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Re: [TowerTalk] Lightning Advice/Ground Potentials

To: Bill Turner <>
Subject: Re: [TowerTalk] Lightning Advice/Ground Potentials
From: Jim Lux <>
Date: Tue, 11 Sep 2007 21:42:45 -0700
List-post: <>
Bill Turner wrote:
> On Tue, 11 Sep 2007 20:32:08 +0000, (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 


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