Electrical Breakdown Characteristics of Soil

The pulsed electrical breakdown characteristics of soil between a hemispherical and a planar electrode have been measured in meter-size geometries for three soil types with water contents from 1 to 7% by volume. The threshold electric field for breakdown is a function of the soil type and the water content, varying from about 1 to 2 MV/m locally near the hemispherical electrode. The delay time (tD) between application of the voltage and the occurrence of the breakdown is primarily a function of the applied voltage, which varies approximately as tD-3/8 for these samples. After breakdown, the impedance of the soil decreases with time until it reaches a minimum impedance of 0.2 to 5 ¿/m for these meter-size samples, depending on the current that the pulser can drive through the arc. The discharge arc prefers to travel underground rather than on the surface of the soil unless there is some low-impedance object on the soil surface.