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As a safety measure, all electrical equipment must be connected to a direct path to the ground, and this path is usually supplied by a conducting rod pounded into the earth. An ideal system would have zero ground resistance, which is the amount of electrical resistance this path has to the flow of electricity. No system is ideal, however, so there is always a small amount. Regulating agencies have different standards regarding the maximum acceptable value for ground resistance to ensure equipment and operator safety, but the US National Electrical Code (NEC) and telecommunications industries both specify 5 ohms as a maximum value.
Ground resistance is dependent on three factors. They include the resistivity of the ground rod, the contact resistance between the rod and the surrounding earth, and the resistivity of the earth surrounding body of earth. Ground rods are usually made of highly conductive material so their resistivity is very low, and the contact resistance is often negligible, provided no contaminants like paint or grease are present. The ground surrounding the rod typically provides the highest resistance, but ground resistivity decreases with increasing distance from the rod.
Before measurements of ground resistance are conducted with a ground rod, soil or ground resistivity testing is usually conducted with a series of four evenly-spaced stakes. A current is applied across the outermost pair of stakes and the voltage drop across the inner pair is measured with a ground resistance meter. The meter is calibrated to automatically convert the voltage reading to resistance, measured in ohms. A ground resistance meter also automatically selects the testing frequency with the least amount of noise to compensate for surrounding ground currents and their harmonics.
There are three methods to conduct a ground resistance test around an existing ground rod. In the first, called fall-of-potential measurement, a current is applied between the ground rod, which has been disconnected from its connection, and a stake, and the voltage drop is measured at a second stake placed midway between them. The second method, called selective measurement, is similar except a clamping ring is applied to the rod so it doesn't have to be disconnected. In the third method, called stakeless measurement, ground resistance is determined with two clamps placed around the ground rod or the connecting wire. It is the easiest to conduct because it doesn't involve driving stakes, but it works only in systems that have more than one ground rod.