The influence of the diameter on contamination flashover/withstand voltage of cylindrical insulators was investigated both for AC and DC. The increase in diameter of a cylindrical insulator significantly lowers the contaminated flashover/withstand voltage. The increase in diameter also lowers the contaminant deposit density on the insulator surface under the same contamination atmosphere. The decrease in the contamination flashover/withstand voltage is more than that implied in an existing IEC publication. The quantitative assessment of the contamination characteristics presented improves the reliability in the contamination design of cylindrical insulators having different diameters. >