AEMC 4500 - Page 21

Digital Ground Resistance Tester Model 4500 19
0
2.5
5
10
15
20
30
> 109
250,000
165,000
53,000
19,000
12,000
6,400
> 109
150,000
43,000
18,500
10,500
6,300
4,200
Resistivity, Ω-cm
Moisture content,
% by weight
Top Soil Sandy Loam
Table 6
Theresistivityofthesoilisalsoinuencedbytemperature.Table7shows
the variation of the resistivity of sandy loam, containing 15.2% moisture,
with temperature changes from 20° to -15°C. In this temperature range the
resistivity is seen to vary from 7200 to 330,000 ohm-centimeters.
20
10
0
0
-5
-15
68
50
32 (water)
32 (ice)
23
14
7,200
9,900
13,800
30,000
79,000
330,000
Resistivity
Ω-cm
Temperature
˚C ˚F
Table 7
Because soil resistivity directly relates to moisture content and tem-
perature, 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 7 below. Since both temperature and moisture
content become more stable at greater distances below the surface of the