Over time, corrosive soils with high moisture content, high salt content, and high temperatures can degrade ground rods and their connections. So although the ground system when initially installed had low earth ground resistance values, the resistance of the grounding system can increase if the ground rods are eaten away. That is why it is highly recommended that all grounds and ground connections are checked at least annually as a part of your normal Predictive Maintenance plan. During these periodic checks, if an increase in resistance of more than 20 % is measured, the source of the problem should be investigate , and make the correction to lower the resistance, by replacing or adding ground rods to the ground system.
IS: 3043 – 1987 (Reaffirmed 2001). Indian Standard CODE OF PRACTICE FOR EARTHING. (First Revision).Second Reprint February 1998. UDC 621.316.99.006.76.
SECTION 9: Miscellaneous installation and Consideration
34. Maintenance of Earth Electrode
34.1 It is recommended that periodic check tests of all earth electrodes shall be carried out. Records should be maintained of such checks.
34.2 Where earth-leakage circuit breaker is employed, a check shall be kept on the associated earth-electrode by periodically operating the testing device which is embodied in the earthed leakage circuit-breaker.
34.3 The neighboring soil to the earth electrode shall be kept moist, where necessary, by periodically pouring water through a pipe where fitted along with it or by pouring water in the immediate vicinity of the earth electrode.
34.4 Substations and generating stations
34.4.1 Record shall be kept of the initial resistance of substations and generating station earth electrode and of subsequent tests carried out.
34.4.3 Periodic visual inspection of all earth electrode connection wherever available, shall be carried out to ensure their rigidity and other signs of deterioration.
National Electrical Code 2011 (First Revision). SP 30:2011. Bureau of Indian Standards.
SECTION 14: EARTHING
4.1.5 The electrode shall be kept free from paint enamel and grease.
IEEE Std 80-2000 (Revision of IEEE Std 80-1986). IEEE Guide for Safety in AC Substation Grounding.
SECTION 14: Evaluation of ground resistance
14.5 Soil Treatment to Lower Resistivity
It is often impossible to achieve the desired reduction in ground resistance by adding more grid conductors or ground rods. An alternate solution is to effectively increase the diameter of the electrode by modifying the soil surrounding the electrode. The inner shell of soil closest to the electrode normally comprises the bulk of the electrode ground resistance to remote earth. This phenomenon is often utilizes to an advantage, as follows.
- Use of sodium chloride, magnesium and copper sulfates or calcium chloride to increase the conductivity of the soil immediately surrounding an electrode. State or federal authorities may not permit using this method because of possible leaching to surrounding area. Further the salt treatment must be renewed periodically.
34.4.2 Normally annual measurement of earth resistance of substation shall be carried out but local circumstances in the light of experience may justify increase or decrease in this interval but it should not be less than once in two years.