The phenomena of late breakdowns in vacuum interrupters are being discussed for almost twenty years but are still not fully understood and generally controversially discussed. Initially interpreted as a sign of distress of dielectric properties, this phenomenon is considered today an intrinsic behavior of vacuum interrupters than can be mastered to reach highest performances as very low restrike probability circuit breakers up to 40.5 kV. Previous results indicate that the most probable source for late breakdowns is particles released from the internal surfaces in the vacuum interrupter. This paper aims to deepen our understanding of late breakdown by correlating experimental results obtained with a low-energy in-house test circuit and those obtained at higher power test stations with different kinds of contact materials. Emphasis will be given on the experimental work and analysis of performances of various contact material. Explanation to get a better understanding of the late breakdown phenomena will be further given.
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