Factors affecting electrical tree testing

In an attempt to produce definitive experimental procedures for the assessment of treeing properties of polyester and epoxy resins, the influence of a variety of mechanical, electrical and geometrical conditions are investigated. The investigations are performed using point-plane specimens and the influence of the frequency of the applied voltage, the method of specimen preparation and the needle tip radius were examined. Consideration must be given to the correct preparation of the insulating resin and to the needle geometry in standard pin-plane assemblies. The cure and post-cure of the resin must be optimised to produce the most stable and reproducible specimens. Both under-cured and over-cured resin specimens have a lower resistance to electrical treeing. The presence of high internal mechanical strain in a specimen reduces the electrical tree initiation and growth times. Acceleration of 50 Hz treeing tests by the application of 1 kHz voltages should not be undertaken. Residual trapped charge in previously formed channels causes the mechanism of tree development at 1 kHz to be different to that at 50 Hz. Interpretation of such accelerated ageing procedures is unlikely to be of practical use.