IEC 61251-2015 pdf Electrical insulating materials and systems – AC voltage endurance evaluation

5 Test methods 5.1 Introductory remarks Different methods of carrying out the VE test can be used. The differences concern the way of applying voltage (constant or increasing with time), the frequency (service or higher) and the time at which the test is interrupted (the time to dielectric breakdown for all sample specimens (complete life tests) or a shorter time for some of the specimens of the sample (censored life tests). In general to enable comparisons to be made, the type of ageing cell or test object shall be the same, whatever the choice of the parameters above. However, with respect to the choice of the frequency of the applied voltage, the amount of heating from either dielectric loss or from partial discharges shall be such that the temperature rise from these causes is less than 3 K. When testing materials, the ageing cell or test object should result in a uniform electric field. This can be achieved by electrodes having a flat surface rounded at the edges. To avoid partial discharges and flashover along the specimen surface, the specimen shall extend a suitable distance beyond the edges of the electrodes. If preliminary tests indicate that this extension beyond the electrodes is not enough to avoid partial discharges and flashover, the electrodes shall be immersed or embedded in an appropriate dielectric having the same or higher permittivity than that of the material under test. The form and processing of the specimen will depend on the purpose of the test. For research purposes, internal degradation studies as a function of cavity size and shape have been performed. However, this lies outside the scope of this International Standard. Evaluation and comparison of materials from the point of view of degradation by external discharge are dealt with in IEC 60343. For insulation systems, the test objects shall represent adequately the form taken in service and be determined by the relevant IEC equipment committee.