AEMC 3640 User Manual page 20

Temperature
°C
20
10
0
0
-5
-15
Because soil resistivity directly relates to moisture content and temper-
ature, it is reasonable to assume that the resistance of any grounding
system will vary throughout the different seasons of the year. Such varia-
tions are shown in Figure 6 below.
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
18
°F
68
50
32 (water)
32 (ice)
23
14
80
60
Curve 1
40
20
0
Resistivity
Table 3
Curve 2
Figure 6
Digital Ground Resistance Tester Model 3640 and 4610
Ω-cm
7,200
9,900
13,800
30,000
79,000
330,000