ELECTRICAL FATIGUE AND DIELECTRIC STRENGTH IN TEFLON TFE INSULATION,

All high-voltage failures of Teflon insulation are initiated in and propagated from defects in the insulation. Teflon insulation containing no gross voids or foreign particles is capable of withstanding continuous testing for many hours under the conditions specified by MIL-C-915. Intermittent application of the voltage lengthens the time to failure, indicating partial reversibility of the damage caused by over-voltage testing. Time to failure decreases sharply as the voltage is increased beyond that specified. Pigmentation generally decreases dielectric strength. Other defects may mask the effect of pigment. Major defects are usually detected by the AC overvoltage test, but a fault which is not quite large enough to fail during the test will become a more likely site for failure in service, because of the propagation of breakdown channels during the brief overvoltage test. A subsequent test may find such a fault, but will in turn enlarge still smaller faults, making them the sites of possible in-service breakdown. If the breakdown channel length is a large fraction of the thickness of the insulation, measurable ionic discharges will occur at the rated voltage. The rate of erosion by these low-energy discharges will be low, probably negligible, and failure will probably occur during a switching surge, rather than during steady-state operation. The number of internal discharges which will cause failure of TFE insulation at a fault of given magnitude is reached more rapidly as the frequency is increased.