Flashover mechanism of polluted insulation

When a leakage current flows over the wet polluted surface of an insulator, high-resistance dry bands form. Discharges across these dry bands usually become extinguished but, exceptionally, may develop into a flashover of the insulator. The formation of dry bands and the subsequent growth of discharges on the polluted surface of a flat-strip insulator have been studied by scanning the voltage distribution along the strip at high speed. The behaviour of an arc rooted on a water surface has been investigated and the voltage gradients in the columns of arcs burning both in air and steam have been measured. Experiments show that when a water surface flashes over the arc burns in an atmosphere of steam, and that the condition for an arc to propagate over a resistive surface is that the voltage gradient on the surface exceeds that in the arc column. On this basis the flashover voltage of a water column is predictable to within 5%.