The resistivity can be obtained with the use of the readings from an earth megger where:
The depth of burial of the electrode in the ground d is assumed to be negligible compared to the spacing between electrodes. In this case:
The less the resistivity of the soil the better it is for earthing. The factors that determine this value are:
Type of soil: Alluvial and clay soils record less resistivity compared to loam and sandy soils. Soils with high amounts of granite and igneous rocks register the highest resistivity to current flow.
Composition of the soil: Some soils have a high electrolyte concentration, reducing resistivity. The electrolytes come from salts present in the soil such as Sodium Chloride, Calcium Chloride, Copper Sulphate etc.
Moisture content: Water increases the conductivity of the soil hence reducing resistivity. Soils in wet climate conditions register lower resistivity than soils in dry conditions.
Temperature: A decrease in soil temperature especially around the freezing point of water increases resistivity considerably. Temperature changes above 200C have negligible effect on the earth resistivity.
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