High Current Interruption of Vacuum Interrupters and Voltage Breakdown during Recovery

The phenomena during recovery of voltage after current interruption of vacuum interrupters are reviewed. Metal vapor, post-arc plasma, droplets and the effects of a molten contact surface may be involved in breakdown after current-zero. Sintered CuCr contact material has proven to be the one and only choice for vacuum circuit breakers. What justifies the better performance compared to other materials and in particular pure copper? Many investigators have tried to get more clarity on the process of breakdown initiation after current-zero, however, hardly succeeded in the past 50 years. This paper gives a description of the main findings. It summarizes the condition of vacuum interrupter contacts after high-current interruption for CuCr and Cu contacts. Measured breakdown probabilities and breakdown rates are evaluated and discussed. Small metal droplets ejected during arcing and after current-zero seem to play only a minor role as well as solid protrusions. The largest impact originates from the contact surfaces melted by the arc remaining liquid for a considerable time after current-zero. Large hot droplets and protrusions drawn out of the liquid are suspected to initiate breakdown at gross electric field strengths exceeding 8 kV/mm. Key parameter is the surface temperature and the properties of the liquid contact material. If the metal vapor density exceeds $10^{22}/{\mathrm {m}}^{3}$ at higher surface temperatures-equivalent to a line integral of the density across the contact gap of $4 x 10^{19}/{\mathrm {m}}^{2}$, a Townsend discharge in metal vapor is feasible.