If a number, N, of equal length vertical ground rods (with tops flush with the surface) are separated equally
along a straight line and connected together by an insulated conductor at the tops of the rods, the resultant
resistance will be somewhat greater than l/N times the resistance of single isolated rod. For N rods of length
at spacing s, the total resistance RN is given by
where r is the radius of each rod.
18.104.22.168 Square Array of Vertical Rods.
The resistance of a square array of rods is
Figure 2-12 shows the value of K for a square array of N equally spaced, equal length rods at spacings up to 10
times a rod length. The distance from a rod to its closest neighbor in the array is s, and the various curves in
Figure 2-12 correspond to values of s, stated as integral multiples of rod length. To illustrate the use of Figure
2-12, consider a 5 by 5 array of 25 rods, each spaced one length from its closest neighbor. From the s =
curve, it is found that the resistance ratio is 2.8 for a 25-rod group. The parallel resistance of the 25 rods is
therefore 2.8 times one twenty-fifth (1/N) of the resistance that one of the rods would exhibit if isolated.