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 electrolytes, which consist of moisture, minerals and dissolved salts. A dry soil has high resistivity if it contains no soluble salts.
Factors Affecting Soil Resistivity
Soil resistivity directly relates to moisture content and temperature, it is reasonable to assume that the resistance of any grounding system will vary throughout the different seasons of the year. Since both temperature and moisture content becomes more stable at greater distance below the surface of the earth, it follows that the grounding system, to be most effective at all times, should be constructed with the ground rod driven down a considerable distance below the surface of the earth. Best results are obtained if the ground rod reaches the water table.
Seasonal variation of earth resistance with an electrode of 3/4 – inch pipe In rather stony clay soil. Depth of electrode in earth is 3 ft for curve 1 and 10 ft for curve 2.