Influence of the solid material nature on the inception of creeping discharges in air

An experimental study of discharges in air at atmospheric pressure under impulse voltage, creeping on insulating solid surface is presented. The objective is to get a better understanding of creeping discharge by studying streamer initiation and propagation, and by looking at the influence of the nature of the insulating solid. Several solid materials, of different chemical natures and permittivity are investigated. Pre-breakdown streamers are investigated by high speed visualization and transient current measurements. In the presence of insulating solids such as epoxy resin, PPA/glass fibers and polycarbonate, surface streamers appear during the voltage rise, and the time delay to appearance is correspondingly rather short. With two materials of polyolefin and fluorine modified polyolefin families, characterized by lower permittivity, the first creeping streamer appears at the maximum voltage after a much longer statistical time delay, very close to that obtained in air without solid.