20
Digital Ground Resistance Tester Model 4500
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.
0
Curve 1
Curve 2
Figure 7
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
groundingcanbeobtainedonlyatconsiderableexpenseandwithanelab-
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 8
shows the substantial reduction in resistivity of sandy loam brought about
by an increase in chemical salt content.
0
0.1
1.0
5
10
20
10,700
1,800
460
190
130
100
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
Resistivity
(Ohm-centimeters)
Table 8
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