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MILHDBK419A
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
(225)
where r is the radius of each rod.
2.6.2.2 Square Array of Vertical Rods.
The resistance of a square array of rods is
(226)
Figure 212 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 212 correspond to values of s, stated as integral multiples of rod length. To illustrate the use of Figure
212, 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 25rod group. The parallel resistance of the 25 rods is
therefore 2.8 times one twentyfifth (1/N) of the resistance that one of the rods would exhibit if isolated.
227


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