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.
From the application note above it becomes apparent that the accepted practice for the Fall of Potential method is to run test leads away from the ground to be tested to be well outside the “sphere of influence” of the building ground grid, so testing electrodes must be placed at long distances away from the ground rod being tested. Ideally 10x the distance of either the diagonal dimension of the building, or 10x the length of the ground rod, which can be impossible to achieve in high density building environments. This can force the application of the clamp-on method as the only practical solution when the 3P Fall of Potential method is impracticable.
A good strategy to follow wherever possible when measuring a distributed ground system is:
1) Disconnect the ground rods from the building grounding system and perform the Fall-of Potential method.
2) Then reconnect ground rods to the building grounding electrode system, and use the clamp-on method to compare. The results should be close - within a few ohms. These measurements can then be used in the following months and years to verify that the grounding has not changed.
Not sure what approach to take? Then please talk to us before you buy or rent, call: 408 988 1346
MRU-30 Earth Ground Resistance and Resistivity Meter for Rent. $ 320 / month. $250 / 2 weeks (local pick up). Full kit includes transmit & receive current clamps.
MRU-200-GPS Earth Ground Resistance and Resistivity Meter for Rent. $ 399 / month. $250 / 2 weeks (local pick up). Full kit includes includes transmit & receive current clamps.