Role of soil resistivity in Earthing

Earthing plays an important role in generation, transmission and distribution for the safe and proper operation of any electric power system. While soil resistivity directly affects the design of a grounding system, it is advised to take those measures that would help us obtain the lowest soil resistivity in order to achieve the most economical earthing installation.

Effect of moisture on soil resistivity:

Soil resistivity has a direct effect on the resistance of the grounding system. Moisture content is a significant factor in determining the resistivity of the soil. The soil resistivity remains low if the moisture content of the soil is high. However, above 20% moisture, the soil resistivity is affected to a very small extent. The drier the soil, the higher the resistivity. Also, the resistivity increases
predominatly at higher freezing levels. However moisture alone is not the only predominant factor in the low resistivity soils. Because ironically pure water is of high resistivity as there are no ions to conduct the electric charges.

Effect of temperature on soil resistivity:

The temperature of the soil also has an impact on soil resistivity. When the temperature of soil is high, the resistivity of that soil will be low which explains that soil has negative temperature coefficient of resistance with an exception for temperatures above the freezing point.

Solution to obtain desired soil resistivity:

Sometimes even when large number of electrodes are installed to get low earth resistance to earth, it may fail to produce a sufficient low resistance. In such cases, to reduce the soil resistivity, it is necessary to artificially treat the soil with substances that are highly conductive in its water solution. That would help in acting as a conducting electrolyte throughout the region surrounding the earth electrode thereby achieving a low resistance.

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