Earthing is a process of giving a dedicated path for the flow of high current during an abnormal condition to the ground by using the earthing electrode. In order to achieve efficiency, the earthing material should have a high current carrying capacity and
As mentioned by IS3043, the resistance offered by an earth electrode is electrolytic in nature and it depends on,
1.Resistance of the (metal) electrode
2.Contact resistance between the electrode and the soil
3.Resistance of the soil.
The first two factors can be neglected as the values are generally very small fractions of ohm.
Corrosion mainly depends on the properties of soil (i.e) resistivity of the soil. If the resistivity of soil is low, corrosion rate will be high.
IS3043:2018 has given the correlation between resistivity of soil and it’s corrosion under
clause 13.6.1 and the details are as follows.
1. To avoid galvanic corrosion, the metals should be coated with a corrosion- resistant coating (E.g., Zinc on mild steel, Tin on copper or we can use bi-metallic strips.
2. IS3043 states that Steel corrodes about six times as fast as copper when placed in the soil. Hence steel electrodes should have higher cross sectional area than that of copper electrodes. For Example, (600x600)mm copper plate should have 3.15mm thickness, whereas thickness of (600x600)mm steel or GI plate should be 6.3mm.
3. The use of coke breeze as infill is not recommended as it may result in rapid corrosion.
4. For cathodic protected structure, the copper should be avoided as an earthing electrode. If we use copper, then it will be faded-off because of its ability to become cathodic to the protected structure.
IS3043:2018 under clause 13.6.1 has provided details about the corrosion allowance in cross sectional area of earthing electrode for different soil conditions.
* For soils having resistivity of (25 to 100)Ω.m, corrosion allowance of 15% on cross
sectional area shall be considered.
* As the corrosion rate and resistivity are inversely proportional, the corrosion allowance would be 30 percent if the resistivity of soil is less than 25Ω.m.