Of all the issues influencing the selection of a particular insulator from among the various alternatives available, perhaps the most critical is life expectancy. No other parameter, it seems, so profoundly affects the economics and ultimate operating performance of an electrical installation. And, no variable has had so much emotion attached to it when it comes to the on-going industry debate about the merits of ceramic versus non-ceramic insulator technologies.

But, what really is the life of an insulator? Is it the potential number of years the insulator can be in-service under ideal conditions? Or, is it the average actual number of years it has been in-service before having to be replaced for whatever reason. The former definition is statistically irrelevant since it assumes a range of givens some of which may not prove true. For example, if an earthquake destroys a recently-installed porcelain insulator at a station, does this count in calculating average life expectancy? The latter definition is therefore clearly of greater significance to the insulator user.

Recently, someone in the industry offered the opinion that it has been well-documented that porcelain insulators can last for many decades. Similarly, it has been said that glass insulators should also have a life of similar duration. On the other hand, doubt about the expected lifetime of composite insulators due to premature ageing is perhaps the major factor which still limits their more widespread use.

It seems that perhaps a double standard is at work when making the comparison of life expectancies of ceramic and non-ceramic insulators. For example, in some environments excessive pin corrosion on a cap and pin type disc can result in a dropped line. Similarly, there are many instances every year of glass and porcelain insulators which have failed simply because a rock has struck and shattered them. Or what about when insulators are taken out-of-service and replaced with another type because the environmental conditions have changed in some way.

Yet, many people do not see these not so common occurrences as examples of a limitation in the expected service life of ceramic insulators when, in fact, they clearly are. Any event which leads to the sooner-than-expected end-of-life of an insulator is, in principle, an indication of failure. It is certainly not only premature ageing.

So, where does this leave users who are looking for the best possible choice of insulators? Hopefully, it should leave them better informed that their selection of insulators should take into account all service conditions which might tend to limit insulator lifetime. They should not rely solely on generalized notions about how long a given insulator material should be expected to perform, all things being equal.

Perhaps it might even be useful to re-define the common notion of insulator lifetime as being not how long an insulator has been proven to last under ideal conditions but rather how long it actually lasts under real-life conditions.