One of the most important parts of electrical maintenance is testing the quality of the grounding system. Ground electrodes provide a safe path to earth for dissipating lightning strikes, faulty currents, EMF/RFI signals and static charges. However, as is the case with insulation resistance, ground resistance can also deteriorate as a result of catastrophic events or environmental conditions. Or, the needs of the installed ground system may change with the expansion of the facility.
There are many risks that can arise from the deteriorating of grounding systems, including the risk of equipment damage, electrical shocks, disrupting in the performance of sensitive, equipment, building up of heat and eventually fire. With that said, frequently performing ground resistance tests using a clamp on earth ground resistance tester to ensure the grounding system is at optimum levels is a very important task of electrical maintenance.
The reason why a clamp on earth ground resistance tester is preferred over the fall of potential method of testing is that it’s a time-saving and effective method where you don’t need to place probes in the ground or disconnect the grounding system to make measurements. This method is based on Ohm’s Law, where resistance equals voltage divided by current. The clamp utilises a transmit coil that applies voltage and a receive coil, which then measures current. In other words, the instrument applies voltage to a circuit, calculates the resistance and measures the resulting current flow.
As briefly aforementioned, the biggest advantage of this ground resistance testing method is that it’s easy and quick. You don’t have to disconnect the ground electrode from the system in order to perform the test, and you don’t need to connect any cables and drive any probes into the ground. Further, you can test bonding and connecting resistance as well, and in case you weren’t aware, great grounding needs to be followed up by bonding, which basically represents having a continuous low-impedance way to the ground.
This method requires a full electrical circuit to measure, simply because there aren’t any probes to set up in the desired circuit. The operator performing the test needs to make sure that the earth is included in the return loop, as the clamp tester measures the entire resistance of the loop the signal is taking, and all elements of the loop are measured in series. The clamp on method assumes that only the resistance of the ground electrode that’s put under the test contributes most. In other words, the more returns and the smaller the contribution of external elements to the reading – the greater the accuracy of the test.