Am Sunday 04 October 2009 10:57:22 schrieb kevin-use...@horizon.com: > Consider a lightning strike to be the closest thing to an ideal current > source you are going to encounter. You *cannot* stop it with series > impedance alone, no matter how high; you have to provide it with a very low > shunt impedance. > > Likewise, remember that it's not the volts that kills you, it's > the amps. It doesn't matter if your entire equipment bench bounces > 1 MV, as long as all potential *differences* are small emough that > no damaging currents flow. > > The basic layout is the "moat and drawbridge" illustrated at > http://www.sigcon.com/Pubs/news/7_02.htm > > Lightning will take the lowest-inductance path(s) to ground. Your goal > is to make sure that all paths through your equipment pass over a SINGLE > drawbridge, which is tied together with surge-diverting devices such as > spark gaps, gas discharge tubes, MOVs, transzorbs, etc. > > Separate the stages with a bit of series impedance like unsaturatable > air-core coils. > > In a typical AM transmitter shack, the drawbridge will take the form of > a big well-grounded steel plate on one wall sized to handle kilo-amp > currents without damage. All wires, WITHOUT EXCEPTION, entering or > leaving the shack pass through feedthroughs in that steel plate. > > Any damaging current would have to pass over the drawbridge, loop through > your equipment, and back to the exact same drawbridge to ground. > This is a dead-end path that current is not going to flow through. > > It's not hard to make the high-frequency impedance of that loop something > like 1M times larger than the direct path through the steel grounding > plate. > > Then, of a large but not extraordinary bolt of 100 kA, only 100 mA goes > through your equipment. Suddenly, it's a lot less threatening. > > > But the secret is making sure that *every wire* to your protected > equipment has a low-impedance path to *every other* wire. It's not a > matter of protecting them individually, because the protection does not > STOP lighting current. You have to tie them all to the SAME ground point. > > For example, if you have equipment plugged into two different surge > suppressors, you can have lightning pass in one and decide to send half > of its energy out through the other via your equiment. On the way, > it lets out all the magic smoke. :-( > > You have to consider the impedance between each possible pair of wires. > Where is the shunt path, and why is its impedance many orders of magnitude > lower than the path through the protected equipment? > > Just one little wire that's not tied into the system provides a path > that will let damaging currents come in through any other wire, no matter > how well "protected" they are. > Well, that's one of the best explanations regarding lightning and electronic equipment i've seen in many years. May i qoute that?
Anyways, as you already stated: The only way to protect your equipment from lightning strikes is to provide a path to ground of reasonably low impedance for the lightning. Everything else is just cosmetics and doesn't help you in case a lightning strikes. Greetings, Florian _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.