Discussing with staff from Kenitra regional office a section of 220 kV transmission line now equipped with composite insulators due to former heavy incidence of vandalism.

New specific tests have therefore had to be developed. Now, thanks to these tests, it is possible to separate good composite insulators from those which will not provide reliable service over a sufficiently long period of time. However, these new tests are still not able to discriminate between good, very good and excellent composite insulators, especially under severe or highly unusual service conditions.

An electric utility may therefore well decide to dispense entirely with carrying out these difficult and costly tests. Rather, they might choose to follow a slower road which calls for the evaluation of composite insulators through trial installations on their network. For some utilities, such pilot installations lines may be the preferred or even the only method ever considered.

For the ONE, the move to composite insulators has been done with caution, installing them gradually in sections of lines where pollution was severe and where vandalism was a problem.

This appears to be exactly what the state-owned utility of Morocco, the Office National de l’Electricité (ONE), has decided to do. Over the years, the ONE has faced two serious problems in the operation of its network: marine and/or industrial pollution; and stone-throwing vandalism causing a high incidence of breakage of glass insulators [see INMR Vol. 8, No. 6].

Satisfactory performance under most types and levels of pollution can be obtained by selecting an insulator with the proper shape and by choosing a sufficient specific leakage distance. This is true for both conventional and composite insulators. In addition, for composite insulators it is also important to consider the weathershed material.

The stone-throwing problem is more difficult to resolve. Since it is not feasible to eliminate it by removing the stones themselves (or of the stone throwers!), ONE thought that it may be possible to take away the incentive to throw stones. When a stone hits and breaks a glass insulator, there is an immediate noticeable event for the stone-thrower - i.e. the glass shatters.

Fortunately, unless other discs on the same string have already been broken, this event will not cause a flashover nor service interruption. Later, inspection by utility linesmen will detect the broken insulator and it can be quickly decided if replacement is urgent or not. By contrast, when a stone hits a composite insulator, it produces a non-event - i.e. the stone just bounces off the insulator usually without causing any damage to the weathershed, a result which can if necessary be verified in the laboratory. What is important is that the non-event nature of the stone-throwing is immediately noticed in the field and seen as a discouraging lack of reward for the stone thrower.