Earth Ground Resistance Testing for Low-Voltage Power Systems

A quality connection to earth through the grounding electrode system for a commercial or industrial facility's power system is necessary for: 1) providing a low impedance path for lightning stroke current dissipation, 2) the reduction of "Step" and "Touch" potentials under line-to-earth fault conditions, and 3) the dissipation of electrostatic charges. Previously accepted test methods for resistance measurements of a grounding electrode system may indicate incorrectly that high or low resistance connections have been established. Testing of several grounding electrode systems was conducted by the author. This paper will present results of these tests to illustrate deficiencies in grounding electrode resistance test methods that have been accepted previously. Inaccuracies of various test instrument principles will also be discussed. The author found that close attention to the test procedure must be given to ensure that an accurate measurement will be achieved. This paper will provide recommendations on measure- ment techniques to minimize errors and ensure valid test results.