Earth Ground Resistance Testers for measuring ground resistance and resistivity. We'll help you choose.
The tester is clamped around a ground conductor, usually on the cable that is connected to a ground rod, and close to the ground rod. It uses two coils, one generates a voltage (an E-field) at a frequency typically in the range 2 – 3 kHz. The E field induces a current into the grounding system which is sensed by the second coil which measures the induced current. From Ohm’s law, the tester can calculate the impedance from the E and I values.
Be aware of the limitations in distributed ground systems. The clamp-on type tester is typically used to test the grounding plate (butt-plate) of power poles. The grounding system as a whole must be taken into account since the pole being tested, Rx, shares a ground “path” with other power poles in the neighborhood, represented by R1 to RN. Or in the case of a pylon, the other three legs plus other pylons in the neighborhood. The method has been applied to distributed ground systems, such as a buried ground grid for buildings; a good example is that of a data center whereby the grounding grid, usually a mesh of conductors, or a long conductive loop around the perimeter, is buried in the ground or in the foundation. A grounding electrode (ground rod) is connected to the grounding grid and it is at this connection point that the ground tester can be clamped to test ground resistance.
Note 1: This method requires that a path for current must exist – it should not be used to measure an isolated, free-standing, ground rod; i.e. if the ground rod is not connected by a cable to the grounding grid.
Note 2: The induced field is an alternating one so using the tester with long lead lengths may cause incorrect measurements because the self-inductance of the leads will interact with the alternating fields creating inductive reactance (e.g., higher ohmic values will occur).
Note 3: The tester should be placed around the ground rod and close to the soil.
The Fall-of-Potential method ideally should be used for individual ground rods and when distance and space permits. When this is impracticable use the clamp-on tester.
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