42 www.megger.comSECTION IIIAccurately Measuring Earth Resistance for Large Ground SystemsLarge ground systems, such as those found in substations and power stations, are an important part of the protection of the electricity supply network. They ensure that fault current will enable protective devices to operate correctly. A substation must have a low ground resistance to reduce excessive voltages developing during a fault which could endanger safety of nearby people or damage equipment. When installing a ground system the resistivity of the surrounding soil should be measured. Inaccurate resistivity tests can lead to unnecessary costs in the design of the system.After installation it is vital to check that the electrical grounding system meets the design criteria and should be measured periodically to ensure corrosion or changes in the soil’s resistivity do not have an adverse effect. Ground networks may not appear faulty until a fault occurs and a dangerous situation arises. To obtain a sufficiently low value of ground resistance, ground systems may consist of an earth mat covering a large area or many interconnected rods. Suitable test techniques must be used for large systems to ensure that valid readings are obtained. This is unlike a small single ground rod (for example, a lightning protection system or residential ground) which can be simple to test.Testing Challenges in Large Ground SystemsSecuring valid measurements when testing large ground systems requires that proper techniques and instrumentation be used. The nature of substation and power station grounding systems and related conditions make testing far more complex than on a simple ground rod. Following are the three key challenges in testing substation ground systems:1. The physically large area of a substation/power station ground system results in a large “resistance area” and, consequently, long distances to the test probes; ideally, the current test probe should be placed 10
