At 02:15 PM 8/12/2003 -0400, you wrote:
>I'm going to add one more comment to the discussion, and then I'm not going
>to post any more on it, due to the sensitive nature of the topic.
Well, it *is* an important topic...
>I don't think that I can build ANYTHING reasonable that will survive a
>direct hit from a stroke of lightning. The currents involved are simply too
>massive.
Actually that's a common myth. You can make something "lightning proof"
for most *direct* strikes, but it is extraordinarily tedius. Think
commercial broadcast stations. Some cut corners, but others who don't
stay on the air even after being struck. More on that below...
But the key, as Stuart pointed out, is the ground *system*. An 8 foot
rod doesn't cut it. A thorough system may involve, say, two rings of
2-inch copper strap, 50 foot radials bonded (via thermite weld) to the
strap, and ground rods every 16 feet on the radials. The Polyphaser
web site has a lot of info on this subject, and not just about their
own products. I.C.E. also makes quality equipment for use in lightning
damage mitigation.
Another important consideration is the "ground window" where all your
equipment grounds get tied to. No daisy-chaining of grounds allowed,
and all suppressors get mounted directly on the ground window. (Basically
a sheet of copper.) All wires that go to your protected gear must pass
through a supressor on the ground window. Even a single wire not going
through that will *totally* defeat your suppression system.
It has already been said, but bears repeating: The idea is not to hold
your equipment at "ground" potential -- whatever that is -- but to let
it rise and fall with the strike with no differential currents among the
equipment.
I noted above that you have to be really anally retentive to properly set
up a system: Consider these two towers with coaxial line comeing down...
One is bonded to the tower at the 8 foot level and goes across to the
equipment hut ground window, the other goes to ground level, is bonded
there, and then runs up the wall of the equipment hut to the ground window.
The former won't survive a direct strike, but the latter will. Why? Because
the inductance of the tower when the strike hits will cause a several
thousand volt differential between the base of the tower and where the coax
is bonded, and it is probable that the coax provides a lower impedence
path to ground. Ouch.
Polyphaser has that example more thoroughly written up, either in one of
their catalogs or online papers.
One last thing... beware of any products that connect a gas tube to the
center conductor of a coax but the other side of the gas tube ties only
to ground but not to the shield. Because of different velocity factors
of the shield and the center conductor, you will in fact cause *more*
damage with one of these supressors. It's not magic, it's just thinking
different. Like working with microwave requires thinking different.
73,
Eric, KB0YDN
--
Eric F. Richards
efricha@dimensional.com
"The weird part is that I can feel productive even when I'm doomed."
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