This task is not easy to solve because the diffusion of LMWs leads to encapsulation of conductive and non-conductive pollution layers, but IEC60815 and IEC60507 (which are the only available recommendations for this subject) give some guidance. But the results obtained with these methods should be regarded critically. The ESDD (equivalent salt deposit density) and NSDD (non soluble deposit density) that are obtained in accordance with IEC60507 and IEC60815 may lead to misinterpretation of the real pollution severity. Normally, decreased values for ESDD and increased values for NSDD are found on silicone rubber insulator housings compared with ceramic or glass surfaces. Currently this subject is under discussion in the IEC working group that works on the revision of IEC60815. The new (revised) standard should also focus on the ESDD and NSDD values for composite insulators. The method of taking samples and measuring ESDD/NSDD may be modified. For practical applications we recommend to measure ESDD and NSDD on reference insulators or take it from existing ceramic or glass insulators installed in the same location or in the vicinity. Experience has shown that the choice of the creepage distance in respect to polluted conditions is correct, if the same specific creepage distances are chosen for composite insulators as recommended in IEC60815 for the specific pollution severity. The only exception is that the values for ESDD and NSDD must not be taken from insulators with silicone rubber housings.

No. At the moment there are no general laboratory (artificial) test procedures available that simulate real service conditions. There are some approaches, for example, the STRI test for combined marine desert pollution profiles or the clean fog test for tropical climatic condition with permanent humidity. The problem of laboratory tests is that they should have an accelerating character to avoid long time consuming tests. Exactly this accelerating character is the problem of most artificial pollution tests because much higher electrical field stresses and more severe artificial test conditions are applied on the composite insulators than the existing ones found under real service conditions. Often a wrong approach with non-realistic test parameters leads to misinterpretation of test results and therefore to the choice of a wrong insulator design. Salt fog tests, for example, are developed with the intention to test the tracking and erosion performance and the resistance of the material. They do not simulate real service conditions and therefore results from salt fog tests are often be misinterpreted.

No. The ESDD (equivalent salt deposit density) and NSDD (non soluble deposit density) that are obtained in accordance with IEC60507 and IEC60815 may lead to misinterpretation of the real pollution severity. Normally, decreased values for ESDD and increased values for NSDD are found on silicone rubber insulator housings compared with ceramic or glass surfaces. Currently this subject is under discussion in the IEC working group that works on the revision of IEC60815. The new (revised) standard should also focus on the ESDD and NSDD values for composite insulators. The method of taking samples and measuring ESDD/NSDD may be modified. For practical applications we recommend to measure ESDD and NSDD on reference insulators or take it from existing ceramic or glass inuslators installed in the same location or in the vicinity. Experience has shown that the choice of the creepage distance in respect to polluted conditions is correct, if the same specific creepage distances are chosen for composite insulators as recommended in IEC60815 for the specific pollution severity. The only exception is that the values for ESDD and NSDD must not be taken from insulators with silicone rubber housings.

One should distinguish into procedures that check the T&E behavior (damages due to artificial aging mechanisms, for example, IEC61109 salt fog test 1000h) and into procedures that check the insulation and flashover performance under artificially simulated “real service“ conditions (deposit layer method, clean fog tests) and individual adapted methods (STRI marine desert pollution test). Both kinds are in any case “artificial“ and in almost all cases “accelerated“. Due to these approaches it is difficult to compare results from laboratory testing with those obtained from real service applications. The problem is that the main aging mechanisms are dependent on voltage and pollution layer conductivity in a non-linear way. Results obtained from artificial aging tests should therefore always be regarded critically.

The best way to test is to collect experience by evaluating insulator aging under real service conditions. Of course, this can be very time consuming and uneconomical. Assumed that all design tests acc. ANSI C29.11 and/or IEC61109 are passed successfully, a long-term evaluation of the insulator design is only possible after at least 3-5 years (10 years is preferred) of service. We recommend periodically scheduled inspections and investigations of insulators taken from service.