Breakdown in air along insulating solid surfaces of different natures, parallel or perpendicular to the field direction

A study of creeping discharges in air under impulse voltage is presented. A large variety of insulating solids, either dry of wet, are investigated with two field geometries, parallel or perpendicular to the solid surface. Pre-breakdown streamers are investigated by high speed visualization and transient current measurements. Breakdown is primarily affected by the solid permittivity, whereas solid chemical nature and presence of water have a negligible influence. The influence of permittivity is larger in perpendicular field configuration, and very weak in parallel. In the latter case, the initiation and propagation of streamers are favored along the solid surface. However, streamers responsible for the transition to breakdown propagate above the surface. This provides an explanation for the small influence of the solid nature on breakdown voltage in parallel field.