Breakdown between water drops on wet polymer surfaces

This paper describes the results of experiments made to examine the nature of discharge phenomena between water drops on polymer surfaces. Small samples of high temperature vulcanised silicone rubber were subjected to electric stress and exposed to clean and salt fog. The resulting surface activity was observed with a high-speed video camera. Water drops on the surface were found to vibrate in a variety of ways and were pushed away from the high electric stress regions to low stress regions. The distribution of the water drops on the surface of the samples appeared to follow equipotential lines between the electrodes. Water drops on the surface grew larger by absorbing droplets from the fog or by coalescing together. Depending on the size of water drops, the process of water drop coalescence took up to several cycles of the 50 Hz supply voltage. During this period the shape of water drops was observed to change to shapes more likely to intensify the electric field around them and thus initiate discharges at the solid/liquid/gas interface. When several water drops simultaneously coalesce, they can bridge substantial lengths of insulation and there may be an increased risk of flash-over. The conductivity of the fog is shown to have a marked effect on the mode of discharge development. Salt crystals deposited on the surface by salt fogs greatly increase the conductivity of water drops during drying and wetting of the polymer surface.