Getting Down to Earth 43times the maximum distance on the ground system (e.g, 3000 ft for a 300 ft2 ground grid) to find the “flat” portion of the characteristic resistance curve.2. The large “resistance area” typically gives ground resistance values of less than 0.5 Ω; test instrument resolution is critical if small variances in readings are to be observed; if the test instrument does not have suitable resolution, instrument errors can overwhelm the results.3. Large electrical networks contain noise consisting of the frequency of the power utility and its harmonics, plus high frequency noise from switching, etc., and induced signals from other sources; the ground tester must retrieve and analyze a small test signal in a much larger test environment; most ground testers only inject a single frequency (usually 128 Hz) which is adequate in most situations because it avoids harmonics of standard line frequencies; unfortunately, it is often not adequate in substations; this type of interference can cause significant measurement errors.Addressing the Testing Challenges in Large Ground SystemsIn the ideal world, testing a large ground system would be conducted in complete accordance with the Fall-of Potential Method. Unfortunately, the large “resistance areas” found in large ground systems may make it unfeasible or even impossible to carry out this test. As noted above, setting the current test probe 10 times the maximum distance of the ground system can require leads to be many thousands of feet. In these situations, the Slope Method can be used effectively because it does not require the user to find the “flat” portion of the curve or to know the electrical center as a point from which to measure. Readings are taken at 20 percent, 40 percent and 60 percent of the current probe distance and fit into a mathematical model of the resistance characteristic. Appendix III provides a detailed explanation of the Slope Method, including relevant tables.
