Bill Turner wrote:
> ORIGINAL MESSAGE:
>
> On Tue, 11 Sep 2007 20:32:08 +0000, kb9cry@comcast.net (Phil Camera)
> wrote:
>
>> OK, so now lets bond everything together, outdoors. Now when we ge the
>> induced energy, everybody (grounds) all rise and fall in that voltage
>> together and there is no voltage difference inside of your equipment and
>> voila, no current and no zapped electronics.
>>
>> Quite simple if you think about it.
>
> ------------ REPLY FOLLOWS ------------
>
> Simple, but wrong.
>
> There is no such thing as "no voltage difference" when you are talking
> a million amps or so.
Doubt you're going to see a million amps. really, really big strokes
might be 100kA, but that's in the stroke, not in some grounding wire
(the tower might carry that, but as soon as you start dividing the
current up among various paths to "earth" the current in any one
conductor gets smaller). The peak current can also decrease because of
the inductance of the conductors spreading the pulse out.
Most lightning is in the 10-20 kA range. The long duration continuing
current that starts fires is actually in the hundreds of amps range.
100kA strokes are rare.
See, e.g. M. Uman, "Lightning" or anything by Uman and Rakov in the last
10 years.
A lot of "lightning damage" is from induced currents, rather than direct
conduction. That several kiloamps with a few microsecond rise time
flowing through the lightning down conductor has a pretty hefty magnetic
field, and if you have a loop with a couple square meter area that
intercepts that field connected to a sensitive circuit, it fries. Folks
fooling with small Marx banks, medium sized Tesla coils, and medium
hobby Van deGraaff generators find this out real fast. The energy in
the "bang" is low in all these (a few joules), but they are notorious
electronics killers at short range (garage door openers are a common
victim).
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