Effects Of Soil Resistivity On Ground Electrode Resistance - AEMC 3640 User Manual

The NEC 2008 250.52 (A)(5) requires a minimum of 8 ft (2.4m) to be in
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contact with the soil. The most common is a 10 ft (3m) cylindrical rod which
meets the NEC
for steel rods and 1/2" (1.27cm) for copper or copper clad steel rods NEC
2008 250.52(A)(5)(a)(b). Minimum practical diameter for driving limitations
for 10 ft (3m) rods are:
• 1/2" (1.27cm) in average soil
• 5/8" (1.59cm in moist soil
• 3/4" (1.91cm) in hard soil or more than 10 ft driving depths
4.1.2

Effects of Soil Resistivity on Ground Electrode Resistance

Dwight's formula, cited previously, shows that the resistance to earth of
grounding electrodes depends not only on the depth and surface area of
grounding electrodes but on soil resistivity as well.
Soil resistivity is the key factor that determines what the resistance of a
grounding electrode will be, and to what depth it must be driven to obtain
low ground resistance.
The resistivity of the soil varies widely throughout the world and changes
seasonally. Soil resistivity is determined largely by its content of electro-
lytes, consisting of moisture, minerals and dissolved salts. A dry soil has
high resistivity if it contains no soluble salts.
Soil
Ashes, cinders, brine, waste
Clay, shale, gumbo, loam
Same, with varying proportions
of sand and gravel
Gravel, sand, stones with
little clay or loam
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code. A minimum diameter of 5/8" (1.59cm) is required
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Resistivity, Ω-cm
Minimum
590
340
1,020
59,000
Table 1
Digital Ground Resistance Tester Model 3640 and 4610
Average Maximum
2,370 7,000
4,060 16,300
15,800 135,000
94,000 458,000
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