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

[jimlux]

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