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[Towertalk] grounding system

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Subject: [Towertalk] grounding system
From: (Tom Rauch)
Date: Mon, 29 Jul 2002 08:08:46 -0400
> At 05:03 PM 7/28/02 -0400, Glenn Little wrote:
> >Acording to some data that I got from a surge suppressor company
> >concerning lightning induced voltages, a lightning strike one Km away
> >will induce 200 volts per meter of wire.  The second floor of you
> >house appears to be about 10 feet or about 3 meters above actual
> >ground. That three meter ground wire could have 600 volts on it from
> >a strike one Km away.  If the strike is 100 meters away, the figure
> >goes to 1KV per meter of wire.
> These numbers seem a bit extreme, or at least a bit categorical.  How
> many times have you seen a lightning bolt and counted less than 3
> before the thunder came.  Did it fry your phone?  Your modem?  Your
> radio?

Part of the problem with taking things literally is people who market 
things always try do the best job they can in pitching the need for 
their devices. What we read, even though traceable to facts, are 
often the extreme.

The induced voltage would be influenced by many things including what 
is around the conductor, the impedances loading the conductor, the 
density and angle of the strick, the position of the conductor, and 
so on.

For example the guy lines on my 300 foot tower will often "pop" 
across the insulators with distant flashes, yet I can have a receiver 
on and running and connected to that tower without ANY lightning 
protection and nothing is hurt.

There is a lot of available potential across an open circuit with a 
conductor high and in the clear, but not much current available to 
drive any load. Voltage decreases dramatically even with extremely 
high values of load resistance.

99% of all of this is how you route the wires into the house and how 
the connections are all made. 

> I suspect these numbers could represent worst case, but there are too
> many variables (orientation of wire, neight above ground, etc.) for me
> to believe that one voltage fits all.

Absolutely Pete.

If that were true, I'd be in deep "poo" here. I have 4 or 5 miles of 
antenna wire spread over a half-mile square area, and 100-300 foot 
tall antennas. I leave everything connected all the time, and don't 
have a single lightning arrestor in any feedline or control cable. I 
live next to the worse area in the USA for lightning, we have 
afternoon thunderstorms like clockwork during the summer.

99% of this is how you route cables and how you wire things.73, Tom 

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