A major difference between the specifications for traditional ceramic cap and pin suspension insulators and composite polymer insulators is the requirement of a routine mechanical and electrical (M&E) test for the former type. In other words, each porcelain cap and pin unit (or bell) undergoes an M&E test before leaving the factory. This not only helps in eliminating obviously defective units but also assures the customer (more specifically the installation crews) that the insulators will not immediately fail, either electrically or mechanically, upon installation. This is especially important when performing live line maintenance on the system.

By contrast, the requirements for composite polymer insulators are only that they be tested mechanically on a routine basis. Basically, this means that electrically defective insulators, which might fail immediately upon application of voltage, can remain undetected at the factory. Needless to say, this presents a safety concern to personnel. But, is it a serious issue?

Requiring that a routine electrical test be added to the specification for composite polymer insulators would create additional costs which would ultimately be passed on to users.

Let’s examine the reasons for the difference in these specifications. The physical dimensions of the porcelain cap and pin suspension unit facilitates the application of a fixed test voltage during manufacturing. For composite polymer insulators, where there are no intermediate metal parts, the length of the insulator can vary from only a few centimeters to five meters or more, depending on voltage class. It would therefore be extremely expensive, although certainly not impossible, to have a variable high voltage power supply to test these units.

This seemingly serious shortcoming in the specifications has apparently not caused too much concern to users. The service record of composite polymer insulators has, for the most part, been very good. Of course, one could require that a routine electrical test be added to their specification. But this would then create an additional cost to manufacturers, which no doubt would ultimately be passed on to users. Considering that live-line insulator replacement is a rare event, some users perform electrical tests on their own batches of such insulators earmarked for this change-out practice.

Would it be possible to visually exam the internal parts of a composite polymer insulator without taking it apart, say by X-rays? I am thinking of something similar to that used at airports for checking carry-on baggage. If something such as this could be developed, it might eliminate the need for routine electrical testing. This process could also help identify defective interfaces, seals, internal cracks, etc., which we know are responsible for the majority of failures of composite polymer insulators.

We may also need to look into the medical field for help. If there are machines, such as magnetic resonance imaging, which can look into the human body without surgery, it should similarly be possible to develop something for insulators. The real challenge will be to make it cost-effective and quick for implementing on the assembly line.

Dr. Ravi S. Gorur
ravi.gorur@asu.edu