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Re: [TowerTalk] ground rod spacing

Subject: Re: [TowerTalk] ground rod spacing
From: Jim Brown <>
Date: Thu, 30 Dec 2010 11:40:58 -0800
List-post: <">>
On 12/30/2010 8:36 AM, Jim Hoge wrote:
> What does the NEC have to say about the number of ground rods used in a single
> point grounding system where the building is surrrounded by a ground wire?

NEC has remarkably little to say about the spacing of ground rods. It 
requires a minimum of one rod, and and if the resistance to earth 
exceeds 25 ohms, a second rod is required.  NEC also requires that ALL 
grounds in a given building be bonded together.In other words, you can 
have as many grounds as you like, but they must all be bonded together. 
The word "bond" in this context means a low impedance conductor that can 
carry any current that may be imposed, and that is physically robust. 
That means short and fat.

> Is there a required minimum and maximum spacing between the ground rods?

There is,considerable published science on the topic of the spacing of 
ground rods.  A good "rule of thumb" is that they should be spaced a 
distance roughly equal to their length.  That's because lightning is a 
high frequency event, and the inductive coupling between adjacent rods 
reduces their effectiveness.

> Is there a required depth the perimeter wire is to be buried?

In general, deeper is better so that the wire has a better chance of 
being in contact with moist earth, and because it is less likely to be 
disturbed by a gardener.  Since I don't use them, I don't know what, if 
anything, NEC has to say on the subject.

In all of this, remember that the primary function of a connection to 
EARTH is lightning protection.  The proper bonding of all such 
connections, and of all wring within a building, are a critical part of 
that protection system, and also provide protection from electrical 
shock and fire hazard.

Connections to EARTH do NOT generally make antennas work better, nor do 
they generally reduce noise or RFI.   There are, of course, a few 
exceptions --  Beverages needs either an earth connection or radials, 
and an earth connection can be helpful as the shunt element with a 
common mode choke on a telephone line.

73, Jim Brown K9YC

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