The flashover processes of polluted insulators under both DC and AC voltages were observed with a high-speed camera. Physical and mathematical models to describe the DC and AC flashover are proposed. A formula expressing the resistance of the remainder pollution layer of a polluted insulator is derived. As a result the flashover criteria of a polluted insulator can be calculated directly, and it is unnecessary to convert a real insulator of complex shape into an equivalent rectangle. Computer programs used to calculate the flashover criteria of polluted insulators under DC and AC voltages have been developed. Some arc phenomena, such as the arc-bridging between the sheds or ribs of insulators, the drift of the arc from the insulator surface, and the number of arcs in series before flashover, were considered in the computer programs. Experiments to determine the flashover voltages of some insulators with complex shapes were performed.
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