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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