5.1 This test method is useful in research and quality control for evaluating insulating materials and systems since they provide for the measurement of the endurance used to compare different materials to the action of corona on the external surfaces. A poor result on this test does not indicate that the material is a poor selection for use at high voltage or at high voltage stress in the absence of surface corona; surface corona is not the same as corona that occurs in internal cavities. (See Test Methods D3382.)
5.2 This test method is also useful for comparison between materials of the same relative thickness. When agreed upon between the buyer and the seller, it is acceptable to express any differences in terms of relative time to failure or the magnitude of voltage stress (kV/mm or kV/in.) required to produce failure in a specified number of hours.
5.3 It is possible for this test method to also be used to examine the effects of different processing parameters on the same insulating material, such as residual strains produced by quenching, high levels of crystallinity or molding processes that control the concentration and sizes of gas-filled cavities.
5.4 The data are generated in the form of a set of values of lifetimes at a voltage. The dispersion of failure times is analyzed using one of the methods below:
5.4.1 Weibull Probability Plot.
5.4.2 Statistically (see IEEE 930-1987 for additional information), to yield an estimate of the central value of the distribution and its standard deviation.
5.4.3 Truncating a test at the time of the fifth failure of a set of nine and using that time as the measure of the central tendency. Two such techniques are described in 10.2.
5.5 This test method intensifies some of the more commonly met conditions of corona attack so that materials are able to be evaluated in a time that is relatively short compared to the life of the equipment. As with most accelerated life tests, caution is necessary in extrapolation from the indicated life to actual life under various operating conditions in the field.
5.6 The possible factors related to failures produced by corona are:
5.6.1 Corona eroding the insulation until the remaining insulation can no longer withstand the applied voltage.
5.6.2 Corona causing the insulation surface to become conducting due to carbonization, so that failure occurs quickly.
5.6.3 Forming of compounds such as oxalic acid crystals causing the surface conductance to vary with ambient humidity. It is possible conductance will be at a sufficient level to reduce the potential gradient at the electrode edge at moderate humidities, and thus cause either a reduction in the amount of ......