AEMC 3640 - Page 22

20 Digital Ground Resistance Tester Model 3640 and 4610
Toassisttheengineerindeterminingtheapproximategroundroddepth
required to obtain a desired resistance, a device called the Grounding
Nomographmaybeused.TheNomograph,shownonthefollowingpage,
indicatesthattoobtainagroundingresistanceof20ohmsinasoilwith
aresistivityof10,000ohm-centimeters,a5/8"ODrodmustbedriven20
ft. Note that the values indicated on the Nomograph are based on the
assumption that the soil is homogeneous and, therefore, has uniform
resistivity(Figure7).TheNomographvalueisanapproximation.
Grounding Nomograph
Ground Rod
Resistance-Ohms
Soil Resistivity
(Ohm-centimeters)
Rod Depth
Feet
Rod Diamete
r
Inches
R
P
D
DIA
20
30
40
90
50
60
70
80
100
15
10
1
5
2
4
3
5/8
1/4
1/2
3/4
1
1.5
8
7
6
5
4
3
2
10000
15000
100000
50000
40000
30000
20000
3000
500
1000
2000
4000
5000
20
30
100
90
80
70
60
50
40
15
1
2
3
4
5
6
7
8
9
10
K
Figure 7
1. SelectrequiredresistanceonRscale.
2. SelectapparentresistivityonPscale.
3. Lay straightedge on R and P scale, and allow to intersect with K
scale.
4. MarkKscalepoint.
5. LaystraightedgeonKscalepointandDIAscale,andallowtointersect
withDscale.
6. PointonDscalewillberoddepthrequiredforresistanceonRscale.