Electrical equipment must be connected to earth with low resistance. Regulatory agencies specify the maximum acceptable value for ground resistance, usually 5 ohms. Soil strength affects resistance, which is measured with an earth resistance meter using different methods including stakeless measurement.
As a safety measure, all electrical equipment must be connected to a direct path to earth, and this path is usually provided by a conductive rod driven 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’s always a small amount. Regulatory 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 the telecommunications industries both specify 5 ohms as the maximum value.
The strength of the soil depends 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 surrounding earth body. Ground bars are usually made of highly conductive material, so their resistivity is very low and contact resistance is often negligible, provided there are no contaminants such as paint or grease present. The soil surrounding the rod typically provides the highest resistance, but the resistivity of the soil decreases with increasing distance from the rod.
Before taking ground resistance measurements with an earth stake, soil or soil resistivity testing is usually done with a set of four equally spaced stakes. A current is applied across the outermost pair of rods and the voltage drop across the inner pair is measured with an earth resistance meter. The meter is calibrated to automatically convert the voltage reading into resistance, measured in ohms. An earth resistance meter also automatically selects the test frequency with the least amount of noise to compensate for surrounding earth currents and their harmonics.
There are three methods of conducting an earth resistance test around an existing earth rod. In the first, called potential drop measurement, a current is applied between the earth rod, which has been disconnected from its connection, and a rod, and the voltage drop is measured in correspondence with a second rod placed halfway between they. The second method, called selective metering, is similar except that a locking ring is applied to the rod so it doesn’t have to be disconnected. In the third method, called stakeless measurement, the earth resistance is determined with two clamps placed around the earth rod or connecting wire. It is the simplest to drive because it does not involve driving stakes, but only works in rigs that have more than one ground stake.
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