Nature of late breakdowns in VI's after short circuit interruption

Late breakdowns in vacuum interrupters after short circuit current interruption are studied with three methods. 1. High speed photography of the contact surface and the inter-electrode space to correlate the position of the restrike with respect to the prevailing conditions on the contact surface : 2. Spectrography during arcing and restrike to evaluate ionized particle densities and electron density ; and 3. A double imaging system to calculate the trajectories of droplets during the TRV phase. We found 95% of the restrikes to be associated with hydrogen in the inter-electrode gap at current zero, and remaining hot spots on the edge of the contact surface. We conclude the breakdown to be of a mixed Paschen type: partially in metal vapour of high density around hot spots and ignited by thermal electron emission, and partially in hydrogen gas which accumulates during arcing in the interrupter.