Surface discharge processes of solid insulator under negative impulse voltage in atmospheric air

The breakdown phenomena on the surface of a acrylic plates, 1 to 10 mm thick, were investigated under negative impulse voltage in atmospheric air. The 50% flashover voltage V₅₀ was measured, and the discharge phenomena were observed with an image-converter camera, in the creeping distance d of 0.4 to 10 cm. V₅₀-d characteristics are classified into two regions, C´ and A. The creeping distance in the C´-region is d=0.4 cm-d_c and in the A-region it is d =d_c-10 cm, where their boundary distance d _c depends on the thickness of the acrylic plate. In the C´-region the positive leader leads to the flashover as in air gaps, while in the A-region the negative leader plays an important part in the flashover