Experiences with diagnostic methods and principles applied to insulator strings subjected to live line work are shown in the following. If lightning is probable, LLW is usually forbidden or interrupted.

Experiences with diagnostic methods and principles applied to insulator strings subjected to live line work:
It is an important rule that LLW is always performed in fair weather, excluding conditions that could lead to pollution flashover, thus only the dielectric performance under transient voltages is of concern. If lightning is probable, LLW is usually forbidden or interrupted. So, only long front overvoltages are of concern, which are simulated in the laboratory by Switching Impulses (SI) [19], [20]. Many types of possible deteriorations of composite insulators in service do not affect the SI dielectric strength and are not of importance with respect to LLW feasibility. Examples of these defect types are pure external surface phenomena (chalking, color changes etc. [20]). Even shed damages (punctures, splitting etc.) may not be critical enough to cause a reduction of the dielectric strength under dry conditions required for LLW

[19], [20]. Only conductive or partially conductive defects can affect the SI strength. Tracking type defects will always be potentially critical, since they are conductive or partially conductive even in dry conditions and are often hidden inside the insulator. However, the insulator can also become partly conductive following moisture ingress in the core and at the interface.

This occurs, for example, when moisture penetrates through the punctures/splitting of the sheath towards the rod, thus increasing the conductivity of the rod or part of it [20]. Therefore, prior to LLW, the condition of the insulators of the span(s) subjected to maintenance are to be evaluated in order to detect any possible risk of flashover. Specific procedures are applied for this requirement, which is focused only on personnel safety. More specifically, the goal is to detect the quite large conductive or semiconducting type defects, since only large defects (e.g. 20-30% of the HV EHV insulator [19], [20]) create risk during the LLW. This specific requirement is very important for the assessment of the feasibility and suitability of the available diagnostic methods.

LLW on composite insulators is carried out by many companies in the world, adopting different diagnostic tools, selected on the basis of their specific experience and taking into account the characteristics and the known weaknesses of the insulator generations installed in their system. In spite of the fact that R&D activities are going on to verify the efficiency of the different diagnostic methodologies, in many countries, like Australia and South America, LLW on composite insulators is still based mainly on careful visual inspection. Combined methodologies are proposed in other countries. As an example (Fig. 10) the Italian procedure [30] supplements visual inspection from ground level with both UV-camera and electric field measurements while in Canada, visual inspection is supplemented by thermal observation.

Conclusions:
The many advantages of composite insulators have led to their wide use in transmission lines all over the world. The reliability of the present generation of composite insulators is considered to be similar to that of ceramic cap-and-pin insulators.
An essential issue that limits an even wider application of composite insulators is the concern about the assessment of their conditions in service and especially before the application of live line working (LLW) techniques.
The applicable diagnostic principles did not significantly change during the last 15 years. However, significant progress has occurred in the development of diagnostic tools and in the interpretation of measurements.
On-line diagnostics, together with periodical tests on samples taken from service, en- able reliable indications regarding the insulator conditions in service and regarding their life estimation to be obtained.
Diagnostics is easier for LLW, since its aim is to identify only large conductive or bsemi-conductive defects which may be critical during the specific linemen activity.
A well chosen combination of available diagnostic methodologies make it possible to identify the absence of critical defects of composite insulators and to carry out LLW safely on overhead lines equipped with composite insulators, in a similar way as it is for ceramic and glass insulators.

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