The aim of diagnostics is, in general, to get relevant information about the state of technical systems . In insulator applications this means that diagnostics should be an aid in making decisions about if and when maintenance or replacement should be done. It is basically associated with reducing costs and minimizing risk of damage to people and property. For insulators made of glass or porcelain, several inspection methods are available to detect faulty insulators in service . In the case of composite insulators, the situation is more complex. Though several techniques have been proposed for assessing the state of insulators in service , it is still difficult to accurately correlate measured parameters with presence of defects .For some insulator users the lack of appropriate diagnostic methods is seen as one of the main disadvantages, and thus work on development of diagnostic methods for evaluation of the composite insulator conditions should

An IEEE task force evaluated a number of methods used to detect faulty composite insulators before installation. A number of insulators containing artificial defects were manufactured and tested. The defects were wires, silver paint, metal shavings or copper dust placed in the interfaces between housing and rod, cracked rods soaked in water, cuts in housing material exposed to salt fog and improperly attached end fittings.

The insulators in imminent danger of failure, i.e. the insulators with embedded wires, were found to puncture when energized at rated voltage. By measuring resistance between two movable electrodes using a megger or a hot stick tester, limited information was obtained, although with help of the hot stick tester one could identify the insulators with buried wires bridging almost the full insulator length. Measurements of leakage current, partial discharges (PD) and radio interference voltage (RIV) could detect insulators with embedded wires as well. RIV was found to be the most sensitive technique since it also could identify insulators containing silver paint. However, the fact that discharges had to be caused by the defects, and not by anything else, restricted applicability of PD and RIV measurements to laboratory environment only. Further, no heating could be observed on the insulators when energizing them even up to 80 % of the dry ac flashover voltage for 20 minutes. This suggested that most of the techniques evaluated would have detected the insulator units that were about to fail within a short time.

Tian et al checked several techniques as means to evaluate the integrity of composite insulation and tools for live line work. By measurements of PDs, performed on 300 mm long segments, faulty insulators were identified through significantly reduced inception voltage and increased apparent charge. IR observations on hollow composite tubes stressed at nominal voltage for 70 minutes showed temperature increases of about 7 ºC on two of seven tubes tested. These two tubes failed later during a subsequent withstand test.