Overhead Line Insulators

Abstract

IN a paper read to the Students' Section of the Institution of Electrical Engineers on November 30, Mr. C. H. W. Clark discussed the design, manufacture and testing of overhead line insulators. It is generally considered that 500,000 volts is the most economical pressure at which to transmit large quantities of electrical energy over long distances. The best material for insulating these lines has been found to be porcelain, as its insulating qualities remain practically the same when exposed to all weather conditions. It has low tensile strength but considerable compressive strength, and so most types of insulator are designed to utilize the porcelain in compression. Electrical failure follows a puncture through the porcelain or by ‘flash-over’ round its surface, which produces an arc short-circuiting the line. As puncture destroys the insulator, it is more serious than flash-over. Insulators are designed with a puncture voltage of about twelve times and a flash-over voltage of about six times the working voltage. Failures occurring in practice are usually due to lightning or to deposits of soot or sea salt on the insulator surface. Lightning affects the design of the transmission line rather than that of the insulators. Often no permanent damage is done by lightning flash-over. The problem of deposits on the surface of the insulators is a serious one and has not yet been completely solved, although many suggestions have been made for improving the standard types. For use near the sea, anti-deposit insulators have long, recessed, protected surfaces. For industrial areas, types with open exposed surfaces which can be cleaned by wind and rain have proved the best. For testing purposes, a percentage of the finished insulators are selected at random and tested for flash-over voltage both dry and in rain (produced artificially by a watering pot), impulse flash-” over voltage, mechanical strength and electrical puncture.