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 of 10 mm during the decrease of current to zero, in the current range from 8-25 kA peak, were characterized. Three groups of arc 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 concentrated cathode root, and (3) diffuse modes. It was shown that the most typical high-current vacuum arc mode during a decreasing current is the local constricted discharge situated on the contact edge, which coexists with a dissipate discharge created by groups of autonomous cathode spots. The threshold current for the transition from the constricted to diffuse arc was measured.