Aging of porcelain suspension insulators under mechanical and electrical stress on EHV AC lines

A small number of failures of standard porcelain suspension insulators used on American Electric Power Company's (USA) 765 kV lines were found with the porcelain shed broken away from the cap and pin assembly, allowing a "donut" of porcelain to fall to the hardware below. The investigation of these failures was approached from many angles to determine the sequence of events leading to the failure. Additionally, it was desired to determine if this phenomena was the result of aging porcelain under electrical and/or mechanical stress.