Resistance of earth is the resistance between infinite earth and earth electrode. This depends upon mainly three factors
- The resistance of the electrode itself,
- The contact resistance between electrode surface and soil,
- The resistivity of soil between the electrode and infinite earth.
The first two factors can be taken as negligible compared to third factor, i.e. resistivity of soil. This is the reason, we generally consider resistivity of the soil only, when we deal with resistance of earth.
Again the resistivity of the soil depends upon the following factors,
- The electrical conductivity, offered by a soil mainly due to electrolysis. That means the resistivity (or conductivity) of a soil is mainly electrolytic in nature. So, concentration of water, salt and other chemical components in the soil largely determines its resistivity.
- The chemical composition of the soil is also responsible for its resistivity.
- The grain size, uniformity of grain distribution and packing of grains in the soil (i.e. inter grain distance) also the factors for resistivity. Since, this factors control the moisture holding capacity of the soil.
- The temperature of the soil can also be a factor but when it is very close to freezing temperature. Below, 0oC, the water contained in the soil begins to freeze, which largely affects the electrolysis process in the soil. It is found that, just below the freezing point the resistivity of soil or earth resistivity is tremendously increased.
So moist soil, which also can hold the moisture throughout the year is preferable for installing earthing system. Wet marshy land, loamy soil land, arable land, clayey land or land with loam mixed with small quantities of sand, gravel and stones are best choice for the purpose. Land with entirely dry sand, gravel chalk, limestone, granite, any very stony ground should be avoided.
It is normal practice to insert an earth electrode well below the ground level, to get a stable moist soil in depth. In that case, the earth resistance of the electrode remains unaltered even the upper portion of the ground is dried out. Sometimes it is found that the earth resistance does not improve enough even the electrodes are buried well enough inside the soil. This is because the soil resistivity is very high at those locations. This soil resistivity can be improved locally by filling the surrounding of electrode by normal salt or by any other similar substance.
There are some highly soluble substances which when dissolve in water, produce highly conductive solution by electrolysis. Common salt (Nacl) is mostly used for the purpose. When the surrounding of the electrode in the soil is filled by common salt, the earth resistivity of the electrode that is the resistance between electrode and infinite earth becomes significantly low. Sodium carbonate (Na2CO3) and copper sulphate (CuS04) salt can also be used for the purpose. Sometime the salt is applied with adequate quantity of charcoal, because charcoal like substances can effectively hold moisture to keep the surrounding wet.
Simple Treatment of Soil for Implanting an Electrode in the Ground
It is found that 90% of resistance between driving rod (electrode) and earth lies within 2 meter radius from the rod. On basis of that a simple treatment of soil is done during implanting an electrode in the ground. One cylindrical basin of 1 meter radius and 30 cm depth is excavated around the driving rod. Salt is applied in that basin. Then the basin is filled with water several times and allowed the water to be soaked by the soil each time. In this way a cylindrical space around the entire length of the driving rod becomes wet with salt water. This significantly improves the earth resistance of the rod (electrode). Generally, this improved conductivity of soil lasts for many years. But till it is recommended to measure the earth resistivity of the electrode yearly or half-yearly and take necessary action towards re-treatment if the resistivity is increased above its desired value.