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 TABLE 3-3
EXPECTED SOIL RESISTIVITIES
Resistivity
OHM-Centimeters
OHM-Meters
Soil Types
Minimum Maximum
Minimum
Maximum
Clay, Moist
1,400 3,000
14
30
Swampy Ground
1,000 10,000
100
10
Humus and Loam
3,000 5,000
30
50
Sand Below Ground-Water Level
6,000 13,000
130
60
Sandstone
12,000 7,000,000
70,000
120
Dry Sand
20,000 7,000,000
70,000
200
Broken Stone Mixed with Loam
20,000 35,000
350
200
20,000 400,000
4,000
200
Dry Earth
100,000 400,000
4,000
1,000
Dense Rock
1,000,000
10,000
Crushed Rock
300,000 400,000
3,000
4,000
(e) Oil circuit breakers require two grounds.
(f) Station fences require complete perimeter ground cables, located three feet outside
the fences, and are connected to the station grid. Ground spacing should not exceed 75 feet.
Fences require a fabric ground for approximately every 75 feet and a post ground for every third
post.
(g) All station gates require connection to both gate posts and copper braid to connect
the hinge posts to the gate.
3.8.1.4 Measurement of Soil Resistivity. An earth resistivity measurement of the soil is
important and should be carried out upon installation of the ground grid to assure accuracy of the
grid placement and to assure no loose ends or bad connections exist. The most common method
used to measure the soil resistivity is the four pin method.
3.8.2 Cable Placement. After the above ground connections are completed, the underground
cable, connecting cable, and ground rods should be placed. Cables should be laid in parallel lines
with reasonable spacing to connect these points. Ground rods should be placed at crossings and
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