Two methods are typically used to measure or test ground resistance—Fall of Potential and Clamp-On. The Fall of Potential test involves imposing a current flow between a remote current probe (Z) driven into the ground and the grounding electrode (X) being tested. A voltage probe (Y) is driven in the ground at varying distances from the grounding electrode on a straight line between the grounding electrode and current probe.
Using Ohm's Law (V=IR), you can calculate resistance (R) by multiplying voltage (V) and current (I). Before starting the test, electrically isolate the grounding electrode by disconnecting the ground conductor from the circuit. The current probe should be separated from the electrode and far enough away from it to avoid overlapping the effective resistance area. The area of interest is the resistance measured within 62 percent of the distance between the electrode and current probe.
Clamp-on grounding test method Electrically isolating the grounding electrode is risky, so the Clamp-On method, also called the Induced Frequency test, was developed as an alternative. The Clamp-On device is a split coil meter with two internal transformers. The jaws of the meter are clamped around the grounding conductor at the appropriate location. The meter then induces a high frequency level of voltage, causing current flow in the conductor. The other meter transformer measures the current flow. Resistance is calculated using Ohm's Law.
The Clamp-On method is considered simpler, faster, cheaper and safer than the Fall of Potential test. It is faster because no disconnecting of the rod from service is required and there is no need for auxiliary ground electrodes. Both methods require trained personnel, but the Fall of Potential test generally requires more interpretation by the individual making the test.