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.

▪ The resistance of the earthing electrode is depends upon the soil resistivity of particular location.

▪ Moisture content is one of the controlling factors in measuring soil resistivity.

▪ It is advisable to conduct earth resistivity tests during the dry season.

▪ As per standards, if moisture content in soil is below 20%, the resistivity increases very abruptly and if it is above 20% there will be little impact on soil resistivity.

▪ The normal moisture content of soils ranges from 10% (in dry season) to 35% (in wet seasons) and an approximate range may accounts to 16 to 18 percent.

❖ Pure water is bad conductor of electricity. Resistivity of soil depends on the resistivity of water which in fact depends on the amount and nature of salts dissolved in it. 

❖ The Resistivity of the soil will be variable from various ranges to 5% of salt content and a further increase in salt in soil gives no appreciable benefits in improving resistivity.

❖ Small quantity of salt with moisture content reduces soil resistivity in large value but it corrodes the earthing electrode which affects the performance of earthing.

▪ When current flows from a ground electrode into the surrounding soil, it is described as flowing through a series of concentric shells of corresponding increasing diameter.

▪ Each and every concentric shells having greater area to dissipate current into the soil, so resistance become less.

▪ When the current dissipation is in larger manner, the current density becomes smaller that the resistance will be very less.

▪ But in high resistive area, the above condition is reverse which leads to higher touch and step potential and also damage the nearby electrical appliance.