On 08/20/2011 02:07 PM, Matt Addison wrote: > On Aug 20, 2011, at 3:09, Pete Carah <p...@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
