6.1 Introduction

The product developments that have taken place since John Looms published, in 1988, his excellent book [1] that covered much of this subject have mainly concerned polymeric materials-especially silicone rubber. Although the principal modifications made to porcelain and glass insulators over this period are those required for high voltage DC applications, much more has -nonetheless -been learned about the performance of such insulators under AC energisation. The discussed proposals of some years ago for ultra high voltage transmission systems resulted in the production of very high mechanical strength insulators of the cap and pin design. However, as such proposals generally failed to come to fruition, at least one insulator manufacturer now has a product that is ready, should a market become available in the future.

The design, the specification for a certain usage and the dimensioning of the insulator for that usage in an outdoor environment are usually dominated by the need to take account of wetted pollution on its surface. Often, the consequential problem is that of flashover across this polluted surface when the leakage current exceeds a certain value. However, some potentially dangerous defects in the structure of such insulators have been known to occur at values of leakage current well below that of the critical flashover limit. The associated mechanisms are quite involved and, although studied in some detail already, are still not fully understood.

Essentially, the troublesome pollution comes in two forms. The soluble components -e.g. salts from the sea and industrial...