The performance of insulators used on overhead lines and overhead distribution lines and in outdoor substations is a key factor in determining the reliability of power delivery systems. The insulators not only must withstand normal operating voltage, but also must withstand overvoltages that may cause disturbances, flashovers and line outages. The reduction in the performance of outdoor insulators occurs mainly by the pollution of the insulating surfaces from air-borne deposits that may form a conducting or partially conducting surface layer when wet.