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Since both temperature and moisture content become more stable at greater
distances below the earth's surface, a grounding system should be constructed
with the ground rod driven a considerable distance into the earth to be most
effective at all times. The best results are obtained if the ground rod reaches the
water table.
80
60
40
20
0
Seasonal variation of earth resistance with an electrode of 3/4" pipe in stony, clay
soil. Depth of electrode in earth is 3 ft for Curve 1 and 10 ft for Curve 2.
In some locations, the resistivity of the earth is so high that low-resistance
grounding can be obtained only with an elaborate grounding system at
considerable expense.
In such situations, it may be economical to use a ground rod system of limited
size and reduce the ground resistivity by periodically increasing the soluble
chemical content of the soil.
Table 4 shows the substantial reduction in resistivity of sandy loam from
increasing the chemical salt content.
The Effect of Salt* Content on the Resistivity of Soil
(sandy loam, moisture content, 15% by weight, temperature 17°C)
Added Salt
% by weight of moisture
26
Curve 1
0
0.1
1.0
5
10
20
Digital Ground Resistance Tester Models 4620 & 4630
Curve 2
Figure 6
Table 4
Resistivity
(Ωcm)
10,700
1800
460
190
130
100
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