Lightning kills underground Ethernet too; PoE wiring/voltages non-trivial!

[Adam Holt]

Just a quick digest below for microdeployments -- some summary excerpts for those who missed both interesting/longer discussions today:

• Power-over-Ethernet (PoE) might seem like magic cure (when installing Wi-Fi Access Points across classrooms not having nearby electrical outlets) but PoE is tough to get installed properly, as Tim has discovered!
• Burying Ethernet cabling in a shallow trench between nearby buildings seems rather popular "to the garage" (quite a number of personal experiences are documented online) however this does NOT eliminate the lightning problem.  What kind of trenching/equipment mitigates lightning risks and to what degree seems to be open for debate across different web sites (if you don't have El Chapo's tunneling team on your side, consider directional wireless/repeaters instead ;)
  

From: Anish Mangal <ani...@umich.edu>
Date: Mon, Jan 11, 2016 at 10:32 PM
Subject: Re: [XSCE] RE: [UKids] Ethernet Cable
To: xsce-devel <xsce-...@googlegroups.com>

Interesting insight Terry.

FWIW, I did some rough testing of a homemade cantenna a couple of months ago. (Rough, because it was not a perfect Line of Sight)

http://people.sugarlabs.org/anish/IMG_20150910_222141.jpg

The measured gain over a omni 5dbm antennas was around 7-8 dbm, which would put the overall gain at about 12-13 dbm. Not bad for a tin can antenna made essentially from spare parts :)

In theory, two of these could form a pretty stable link over a kilometre or so.

---------- Forwarded message ----------
From: T Gillett <tgil...@gmail.com>
Date: Mon, Jan 11, 2016 at 10:29 PM
Subject: Re: [XSCE] RE: [UKids] Ethernet Cable
To: xsce-devel <xsce-...@googlegroups.com>

Underground cabling doesn't add a lot of protection against lightning strike as George has related.

One problem is that when a lightning strike occurs, there are large ground currents flowing from the point of the strike outwards.

This produces voltage differences between different points on the ground - the further apart, the greater the voltage.

If you stand at one point, you experience no problem. But if you straddle two points some distance apart, then you are in trouble.

If you introduce a copper cable between two distant points, when a strike occurs you will see a large voltage between the end of that cable and local ground (to which the equipment is essentially connected). And your gas arrestors just try to tie it all together and so carry the current in the copper conductor.

The voltages and currents produced in lightning strikes are truly awe inspiring.

This is just one mechanism of failure that occurs. A lightning strike is really like a bomb going off. You can describe what happens in general terms, but the detail is extremely variable and difficult to predict with any certainty.

Prevention is generally considered the best strategy.

On Tue, Jan 12, 2016 at 1:18 PM, Adam Holt <ho...@laptop.org> wrote:
On Mon, Jan 11, 2016 at 10:14 PM, T Gillett <tgil...@gmail.com> wrote:

Long network cable runs are a magnet for lightning problems, particularly if they are run between buildings.

And no amount of expensive protection equipment will save you from a nearby strike.

General guideline for lightning prone areas is to keep network cables short as possible and within a single building.

If you need to network to other buildings, consider a wireless technique.

Is shallow/quick trenching also viable?
Or will rats/worms/frost destroy the cable quasi-annually?

On Tue, Jan 12, 2016 at 1:08 PM, Anish Mangal <ani...@umich.edu> wrote:

fwiw, a simple google search reveals that these are prohibitively priced.
http://www.blackbox.com/Store/Detail.aspx/Power-over-Ethernet-Surge-Protector-60-Volt/SP075A

On Tue, Jan 12, 2016 at 8:36 AM, Anish Mangal <ani...@umich.edu> wrote:

Perhaps this is slightly offtopic

Talking to a school a few days ago up in the mountains, they had a lightning strike that knocked out a bunch of electrical equipment. Are there any surge inhibitors for ethernet cables, would using PoE create more of these issues (i.e. the surge inhibitors probably wont work with PoE stuff)? Sure inhibitors are easily available for AC power sockets.

On Tue, Jan 12, 2016 at 3:10 AM, James Cameron <qu...@laptop.org> wrote:

PoE can be loads of fun.  The products vary considerably; both
injectors, cables, and routers.  Most problems can be solved through
measurements, ohms law, and power calculations.  Most problems can be
pre-empted by choosing industry best practice equipment, but that
raises costs.

To avoid all the PoE, there's no substitute for an electrician running
nice thick cables for AC.  Because these cables must meet a code, they
are way thicker than they need to be for a router.  It depends on
where you want to put your money.

--
James Cameron
http://quozl.netrek.org/

--

Anish

--

Anish

--

Anish

--

Unsung Heroes of OLPC, interviewed live @ http://unleashkids.org !

[Adam Holt]

Just a quick digest below for microdeployments -- some summary excerpts for those who missed both interesting/longer discussions today:

• Power-over-Ethernet (PoE) might seem like magic cure (when installing Wi-Fi Access Points across classrooms not having nearby electrical outlets) but PoE is tough to get installed properly, as Tim has discovered!

• Burying Ethernet cabling in a shallow trench between nearby buildings seems rather popular "to the garage" (quite a number of personal experiences are documented online) however this does NOT eliminate the lightning problem.  What kind of trenching/equipment mitigates lightning risks and to what degree seems to be open for debate across different web sites (if you don't have El Chapo's tunneling team on your side, avoid these risks using underground fiber or directional wireless/repeaters instead ;)
  

[Samuel Greenfeld]

If you are going to use any cable outdoors or where it may be exposed to the elements, getting cable rated for the desired outdoor use may be as important as its speed rating.  You don't want the cables getting water in them, causing interesting shorts and ground currents.

Cables routed inside of walls/vents/etc. also often have to be one of a few special types for fire safety and other reasons.

Note that these cables types are not as flexible as the cords running into your computer, so make sure not to bend them too hard.

If you can afford it, it might be worth splurging for a few Ethernet switches which support fiber connections.  That way there is no electrical link on the data line between buildings.

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[Sora Edwards-Thro]

Here in Lascahobas, Ben splurged and hired professionals to install solar panels back in 2014. The roof of the building with the battery/computer charging room was not stable enough, so they installed them on the roof of a building ~10 feet away. They ran a PVC pipe from the solar panel and through a hole in the charging room wall, and they placed the wire inside that. I think they may have also grounded (is that the correct term?) the solar panel itself by connecting it via wire to one of the metal rebar prongs sticking up out of the roof.

Would a PVC pipe protect an ethernet cable that ran between buildings too? Sounds like if not, an ethernet switch (to prevent electrical links, like Sam suggests) or a wireless repeater (to eliminate the need for cables of any kind) are necessary.

Thanks for bringing this issue up, Anish! 

[John Gilmore]

> Would a PVC pipe protect an ethernet cable that ran between buildings too?

No. PVC pipe won't protect your power cables either, except from
mechanical stresses like a tree limb hitting it. If the building on
either end of the power cable gets hit by lightning, the lightning
will be conducted to the other building.

The classic electrical code in the US requires "electrical conduit"
rather than PVC. Conduit is thin walled steel pipe with special
(cheap) connectors between segments. It is not structurally strong --
you can bend it by using a short lever, for example, and cut it easily
with a hacksaw -- but when installed properly, it provides a complete
grounded path from one end of the conduit to the other end. This
grounded path is a better conductor for things like lightning (or
short-circuits caused by bare bits of wire, etc), which makes the
whole circuit safer for the nearby humans -- and protects it from
mechanical stresses like a PVC pipe would.

> Sounds like if not, an ethernet switch (to prevent electrical links, like
> Sam suggests) or a wireless repeater (to eliminate the need for cables of
> any kind) are necessary.

If a cat5 or cat6 Ethernet cable is carrying a lightning strike, it
will destroy pretty much any Ethernet switch that it's plugged into.
The best ethernet switches (for this purpose) *might not* conduct that
strike into all the other Ethernet cables plugged into the switch.
But the average cheap one almost certainly will result in everything
that's plugged into any Ethernet cable getting destroyed. I am not
sure how you would find a "best" switch for that purpose.

What was suggested for between-building use was not just an "ethernet
switch", but a fiber-optic Ethernet link. Gigabit ethernet switches
that have one or two slots for a fiber interface are available. You
plug an "SFP transceiver" into one of those slots, plug a fiber into
the SFP, run the fiber to the other building, put a second SFP there,
plug that into a second fiber-enabled switch, and you have a working
Ethernet connection. The beauty of this for lightning is that there
is NO metal connection between the buildings -- the fiber cable is
plastic and glass, containing no wires at all, and the gigabit data is
carried as pulses of infrared light traveling through the glass fiber.
The plastic and glass will not conduct electricity or lightning.
Another advantage is that you can easily get a fiber to carry gigabit
data over 40 to 80 *kilometers*, while a cat5 ethernet cable only
works for perhaps 100 meters.

The cheapest of those fiber-enabled switches still cost about US$300
the last time I looked. The GBICs cost about $50 to $100 at best.
The fiber cable itself is finicky and ideally you would buy it from a
supplier who will cut it to the right length and put the right
connectors on it for you (because doing this in the field requires
custom equipment, trained personnel, and is slightly hazardous with
tiny bits of glass fiber that can get under your skin). Fiber cables
can't be bent as much as cat5 cables (or the glass fibers inside
break) so you have to take some care installing them. Unfortunately
there are no standards for fiber connectors, or rather there are
dozens, so you'd have to pick one connector type (e.g. "LC"
connectors) and get the cable and the SFP that have those connectors.
There are half a dozen 1000Base-something standards for fiber
ethernet, too, for different kinds of fiber cables and different
distances, so you have to specify which standard your SFP's will use.
You may need a pair of cheap attenuators too, if your fiber run is
short (to reduce the intensity of the light in the fiber). Compared
to just getting cat5e or cat6 cables and plugging them into a cheap
and standard switch, fiber is much more complicated and expensive.

If there are power cables running between the two buildings, those
power cables will conduct the lightning anyway, so there is zero
reason to use the expense and complication of a fiber optic ethernet.
The thing that fiber ethernet is really good for is connecting places
that are kilometers apart, with a super high speed (gigabit or faster)
connection.

> > If you are going to use any cable outdoors or where it may be exposed to
> > the elements, getting cable rated for the desired outdoor use may be as
> > important as its speed rating. You don't want the cables getting water in
> > them, causing interesting shorts and ground currents.

UV-resistant cables are needed outdoors (the outer cable plastic
doesn't break down when exposed to ultraviolet light from the sun for
a year).

> > Cables routed inside of walls/vents/etc. also often have to be one of a
> > few special types for fire safety and other reasons.

These are called "plenum rated". Their special property is that when
they burn (e.g. when the building catches fire), they don't release
toxic gases that will hurt the people/kids who are breathing the air
nearby.

John Gilmore

[Samuel Greenfeld]

On Tue, Jan 12, 2016 at 4:55 AM, John Gilmore <g...@toad.com> wrote:

The cheapest of those fiber-enabled switches still cost about US$300
the last time I looked.  The GBICs cost about $50 to $100 at best.
The fiber cable itself is finicky and ideally you would buy it from a
supplier who will cut it to the right length and put the right
connectors on it for you (because doing this in the field requires
custom equipment, trained personnel, and is slightly hazardous with
tiny bits of glass fiber that can get under your skin).  Fiber cables
can't be bent as much as cat5 cables (or the glass fibers inside
break) so you have to take some care installing them.  Unfortunately
there are no standards for fiber connectors, or rather there are
dozens, so you'd have to pick one connector type (e.g. "LC"
connectors) and get the cable and the SFP that have those connectors.
There are half a dozen 1000Base-something standards for fiber
ethernet, too, for different kinds of fiber cables and different
distances, so you have to specify which standard your SFP's will use.
You may need a pair of cheap attenuators too, if your fiber run is
short (to reduce the intensity of the light in the fiber).  Compared
to just getting cat5e or cat6 cables and plugging them into a cheap
and standard switch, fiber is much more complicated and expensive.

It depends on the port count and how managed it is.

Netgear has at least one semi-fancy switch (8 port PoE+2 port SFP) for $150.  But being a PoE switch, it comes with a pretty hefty (24 or 48 Volt IIRC) power supply.

SMC appears to have a similar switch without POE available for ~$110 on Amazon.

I only have experience with the Netgear switch.  Third party GBICs can run $25-$50 each depending on the manufacturer & source.

 

> > Cables routed inside of walls/vents/etc. also often have to be one of a
> > few special types for fire safety and other reasons.

These are called "plenum rated".  Their special property is that when
they burn (e.g. when the building catches fire), they don't release
toxic gases that will hurt the people/kids who are breathing the air
nearby.

I am aware of plenum rated; but I've also seen terms like "riser rated" (not as strict as plenum?) and similar tossed about.

In all cases installation should be done in accordance with the local building codes, if there is such a thing in the area.

[Sora Edwards-Thro]

On Tue, Jan 12, 2016 at 9:09 AM, Samuel Greenfeld <sam...@greenfeld.org> wrote:

In all cases installation should be done in accordance with the local building codes, if there is such a thing in the area.

There probably are building codes! They don't seem to be enforced / followed though. Electrical situation is particularly precarious because people are constantly trying to tap into power lines for free electricity. 

(Random note: my Haitian friends and I are always joking about why they're so afraid of rain. Rain gets blamed for disease transmission here, but maybe I can tell them that their fear is logical if heavy rain / lightning makes these wires dangerous).