Importance of preventing moisture ingress to polymeric insulators

Water ingress between the glass-fibre core and the coating material is considered to be the major cause of failure of polymeric insulators in service. The various failure mechanisms which follow from this trapped water are described briefly. A water-immersion/leakage-current test, which evaluates the rate of ingress of moisture to glass-fibre insulators, is described; these laboratory results are then compared with accelerated life tests and service experience on the range of glass-fibre insulators used on British Railways´ 25 kV overhead line electrification. The water immersion test is shown to predict both the failure time and mechanism accurately.