| Summary: | Onset electric field of soil ionisation is one of the basic parameters to analyse the lightning impulse performance of a grounding electrode, which has been fully investigated based on experiments on hemispherical electrode or rod-to-rod gap. Until now, there has been lack of onset electric field of rod-to-plane gap, which is the real situation for grounding electrode. In this study, the soil ionisation around the grounding electrode is studied by experiment. Method to judge the start of ionisation is discussed. Both the effect of the soil resistivity and that of the radius of the grounding electrode on the onset electric field of soil ionisation are investigated. It shows that the onset electric field decreases as the radius of the electrode increases, while increases as the soil resistivity increases. A formula to predict the onset field with respect to soil resistivity and the radius of the grounding electrode is proposed.
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