Late breakdown phenomena in vacuum interrupters

Late breakdown (LBD) phenomena in vacuum interrupters are being discussed for more than 20 years. This has led to several changes and amendments to the major international standards in the past ten years and thus to clear rules for the interpretation of these phenomena during type testing. The physical nature, however, is still being discussed with two schools of thoughts being pursued. One model is based on field electron emission effects whereas the other concept calls for particle induced breakdowns. This paper aims to widen the understanding of late breakdowns by correlating experimental results of a low-energy in-house test circuit with those obtained at high power test stations. The emphasis of the experimental work is put on the influence of dielectric performance, contact materials and micro-particles on the late breakdown behavior of the interrupters. The two explanation models will be discussed and guidelines on how to improve the design and performance of vacuum interrupters presented.