AEMC 4620 User Manual page 21

Since both temperature and moisture content become more stable at
greater distances below the surface of the earth, it follows that a grounding
system (to be most effective at all times) should be constructed with the
ground rod driven down a considerable distance below the surface of the
earth. Best results are obtained if the ground rod reaches the water table.
Seasonal variation of earth resistance with an electrode of 3/4" pipe in rather 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 at considerable expense and with an elab-
orate grounding system.
In such situations, it may be economical to use a ground rod system of
limited size and to 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 brought
about by an increase in 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
Digital Ground Resistance Tester Model 4620 and 4630
80
60
Curve 1
40
20
0
0
0.1
1.0
5
10
20
Curve 2
Figure 6
Resistivity
(Ohm-centimeters)
Table 4
10,700
1800
460
190
130
100
19