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.