Onset or breakdown voltage reduction by electrode surface roughness in air and SF6

This paper describes investigations on the influence of surface roughness on the onset or breakdown voltage in air and SF6. Applying the streamer theorie of electrical discharges the role of ionization characteristic, gas pressure, shape and size of electrode surface protrusions is discussed. The maximum allowable field gradients of electrode surfaces with hemispherical, spherical, and filamentary irregularities were computed in air and SF6. Pictures of a sparked electrode surface, showing slze and shape of surface roughness, were made using an electron scanning microscope. A comparison between computed breakdown field gradients of rough electrodes and experimental results obtained by employing different electrode configurations in a wide pressure range confirmed the theoretical considerations.