The influence of impurities on the dielectric strengths of bushings

Insulated bushings, housings and supports form an essential and critical part of most high-voltage equipment. They are subjected to high stresses, both mechanical and electrical, arising from internal, design-related conditions and from the environment in which the equipment operates. Electrical failure of the porcelain insulation can precipitate subsequent mechanical failure, leading to complete destruction of the equipment. The recent explosion of a gas-filled current transformer porcelain during testing associated with an equipment-development programme at the CSIR (formerly, Council for Scientific and Industrial Research), has raised serious questions relating to the inherent quality and quality control of such items. Subsequent testing of hollow porcelains of various designs and from several sources gave widely differing values of the material´s dielectric strength-even on units with the same origin. In this paper we briefly describe the adoption of improved routine testing procedures.