For many years now, one of the variables most often considered in insulator selection and design has been the equivalent salt deposit density, better known by its acronym ESDD. This widely-accepted measure of soluble pollution deposition on the surface of an insulator has apparently allowed line designers worldwide to confidently specify creepage distance requirements for different levels of pollution, based on clearly-defined IEC standards. Or has it?

Today there seem to be growing examples that over-reliance on this parameter when specifying insulators may not give the results intended - quite often the reverse.

ESDD measurements are often not performed at the time when this variable actually peaks but rather at times when it is most convenient for maintenance personnel to visit test sites.

In the first place, ample evidence seems to exist that ESDD measurements are often not performed when this parameter actually peaks but rather at times when it is most convenient for maintenance personnel to visit test sites and take readings. In fact, researchers in Sweden claim that one of the major problems in accurately quantifying pollution environments is that in most instances there are either none or only a few measurements taken of ESDD in very severe service areas. Moreover, sometimes those measurements which are taken are performed at the wrong time in the sense that they may miss the actual peak levels attained before a storm has cleaned the insulator.

Similarly, there are examples where line design decisions, based on measured ESDD levels, have had to be substantially modified when actual service experience of the insulation selected did not coincide with performance expectations based on this parameter.

Finally, there is a body of opinion that, when it comes to composite insulators in general, ESDD measurements do not have all that much validity. Noted insulator researcher, Claude de Tourreil observes, “ESDD is not all that valuable for composite insulators because the techniques for measuring it are not applicable and because these insulators react differently to pollution than do ceramic insulators.”

In this issue are articles which touch directly or indirectly upon ESDD and how some utilities have responded to the fact that lines designed with this parameter in mind have nevertheless experienced faults associated with inadequate insulation. These articles also discuss how these utilities have responded, either by better defining their pollution definition criteria or by utilizing different insulator designs.

Marvin L. Zimmerman