Performance of earthing systems of low resistivity soils

In a previous paper [1], the properties of sand with different water contents were investigated under high-magnitude fast-impulse currents, and an equivalent circuit model for soil ionisation was derived. For the hemispherical test cell used, resistance values in the kilo-ohm (Omega) range were obtained. These high values of resistance limited the current magnitudes to less than 300 A before breakdown occurred in the test cell. In practice, however, the resistance of an earth electrode is usually less than 10 Omega and the transient current magnitudes can reach several tens of kiloamperes. It is, therefore, necessary to investigate the behavior of soils under high current magnitudes and compare it with low ac and dc test results. In order,to achieve high impulse current magnitudes with the adopted test cell, low resistivity media were utilized as test soils. In this present paper, a new impulse test data using wet clay and sand mixed with controlled amounts of sodium chloride (NaCl) to obtain a range of low-resistivity materials, was analyzed. The characteristics of NaCl are also investigated under low-magnitude low-frequency and high-magnitude impulse currents in order to understand better its effect on sand-salt soil mixtures. Impulse currents up to 5 kA were used during the test program. These new data would be useful in understanding the characteristics of a low earth resistance value (which is a typical condition at field site) under high impulse conditions. By including the nonlinear effect of the soil under high currents, more accurate modelling of protective devices and their performance could be achieved. In addition, the consideration of the nonlinearity in soils can help the optimum design of earthing systems.