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MILHDBK419A
1.2.2.2 Calculation of Earth Resistance.
Once the most appropriate configuration is chosen for the facility, calculate the resistance to earth for the
configuration. If the calculated resistance meets the design goal (or requirement), complete the design to
include all necessary interconnections. To illustrate this design procedure, assume that a 100 ft x 160 ft
rectangular configuration like that shown in Figure 13 is initially chosen. Further, assume that the soil
resistivity measurements made during the site survey showed an average resistivity of 10,000 ohmcm for the
area. In addition, the site survey indicated that all rock formations are at depths greater than 10 feet; the
water table never drops more than 5 feet below grade; and the frost line extends only to 1 foot below grade.
Therefore, 10foot ground rods are initially selected for evaluation. (The minimum rod diameter required in
MILSTD188124A, para 5.1.1.1.4 is 3/4 inch.)
Determine the resistance of one of the ground rods from Figure 14. First, place a straight edge
a.
between the point marked 3/4 on line "d" and the point marked 10 on line
. Indicate on line "q" where the
Next, place the straightedge between the point just marked on "q" and the
straightedge crosses.
10,000 ohmcm point on the vertical line labeled "Resistivity." Read the resistance as 32 ohms at the point
where the straightedge crosses the vertical line labeled "Resistance."
Assume an initial spacing of 20 feet or twice the rod length (see Volume I, Section 2.6.2) between
b.
rods. Figure l3 shows that 26 rods are required to encircle the structure. Use Figure 15 to determine the
relative lowering of the resistance of one rod that is produced by 26 rods in parallel. (The answer is about 5.5
percent.) Thus the resistance of the 26 rods in 10,000 ohmcm soil is
R = 32 x 0.055 = 1.76 ohms.
Figure l5 primarily applies to ground rods laid out in a straight line or around the perimeter of a site whose
dimensions are large with respect to the rod spacing. If the rods are distributed in a grid pattern, as will
frequently be done for substations, use Figure 16 to estimate the net resistance. In many instances, the
answers provided by Figures 15 and 16 will agree. For this example, the resistance multiplier given by Figure
16 for 26 rods over an estimated area of 16,000 square feet (100' x 160') is 0.056 for a net resistance of
1.9 ohms.
1.2.2.3 Alternate Configurations. Nonideal sites will frequently be encountered. For example, large rock
formations may be present which prevent the uniform placement of ground rods around the site; bed rock may
be relatively near the surface; the water level may drop to several feet below grade; the soil resistivity may be
very high; or architectural and landscape requirements may preclude locating ground rods at particular points.
In such cases, modify the electrode configuration to conform to the constraints while achieving the desired
resistance. Typical suggested alternatives are:
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