Testing the dielectric strength of insulating materials.
according to DIN EN 60243-2:2014-08;VDE 0303-22:2014-08
When a non-homogeneous specimen is subjected to AC voltage, the electrical stress inside the specimen is determined by the predominantly capacitive impedance. With an increasing DC voltage, the potential distribution may still be predominantly capacitively influenced, but it depends on the speed of the voltage increase. With continuous loading with DC voltage, the stationary state with ohmic potential distribution is established. The choice between DC or AC voltage depends on the purpose of the test and to some extent on the intended use of the material. The following currents result from the application of DC voltage: the capacitive current, the current due to electrical absorption (recharging effect), the leakage current and in some cases the current due to partial discharges. In addition, for materials consisting of layers of different types of insulating materials or exhibiting other forms of discontinuity, the potential distribution in the test specimen is also influenced by charges of opposite polarity which, as a result of interfacial polarisation, can accumulate on both sides of interfaces and generate sufficiently strong local fields there to cause partial discharges and/or breakdown of the test specimens. For most materials, the value of the breakdown voltage when subjected to DC voltage is higher than the peak value of the breakdown AC voltage. For many materials, especially for non-homogeneously composed materials, the breakdown voltage at DC voltage will be up to three times the value at AC voltage or even higher. This Part 2 of the IEC 60243 series of International Standards specifies, in addition to the requirements in IEC 60243-1, further requirements for the determination of the dielectric strength of solid insulating materials under DC voltage stress, including methods for the determination of breakdown voltages along the surface of solid insulating materials.