Nanosecond streak photography of discharges on spacer surface in gases

Results of an investigation to clarify the process of surface flashover in gases are reported. Two hemispherical capped rods of 12 mm diameter, separated by a half-cylindrical spacer of 50 mm diameter, formed the samples. The spacer materials were polytetrafluoroethylene, acrylic and nylon. High-speed streak photographs of impulse flashover indicate that the presence of a spacer not only lowers the impulse breakdown voltage but also reduces the breakdown time. The discharge on spacers may be initiated anywhere between the electrodes and develops rapidly into a flashover in ~5 ns for an 18 mm gap