Transition from constricted to diffuse vacuum arc modes during high AC current interruption

Modes of high ac current vacuum arcs between butt contacts at a fixed gap o f 10 mm during the decrease of current to zero, in the current range from 8 -25 kA peak, were characterized. Three groups of are modes were featured: 1 ) constricted without or with a small number of autonomous cathode spots, 2 ) constricted with a considerable number of cathode spots beyond the concen trated cathode root, and 3) diffuse modes. It was shown that the most typic al high-current vacuum are mode during a decreasing current is the local co nstricted discharge situated on the contact edge, which coexists with a dis sipate discharge created by groups of autonomous cathode spots, The thresho ld current for the transition from the constricted to diffuse are was measu red.