Dr. C. de Tourreil, Contributing Editor

Looking at insulators used by ONE with Jacky Aznar of COMADEL.

When designing a system which has to operate in often difficult conditions for up to 40 years and with minimum maintenance, selection of the various components is of critical importance. Indeed, for the system to last such a long time means that each component of the system has to operate reliably for at least that same period. A transmission or distribution electrical network is exactly such a system.

In spite of its apparent simplicity, it is made up of several components some of them highly complex: towers with their various designs and footings adapted to specific terrain; metallic conductors and their accessories; and insulators with their dual mechanical and electrical functions.

Good analytical methods and sophisticated computer programs now exist to achieve the mechanical design of most components for overhead lines, including the insulators. Similarly, they allow the mechanical and thermal behaviour of the conductor and its related hardware to be determined. Laboratory tests then can validate these calculations.

However when it comes to the design of the insulator itself, evaluation of electrical performance under the applicable service conditions is based heavily on the experience of the manufacturer as well as a battery of special tests which can be performed. For conventional line insulators made of porcelain or toughened glass, these laboratory tests have been developed long ago and now permit predictions of both the short-term and long-term performance over a wide range of service conditions.

Early in the development of composite insulators, these same laboratory tests were used to evaluate their performance. However, it was quickly found that they were not sufficient, especially for predicting long-term performance. This was because composite insulators are made from materials whose performance changes both with time and the various stresses to be withstood in service.