[Server-devel] [UKids] Lightning kills underground Ethernet too; PoE wiring/voltages non-trivial!

[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