where an sleeping occupant has the least
warning time; where such a fire can be most fatal.
John Gilmer
> If that were true, then the Christmas tree fire would not have taken
> out the entire house in only five minutes. A conventional circuit
> breaker trips after the fire has started. This fire is why AFCIs are
> now required for bedrooms - where an sleeping occupant has the least
> warning time; where such a fire can be most fatal.
Huh? Unless there is something to limit the current, arcs tend to ramp up
in current demands.
Perhaps I am missing something here but I would be hard pressed to create an
"arc" between HOT and NEUTRAL (or ground) that would NOT trip a conventional
breaker. A "direct" arc would not have anything to limit the current.
John Gilmer
> A short circuit will trip the breaker but this is an arc. Ideally the
> breaker would trip "instantaneously", but that takes a lot of current.
A lot? With a 15 amp breaker, a 30 amp arc should cause a trip within a
few seconds.
> The available fault current in an extension cord may not be high enough
> to trip a breaker on "instantaneous".
Oh!
Now we are speaking the same language. Is the NEC worried that some of
that crappy lamp cord has enough resistance to limit current flow to less
than, say, 30 amps when subject to essentially a direct short?
> If it is high enough, the arc is
> not necessarily continuous and the breaker still may not trip on
> "instantaneous". And an arc is not a short circuit - the current will be
> less than the available fault current.
An arc is quite close to being a short circuit. To a good approximate, an
arc can be modeled as a "short" with a fixed voltage drop that stays about
the same regardless of current. That voltage drop is only a few volts but
even if it were, say, 20 volts, the line cord would have to soak at 100
volts and to get a 100 volt drop in a line cord would take enough amps to
trip a breaker.
> That often leaves the breaker in
> its inverse-time mode. With a constant load of 30A on a 15A breaker, for
> example, the breaker may not trip for 2 minutes.
That long?
OK, then why not require breakers that "magnetic trip" at a current closer
the the rating for bedroom circuits? Or require a shorter "heat" trip
time?
>The time delay likely
> with an arc leaves opportunity for the arc to start a fire. AFCIs were
> developed to detect an arc and provide a fast trip.
>
> The NEC requirement for AFCIs was based on research done by UL for the
> CPSC. When the research was done AFCIs didn't exist. The CPSC, UL and
> the NFPA (author of the NEC) think parallel AFCIs can prevent a
> significant percentage of fires with electrical causes, estimated at
> 40,000 per year.
Just WHAT is that "significant percentage?"
If it's only, say, 10% then I say the AFCI is a WASTE.
w_tom
> Perhaps I am missing something here but I would be hard pressed to create an
> "arc" between HOT and NEUTRAL (or ground) that would NOT trip a conventional
> breaker. A "direct" arc would not have anything to limit the current.
Remember another parameter - time. Conventional circuit breakers
trip after significant energy has been dissipated at the arc. To
better appreciate how circuit breakers and fuses work, lean about a
famous 'I squared t' rule. Does a 20 amp circuit breaker trip or fuse
blow immediately when current is 25 amps? No. Visit application notes
from fuse manufacturers such as Littelfuse to learn that 'I^2t" rule
and to appreciate why fuses / circuit breakers don't trip fast enough
to quash an arc. Notice how long it takes a 20 amp circuit breaker to
trip when conducting 25 amps.
Bud--
>>A short circuit will trip the breaker but this is an arc. Ideally the
>>breaker would trip "instantaneously", but that takes a lot of current.
>
>
> A lot? With a 15 amp breaker, a 30 amp arc should cause a trip within a
> few seconds.
>
Might help if you read
http://www.cpsc.gov/volstd/afci/AFCIFireTechnology.pdf
hereafter called "the paper".
The paper quotes the UL standard as only requiring a trip in 2 minutes
at 200% of rated current.
Looking at the time-current curve for SquareD breakers, at 200% a trip
can take 40 seconds.
Breakers work on a thermal element to trip and it takes longer the lower
the overload - hence "inverse-time element". They also have a magnetic
trip for "instantaneous".
>
>>The available fault current in an extension cord may not be high enough
>>to trip a breaker on "instantaneous".
>
> Oh!
>
> Now we are speaking the same language. Is the NEC worried that some of
> that crappy lamp cord has enough resistance to limit current flow to less
> than, say, 30 amps when subject to essentially a direct short?
>
From the paper: UL field tested 1,590 receptacles in 80 dwellings to
determine the available fault current at the receptacle. 16% of the
receptacles would likely not trip on "instantaneous" with a short at the
receptacle. 44% of the receptacles would not trip on "instantaneous"
with a short at the end of a 6' of #18 cord.
For a SquareD breaker, a 15A breaker may require up to 11 times the
rated current to trip "instantaneously". That is 165A for a 15A breaker.
More than 16% of the surveyed receptacles would not supply 165A.
Not tripping on "instantaneous" puts the trip on the inverse-time curve.
And this was for a short. An arc reduces the current further, slowing
the trip. "Crappy" #18 is widely used for extension cords and appliances.
>
>>If it is high enough, the arc is
>>not necessarily continuous and the breaker still may not trip on
>>"instantaneous". And an arc is not a short circuit - the current will be
>>less than the available fault current.
>
>
> An arc is quite close to being a short circuit. To a good approximate, an
> arc can be modeled as a "short" with a fixed voltage drop that stays about
> the same regardless of current. That voltage drop is only a few volts but
> even if it were, say, 20 volts, the line cord would have to soak at 100
> volts and to get a 100 volt drop in a line cord would take enough amps to
> trip a breaker.
>
An arc may involve material that has been carbonized - which can run on
a lower current. And "parallel arcing faults have erratic current flow"
which reduces the current value.
>
>>That often leaves the breaker in
>>its inverse-time mode. With a constant load of 30A on a 15A breaker, for
>>example, the breaker may not trip for 2 minutes.
>
> That long?
>
Yes - see above.
> OK, then why not require breakers that "magnetic trip" at a current closer
> the the rating for bedroom circuits? Or require a shorter "heat" trip
> time?
>
Lower "instantaneuos" ratings can cause nuisance trips for motors and
lights which draw about 6x full load when they start. This is covered in
the paper - it was considered.
>
>>The time delay likely
>>with an arc leaves opportunity for the arc to start a fire. AFCIs were
>>developed to detect an arc and provide a fast trip.
>>
>>The NEC requirement for AFCIs was based on research done by UL for the
>>CPSC. When the research was done AFCIs didn't exist. The CPSC, UL and
>>the NFPA (author of the NEC) think parallel AFCIs can prevent a
>>significant percentage of fires with electrical causes, estimated at
>>40,000 per year.
>
>
> Just WHAT is that "significant percentage?"
>
> If it's only, say, 10% then I say the AFCI is a WASTE.
>
I would say 10%, 4,000 fires a year, would be well worth it. Consider
deaths and injuries. And the cost of medical for burns and cost of
building loss, which of course we all pay for.
There was a "cost-benefit" analysis done - so the requirement for AFCIs
in bedrooms had some grounding, if you'll excuse the expression.
I think the extension to all 15 & 20A circuits is a lot more
questionable. Particularly since the new series/parallel devices are
barely on the market (if at all) 10 months before they are required for
widespread use.
Incidentally - parallel arcs were considered more dangerous than series
because the current availble was much higher than a series arc.
--
bud--
John Gilmer
"w_tom" <w_t...@usa.net> wrote in message
news:1172298111.1...@t69g2000cwt.googlegroups.com...
1) Why would anyone want a 20 amp circuit in a bedroom?
2) All this ASSumes that suddenly, in the middle of the night, a fault
occurs that draws more than 20 amps but less than, what, 30 amps. A "magic
short" as it were. But this "magic short" is supposed to have the
"signature" of an arc. What we have is a 4 kW heat source. In the
several minutes it takes to trip the breaker, this "magic short" is supposed
to set the house of fire without setting off any of the smoke detectors.
Frankly, it "reads" like Bull Sh*t.
I suspect that some "wise guy" is making BIG money from this nonsense.
Slight OT: Some 30 years ago I used the then relatively new SOFT contact
lenses. Every evening I took them out and cleaned them and they boilding
them in a salt solution. I had no problem for over a year. I made the
salt solution from little tablets given by the optician and mixed it with
bottled distilled water in a small bottle than held the correct amount of
water.
BUT, the federal government decided that commercial "distilled" water mixed
with salt pills (which cost about $.01 each) wasn't "safe." It ordered the
"salt" pills (of a premeasured size) off the market and essentially forced
contact lens wearers to use a "commercial" solution that, amoung other
things, user a Mercury compound as a preservative.
My eyes were quite sensitive to the Mercury so I gave up on contacts.
BUT the "rest of the story" is that the bureaucrat responsible for the silly
rule had relatives who were in the business of "packaging" the mercury
preserved contact lens solution. The bureaucrat ended up doing some hard
time but that didn't do me any good.
When "they" decided that R-12 was bad for the ozone hole, the makers of R-12
quickly had EXPENSIVE replacements. IOW: it was the BIG chemical
companies who are behind the "ozone hole" nonsense.
>
>
>
>
John Gilmer
"Bud--" <remove....@isp.com> wrote in message
news:c3f27$45e010a2$4213ea5c$18...@DIALUPUSA.NET...
> John Gilmer wrote:
>
> >>A short circuit will trip the breaker but this is an arc. Ideally the
> >>breaker would trip "instantaneously", but that takes a lot of current.
> >
> >
> > A lot? With a 15 amp breaker, a 30 amp arc should cause a trip within
a
> > few seconds.
> >
>
> Might help if you read
> http://www.cpsc.gov/volstd/afci/AFCIFireTechnology.pdf
> hereafter called "the paper".
>
> The paper quotes the UL standard as only requiring a trip in 2 minutes
> at 200% of rated current.
>
> Looking at the time-current curve for SquareD breakers, at 200% a trip
> can take 40 seconds.
Frankly, that's "gud enuf."
Your AFCI causes a lot of problems to protect us from "magic shorts"
which draw from 100 to 200% of the breaker rating and have the correct
"signature.
>
> Breakers work on a thermal element to trip and it takes longer the lower
> the overload - hence "inverse-time element". They also have a magnetic
> trip for "instantaneous".
So? The NEC would have specified a class of breakers with the "magnetic"
trip point closed to the rating and a shorter time constant for the thermal
trip. That would have solved the problem cheaply....But NO!
>
Mark Lloyd
The resistance of the conductors would limit the current.
Mark Lloyd
<gil...@crosslink.net> wrote:
>
>"w_tom" <w_t...@usa.net> wrote in message
>news:1172298111.1...@t69g2000cwt.googlegroups.com...
>> On Feb 23, 9:57 pm, "John Gilmer" <gil...@crosslink.net> wrote:
>> > Perhaps I am missing something here but I would be hard pressed to
>create an
[snip]
>1) Why would anyone want a 20 amp circuit in a bedroom?
>
Electric heater as well as lamp, TV, fan etc... could be too much for
15A.
[snip]
Bud--
> "Bud--" <remove....@isp.com> wrote in message
> news:c3f27$45e010a2$4213ea5c$18...@DIALUPUSA.NET...
>
>>John Gilmer wrote:
>>
>>
>>>>A short circuit will trip the breaker but this is an arc. Ideally the
>>>>breaker would trip "instantaneously", but that takes a lot of current.
>>>
>>>
>>>A lot? With a 15 amp breaker, a 30 amp arc should cause a trip within
>
> a
>
>>>few seconds.
>>>
>>
>>Might help if you read
>>http://www.cpsc.gov/volstd/afci/AFCIFireTechnology.pdf
>>hereafter called "the paper".
>>
>>The paper quotes the UL standard as only requiring a trip in 2 minutes
>>at 200% of rated current.
>>
>>Looking at the time-current curve for SquareD breakers, at 200% a trip
>>can take 40 seconds.
>
>
> Frankly, that's "gud enuf."
>
Fires can easily start in 40 seconds of parallel arcing.
And parallel arc doesn't necessarily cause a trip in 40 seconds.
>
>
>>Breakers work on a thermal element to trip and it takes longer the lower
>>the overload - hence "inverse-time element". They also have a magnetic
>>trip for "instantaneous".
>
>
> So? The NEC would have specified a class of breakers with the "magnetic"
> trip point closed to the rating and a shorter time constant for the thermal
> trip. That would have solved the problem cheaply....But NO!
>
"Lower 'instantaneuos' ratings can cause nuisance trips for motors and
lights which draw about 6x full load when they start. This is covered in
the paper - it was considered." Faster thermal trip would probably be an
ever bigger problem with nuisance trips.
--
bud--
John Gilmer
"Bud--" <
> > So? The NEC would have specified a class of breakers with the
"magnetic"
> > trip point closed to the rating and a shorter time constant for the
thermal
> > trip. That would have solved the problem cheaply....But NO!
> >
>
> "Lower 'instantaneuos' ratings can cause nuisance trips for motors and
> lights which draw about 6x full load when they start. This is covered in
> the paper - it was considered." Faster thermal trip would probably be an
> ever bigger problem with nuisance trips.
>
This is getting circular.
The AFCIs are required in "NEW" construction. So far as I know, there
isn't any requirement that they be retrofitted in existing homes.
Yet the justification for them (over a more effective solution) is that a
"new" bedroom would have a room air conditioner and an electric heater,
neither of which (so the claim) could work on a 15 amp circuit.
No matter how you slice it, it's still ....
Again, I bet is 2 or 3 jails the "real reason" will become known. Odds are
that when someone checks our who benefited from the rule someone might end
up in jail.