Chapter 8

Precursory Phenomena and Dielectric Breakdown of Solids 1

8.1. Introduction

The physical stresses to which dielectric materials are subjected do not, a priori, contribute to the early ageing of these materials or to their failure, and it is the fundamental reason for the undertaking of a test which guides the choice of test made. Experiments show that the joint action of physical or physico-chemical parameters leads to an unexpected evolution and breakdown of dielectrics used as electrical insulating materials (as we shall subsequently call them).

The evolution of the materials is related to the growth of stresses, such as temperature (which can have a positive as well as negative effect), high voltage, electrical field, environment (whether on earth or in space, where humidity and pollution in a broad sense can play a part), and radiation across the whole range of frequencies. New terms such as very high voltage and reliability appear and, because of these requirements, the use of natural materials such as wood, natural rubber, paper, etc, is significantly reduced.

Please note, however, that the use of paper-oil (PO) remains, particularly in the insulation of direct current high voltage cables. Due to its physical and chemical properties and thermal stability up to 700°C, muscovite mica is always used for high voltage systems and rotating machines.

Obviously, chemistry has always played a large part in this evolution and as an example, cables (very well described by ...

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