On the physical properties of discharges in sand under lightning impulses

Abstract

Experimental results of discharges in sand and air are presented. A coaxial cylindrical test cell was used. Three different types of sand grains were tested. No formation of large space charge in sand was detected before the breakdown. The breakdown voltages for sands with large resistivities were observed to be close to those in air. However, an exception was found in the fine sand (type C) in which the breakdown took place at voltage levels below the corona thresholds in air. Another peculiar property was also found in Sand C. The critical breakdown voltage was always lower for the negative impulse than for the positive. For high resistivities it was observed that the smaller the size of the particle the lower the breakdown voltage, but at low resistivities this fact was not clear. At lower resistivities of sand the breakdown voltage for all the sand samples approached a common value which was lower than the breakdown voltage in air. The arc resistances in sand were consistently larger than in air. The smaller the size of the sand particle the larger the arc resistance. No clear correlation was found between the arc resistance and the water content of sand (i.e. the sand resistivity).