On 9/3/12 5:13 AM, Larry wrote:
The use of ground rods spaced at 2x the rod length was covered in the
Polyphaser book on lightning and perhaps elsewhere. As I recall (it has
been a few years since I read it) the idea was better dissipation of
charge during a strike. The wire between the rods doesn't dissipate the
charge all that well relatively speaking.
Perhaps they're talking "above ground" for that wire? If the wire's
buried it will work as well as a rod.
Closer than 2x rod length
spacing allows the ground charge to swell and reduces the effectiveness
of the rods in dissipating the charge.
I'm not sure about "swell", but the 2x recommendation is basically a
simple rule of thumb based on the current distribution from a rod. TO a
first order, by the time you're 8 feet away, the current is distributed
over a cylinder 16 feet in diameter, and pretty small.
The actual equation from the IEEE doc for two idealized ground rods of
length "L", radius "a", spaced "s" apart is:
R = rho/(4 pi L)*(ln(4*L/a)-1) +
rho/(4 pi s) *(1 - L^2/(3*s^2) + 2*L^4/(5 * s^4) - ...)
The first term is the resistance of the two rods in parallel, neglecting
the interaction. The resistance of a single rod:
R = rho/(2 pi L) * (ln (4*L/a) - 1)
You don't actually gain much as you get farther than one rod length
apart (formulas from IEEE.. there's one for spaced farther than length,
and one for closer):
rho (ohm*m) 200
L (m) 3
a (m) 0.01
s (m) 1 2 3 4 5 6 7
R 64.6 64.6 64.6 64.6 64.6 64.6 64.6
R2 (s>L) 36.9 35.8 35.2 34.8 34.5
R2 (s<L 41.0 38.1 36.7
The Polyphaser book suggested
several ground radials with ground rods along each radial with the rods
spaced at 2x the rod length.
I wonder if that's for above ground wire? (that is, the ground
connection is only the rods, and not the wire?)
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