Page 740 - Industrial Power Engineering and Applications Handbook

22/700 Industrial Power Engineering and Applications Handbook
Table 22.1 Likely resistivity of soil
Climatic conditions
Type of soil Normal and high Low rainfall or desert condition
rainfall (more than (less than 250 mm a year) Underground
500 mm a year) water salinity
Resistiviry of soil
Rm
(Likely value) (Likely range of values) l
Alluvium and lighter clays, such as sandy 5 a a 1 to 5
and/or muddy soil
Clays (excluding alluvium) 10 5 to 20 10 to 100 -
Marls like keuper marl such as marble 20 10 to 30 5 to 300 -
Porous limestone like chalk 50 30 to 100
Porous sandstone such as keuper sandstone 100 30 to 300
and clay shales
Quartzite's compact and crystalline limestone 300 100 to 1000
such as carboniferous marble
Clay slates and slaly shales 1000 300 to 3000 1000 upwards 30 to 100
Granite 1000 -
Fossil slates, schists, gneiss, igneous rocks 2000 1000 upwards

Based on BS 7430
"Depends up on the water level of the locality

preferred metal for grounding purposes. Choice of Table 22.2 Range of soil resistivity
the metal for the ground electrode would, however,
depend upon the underground municipal services, such Type of soil Average resistivity
as water, sewerage and telephones and also the Rm
structures and foundations of nearby buildings to save
them from corrosion and erosion. Copper, being Wet organic soil 10
galvanic, under damp conditions forms a complete Moist soil 102
1 o3
Dry soil
electrolytic circuit between it and other metals and Bedrock I o4
causes corrosion, which erodes the other metals (an
effect similar to that discussed in Section 29.2.5, while Based on IEEE-80
making a bimetallic joint).
6 Any of the grounding methods may be adopted for
industrial installations, equipment grounding or small nearby locations and an average value of the soil resistivity
and medium-sized sub-stations, depending upon the is determined, as discussed in Section 22.11. The condition
type and condition of the soil. A sandy soil will be of soil, such as its moisture, content temperature and
easy to dig and plate grounding will be easier, while content of salts and other minerals have a large bearing
a rocky soil will present problems in digging and a on its resistivity. Figure 22.6 illustrates the effects of
pipe, rod or strip grounding would be easier. Similarly, such factors on the resistivity of soil. While the temperature
dry soil will require deeper digging, where a pipe, of the soil is a fixed parameter, at a particular location of
rod or strip grounding would be a better choice. the grounding station the soil can be artificially treated
to improve the content of moisture and chemical com-
position, to achieve a lower value of soil resistivity. It
22.2 Resistivity of soil (p) has been found that the resistivity of soil can be reduced
by 15-90% by a chemical treatment with the following
This will depend upon the type and quality of soil and its salts
chemical composition, i.e. the composition of salts and
minerals, content of moisture and normal rainfall during Normal salt (NaCI) and a ~~~~~~~~l and most
the year. Table 22.1 obtained from BS 7430 and Table mixture of salt and soft coke commonly used salts
22.2 from IEEE 80 show the likely resistivity of different Magnesium sulphate (MgS04)
types of soils in ohm-metres. These are only likely values Comer subhate (CuSOA)
for a general reference. It is recommended that where a Caichm chloride'(CaC1;) More common salts
grounding station is to be installed, the soil is tested at Sodium carbonate (Na2C0,)