From IEEE Std 142:
"A useful rule is that grounding systems of 2-24 rods placed one rod
length apart in a line, hollow triangle, circle, or square will provide
a grounding resistance divided by the number of rods and multiplied by
the factor F taken from Table 14. Additional considerations with
respect to step and touch potentials would be addressed by the geometry.
Table 14 - multiplying factors for multiple rods
2 1.16
3 1.29
4 1.36
8 1.68
12 1.80
16 1.92
20 2.00
24 2.16
Placing additional rods within the periphery of a square, circle, or
other shape will not appreciably reduce the grounding resistance below
that of the peripheral rods alone.
"
So, for a practical example.. if you had 3 rods, spaced one rod length
apart, that's the DC resistance of 1.29/3 * one rod = 43% of one rod.
On the next couple pages, they discuss the "smoking" rod issue, where
the current density is too high.
the current should be no more than
I = 34800 * d * L/Sqrt(rho * t)
L rod length in meters
d rod diameter (in meters, I assume)
rho in ohm meter (inverse of conductivity)
t in seconds (for short times)
1 ohm meter = 100 ohm cm - as hams, we're usually looking at
conductivity (sigma) with values like 0.005 Siemen/meter 0.005 S/m =
200 ohm*meter (or 20,000 ohm*cm)
So, for a 8 foot rod (2.43 m) 5/8" in diameter (0.015875 m), we get
about 94 Amps for 1 second..
But for, say, 50 microseconds (the duration of a lightning stroke), you
could multiply by 141 and get 13 kAmps..
Since most lightning strikes are in the tens of kA range (but also
shorter than 50 microseconds at that current), the usual recommendation
to have more than one rod is a good one.
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