Engineering and Consulting Services

Grounding System Testing

After a grounding system is designed and installed, the grounding system resistance should be measured to make sure it conforms to design values. Testing needs to be performed after the grounding system is completely installed but prior to connection to the utility neutral or any other system (cold water, building steel, etc.).

In cases where the measured resistance is too high, the cause can be poor installation or incorrect assumptions on soil resistivity during design. To avoid high resistance due to incorrect assumptions, soil resistivity measurements should be made and incorporated into a site specific design. This additional step will eliminate the need for a specification that reads "the contractor shall install the grounding system as specified. If the measured resistance is greater than 25 ohms, the contractor shall install additional ground rods until reaching 25 ohms or less, at no additional cost to the owner"!

Testing specifications typically call for the fall-of-potential or three point test method. This method requires the test engineer to be able to access "remote" earth; that is, earth outside the area of influence of the grounding system under test. Unfortunately, in heavily populated areas, access to remote earth can be difficult. In this case, an alternative test method is the direct or two point method. The test probes are shorted together and connected to a reference ground. The reference ground should be a ground system with a fairly low resistance (i.e., the utility system neutral). The resistance of the ground system under test will be measured relative to the reference ground, assuming the reference ground to be true earth. In a few instances like a downtown metro area, even the two point method may be impossible. In this particular case, there is no existing test equipment or test procedure that will accurately measure the grounding system resistance.

If the system is a complex electrical system that is already tied to the electrical utility system, tied to underground water, or is extremely large, then the best test method available is the EPRI Smart Ground Multimeter (SGM). The SGM was developed for use by electric utilities to measure the impedance of large electric utility substations that are connected to transmission and distribution systems. The SGM is a computer-based device that takes numerous measurements of voltages produced by current injected by the SGM, and from these measurements, complex computer algorithms calculate the ground impedance of the system under test. The SGM is relatively immune to noise from AC power systems because it injects current and measures voltages at random frequencies which allows the software to digitally filter out external noise.