Flashover characteristics and surface processes under negative impulse voltage in atmospheric air

Breakdown phenomena on surfaces of acrylic plates, 1 to 10 mm thick, were investigated under negative impulse voltage in atmospheric air. The 50% flashover voltage V/sub 50/ and the 50% corona inception voltage were measured, and the discharge phenomena were observed with an image converter camera, in the creeping distance d=0.4 to 10 cm. The V/sub 50/ vs. d characteristics are classified into two regions, C' and A. The creeping distance in the C' region is d=0.4-d/sub c/ cm and in the A region d=d/sub c/-10 cm, where the boundary distance d/sub c/ depends on the thickness of the acrylic plate tau . In the C' region the V/sub 50/ increases at the rate of approximately 18.4 kV/cm independently of tau , and the positive leader leads to flashover as in air gaps, while in the A region the V/sub 50/ increases at the rate of 1 to 4 kV/cm and the negative leader plays an important part in the flashover. These flashover processes are illustrated with typical photographs, the conceptual figures constructed on the basis of photographic observations, and flashover characteristics are explained.>