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Re: [TowerTalk] Ground Rods

To:, <>
Subject: Re: [TowerTalk] Ground Rods
From: Jim Lux <>
Date: Tue, 04 May 2004 09:49:09 -0700
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At 08:07 AM 5/4/2004 -0700, Bill Turner wrote:
On Mon, 3 May 2004 18:38:26 -0600, Grillo's wrote:

>One of the calculations I was looking at can drop impedance from 300 down to 5 ohms with using only 4 of these rods.


Does it really matter to a multi-million volt lightning strike whether
the ground is 300 ohms or five?  The difference would seem to be

This reminds me of the school of thought that says the connection to
your ground rods must be tight.  As if lightning, having traveled
thousands of feet through the air, is going to be stopped by a gap of a
few thousands of an inch.

The voltage in a lightning stroke is immaterial (mostly because it's spread over many thousands of meters of air), but the current is what does the damage (or, in some cases, the di/dt, which induces a voltage in nearby conductors). The goal is to keep the voltages in your system low enough to be reasonably handled.

Figure 25kA for a typical lightning stroke.. 25kA at 5 ohms is 125V.. not enough to flash over in air and well within the transient suppressing ability of many devices.
25kA @ 300ohms is 7500V, more than enough to flashover to adjacent conductors, and well beyond most transient suppression devices.

So.. reasonably low impedance is a "good thing", although the practical difference between 5 ohms and 10 ohms isn't all that signficant.

Jim, W6RMK


See: for "Self Supporting Towers", "Wireless Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions and ask for Sherman, W2FLA.

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