[time-nuts] GPS antenna and lightning
usenet at teply.info
Sun Oct 4 11:22:29 UTC 2009
Am Sunday 04 October 2009 10:57:22 schrieb kevin-usenet at 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
> 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
> 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.
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.
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