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Re: [TowerTalk] copper or galvanized ground rods in red SC clay

To: towertalk@contesting.com
Subject: Re: [TowerTalk] copper or galvanized ground rods in red SC clay
From: jimlux <jimlux@earthlink.net>
Date: Sun, 10 Jan 2016 08:07:40 -0800
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
On 1/9/16 10:37 PM, Jim Brown wrote:
On Sat,1/9/2016 8:10 PM, Larry wrote:
The Polyphaser book on grounding recommended that ground rods be
spaced about twice the length of the rods (e.g., 16 feet spacing for 8
foot rods). Presumably that will help with the charge saturation
during an event.

The reason for rod spacing is to minimize inductive coupling between
them, which reduces their effectiveness.



This is interesting..

I just ran a calculation of the DC resistance of 2 rods vs 1 rod (using the equations in IEEE Std-142, which are also in lots of other places).

for 8ft rod, 5/8" diameter, here's what I got..

two rods, spaced 4 ft is 68% of the resistance of one rod.
two rods, spaced 8 ft is 61%
two rods, spaced 16 ft is 56%

This isn't a huge difference, so I think the "space it twice the rod length" is more of an "easy rule of thumb" than some hard cutoff.

I'd have to think about the inductance effect. You're looking at the mutual L between two parallel conductors spaced by some distance (and then, you also really need to account for the L in the cable connecting them.

L       a       s       R2/R1
8       0.3125  1       0.85
8       0.3125  2       0.76
8       0.3125  3       0.71
8       0.3125  4       0.68
8       0.3125  5       0.66
8       0.3125  6       0.64
8       0.3125  7       0.63
8       0.3125  8       0.61
8       0.3125  10      0.60
8       0.3125  12      0.59
8       0.3125  14      0.57
8       0.3125  16      0.56
8       0.3125  18      0.56
8       0.3125  20      0.55
8       0.3125  24      0.54
8       0.3125  26      0.54
8       0.3125  28      0.54
8       0.3125  30      0.54


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