Field Tests & Measurements
Earth Resistance Measurements
A grounding system can be as simple as a single ground rod, a system of interconnected ground rods/plates or a complex ground grid comprising interconnected horizontal & vertical ground electrodes buried in the ground to establish electrical contact with the earth. Resistance of each of these electrode configurations is known as “Earth resistance” or “Ground resistance” or even ‘Ground impedance”. Field measurements are made:
· To verify the actual Earth resistance to remote earth of the installation
· To verify if the actual Earth resistance is close to the design value
· To determine ground potential rise within and outside the earth electrode area in cases of ground faults
Most common and accurate method for field measurement of Earth resistance is “Fall-of- Potential” or “Three Point” Method according to IEEE Std. 81 and IS: 3043 employing Standard Earth Testers. Experts in SPC are specialized in Fall-of-Potential Method of measurement, be it for large grounding systems like sub stations/generation plants or earth pits in HT/LT installations.
Fall-of-Potential method of Earth resistance measurement
Testing a grounding system to measure its resistance is important since the engineer responsible for designing the Earthing system must ensure that the potential rise in the Earthing system due to ground fault currents doesn’t create a safety risk to people or equipment coming in contact with (touch potential) or in the vicinity of (step potential) the grounding system. Knowledge of the correct value of earth resistance is also fundamental to the protection settings and operation of protective relays.
Field Test by itself may lead to erroneous values of earthing system resistance if proper care is not taken during measurements. A case in point is resistance measurement of large and complex grounding grids. Difficulties also arise in measurements at space-limited installations such as GIS substations. SPC with its experts undertakes field tests in such special earthing installations also by adopting mitigation techniques as specified in the Standards.