Protective devices such as spark gaps and surge arresters are used to divert surges to earth. It is, therefore, necessary that earthing systems are designed with a low-magnitude earth impedance value so that the overvoltage protection devices can divert high fault currents effectively to earth for all types of fault. Power frequency characterization of earthing systems is extensively researched and fairly well understood. However, the behavior of such systems under transient conditions is still not fully clarified. This paper addresses some of these shortfalls. Laboratory experiments were set up to study the characteristics of the ionization phenomenon in various soil conditions. The soil ionization characteristics were investigated using a proposed new concept of pre- and post-ionization resistances. A measurement technique was developed to establish more accurate estimates of ionization electric field threshold, and an equivalent circuit representation is proposed.

In a previous paper , 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

This paper investigates the factors affecting the impulse characteristics of purposely built grounding systems. We installed 2 m × 2 m grounding systems in four different sites with different soil resistivity values. The effect of impulse polarity on soil characteristics was also investigated for different soil resistivity. A circular ring electrode was used as a return electrode in all four sites. For one of the sites, different configurations of grounding systems were installed to allow the study of the effect of ground electrode configurations on soil characteristics. The aim of this study was to quantify the effects of soil resistivity, impulse polarity, and earth electrode configurations on soil electrical properties under high impulse conditions by field measurements. The new data could be useful in understanding the characteristics of grounding systems in various factors under high impulse conditions. It is hoped that by considering these factors, it can help optimize the design of earthing systems.