Vacuum interrupters have some merits such as size and maintenance. They have been used widely up to 72 kV, and expected to be used in the higher voltage class. To improve their dielectric performance, it is necessary to clarify vacuum breakdown phenomena. Micro particles are not negligible especially concerning vacuum interrupters used in the high voltage class. Therefore, the authors have developed optical measuring system for particle behavior in vacuum. In this paper, the authors investigated the causes of particle production and their influence on dielectric performance. The authors focused on two different causes of particle production as follows: (a) contact melting during current interruption, (b) mechanical damage of contacts during closing operation. Concerning “(a) contact melting during current interruption”, it turned out that particles generated during the current interruption are able to be significantly suppressed by current conditioning. Concerning “(b) mechanical damage of contacts during closing operation”, it turned out that some particles were generated by the high-velocity closing operation. The breakdown voltage just after the mechanical damage was about 70% of the conditioning voltage. Closing velocity would have some influence on particle production.
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