One method to estimate the remote earth impedance or resistance of an object such as a grounding system is to complete a Fall-of-Potential test. This requires injection of a signal on the object under test and then profiling the resulting voltage difference between a portable reference and the object as the reference is moved away from the object in a straight-line. The resulting voltage profile is plotted and analyzed using various criteria to establish an equivalent “remote earth” voltage. This voltage divided by the signal current gives the ground impedance of the object.
Typical set up and analysis of a Fall-of-Potential test is described in IEEE Standard 80-2000.