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An experimentwas carried out to investigate the influence of metallic particles on vacuum breakdown characteristics. By laser scattering method, the movement of particles, the lifting voltage of particles and the velocity of impact to the opposite electrode were measured. The particles were made of pure copper, and the particle diameter was 0.1 to 300 µm. The electrodes were parallel plane, 40 mm diameter, and two sets of electrodes (pure copper and stainless steel)were examined.Experimental results showed that breakdown voltage was reduced by the presence of metallic particles. This breakdown voltage reduction depended on the number of particles between elec trodes. The effect of electrodes material on breakdown characteristics was not apparently obser ved. The observation results of the paticle movement showed that the lifting voltage of particles decreased with decreasing particle size. The electrical breakdown of the gap occurred that when the impact velocity of the particles at the opposite electrode exceeded the critical value. We found microparticle-induced breakdown phenomena that could explain the phenomena of long delayed restrikes that is occasionally observed in vacuum circuit breakers.