NANOG Digest, Vol 43, Issue 53

Pete Carah pete at altadena.net
Sat Aug 20 15:06:22 CDT 2011


On 08/20/2011 02:07 PM, Matt Addison wrote:
> On Aug 20, 2011, at 3:09, Pete Carah <pete at altadena.net> wrote:
>
> Note that he wanted to use fiber for lightning protection; the metal
> strip rather negates that...
>
>
> Only if you plug the metal strip into  your equipment. We usually don't do
> that with locate wires (they usually sit unterminated, or maybe grounded,
> depending on site practice).
Never underestimate lightning!!!

Actually unterminated is still bad for the equipment if the conductor
comes within a few inches of it (and lightning hits don't have to be
direct to get damage even from this)...
The best is non-metallic fiber in well-grounded metallic conduit (that
ends at the building entry)  (there are safety considerations here; the
building or at least the entry area has to be built to handle the ground
currents and not mix them into either power or phone lines) if you are
in a serious lightning area.  Sometime look at the external grounding at
a (properly-installed) cell site if you can get close enough to one to
see it.  (and the power folks are even more paranoid, sometimes they
will run a microwave link across a street to avoid conductive
communications links.  Then again, they can make their own lightning,
and they do (usually) plan for it.)  If unterminated to a junction point
at the entry wall, then non-conductive from there to the equipment, it
will be better for the equipment, but lightning hitting the cable
directly will likely explode the strip where it hits, normally cutting
the fiber for a few inches; this can easily happen even if nothing
appears to be grounded (think capacitor; the voltage and current
rate-of-rise makes capacitors and inductors out of things you wouldn't
expect).  And a grounded strip can attract the lightning, even through
an insulating sheath (again, think capacitor).  If you wonder, I have
some darkened parts left over from some satellite receivers that had
been connected to well-grounded antennas; that helps but not always
enough...  If the locate wire is big enough (say 10awg, maybe 12) then
it probably won't explode but even that can't be guaranteed; I've seen a
#4 wire melted half-way through from what was probably a direct hit
(this was in California in an area not known for lightning, even; we
only got a significant storm once or twice every few years.)  And I had
an old 14.4k modem (this was the early 1990's) burned out by a direct
hit at the CO 1.5 miles away (I happened to be near the CO and saw the
hit at the same time my link disappeared so I know that was what did
it...  Got back home and found the modem didn't recover.) (same part of
California.)

-- Pete





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