The Earthing Resistance of any electrode is made up of following factors:
• Resistance of the (metal) electrode,
• Contact resistance range between the electrode and the soil
• Resistance of the soil from the electrode surface outward in the geometry (which is set up for the flow of current outward from the electrode) to infinite earth.
Among these three, Soil Resistivity plays a vital role in performance of Earthing
The Resistance to earth of a particular electrode depends upon the electrical resistivity of the soil where it is installed. Soil resistivity has a direct effect on the resistivity of the earthing system and it is an indication of a given soil’s ability to carry electric current to flow.
Earth conductivity depends on some of the contributory factors on given below
✓ Electrolytic in nature,
✓ Moisture content of soil,
✓ Chemical composition,
✓ Concentration of soil dissolved in water,
✓ Grain size and distribution,
✓ Closeness of packaging.
Hint: Many of these factors may vary locally, occasionally and seasonally as well.
• The temperature coefficient of resistivity for soil will be negative, but is negligible for temperatures above freezing point.
• At about the temperature of 20°C, the resistivity changes about 9 percent for every degree Celsius.
• Below 0°C the water in the soil starts to freeze itself and generates an enormous increase in them temperature coefficient values.
• As the temperature becomes lower, the resistivity increases drastically. Therefore, we recommended the earth electrode should be well below the frost line.