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 V₅₀ 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 V₅₀ vs. d characteristics are classified into two regions, C´ and A. The creeping distance in the C´ region is d=0.4-d_c cm and in the A region d=d_c-10 cm, where the boundary distance d_c depends on the thickness of the acrylic plate τ. In the C´ region the V₅₀ increases at the rate of ~18.4 kV/cm independently of τ, and the positive leader leads to flashover as in air gaps, while in the A region the V₅₀ 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