Outdoor testing of nonceramic insulators

Nonceramic insulators (NCIs) are varied in design, but all essentially consist of three components: a resin bonded glass-fibre rod or tube; covered by a polymer weather shed; and metal end-fittings. Composite (nonceramic) long-rod insulators have been installed on overhead lines as replacements for conventional porcelain or glass cap-and-pin insulators for about twenty years. Unfortunately, problems associated with the early designs had cast doubts on the long-term electrical and mechanical performance of this type of insulator. Hence, composite insulators require two acceptance criteria, (i) suitable electrical properties, including insulation capability, and resistance to tracking or erosion during surface electrical discharges, and (ii) reliable long-term mechanical performance. Outdoor testing of NCIs is required to establish appropriate acceptance criteria, and guidelines for their application. The author discusses the electrical and mechanical properties required by NCIs and then discusses type approval tests being undertaken at the Dungeness Insulator Testing Station