The effect of insulator is insulation and support. Insulator will be damaged after a long time. So insulator inspection is must. Although some composite insulators have been successfully in service for thirty years, also need inspection.

Visual line inspection was the first method and is still the method most commonly applied by utilities. Typically, surface damages can be identified, which could be an indicator for internal defects. The defects are often small in appearance, requiring very powerful binoculars. Visual line inspection needs both an experienced helicopter pilot and an experienced line inspector. Obviously, the upper side of tension string insulators can be visually investigated, which is rather difficult from the ground.

Measuring with a field probe is an accurate, but time consuming method. Initially, this technique was developed for the in-service evaluation of cap and pin insulator strings.
With the increased service time of composite insulators, the field probe has subsequently been modified to also evaluate the new insulator technology. In principle, measuring involves mapping of the electrical field along the insulator. If a defect is found, the electrical field will show an immediate change.

The ambient humidity has a strong influence on the field recordings, which makes interpreting the results difficult at such times.

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