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

Modes of high current vacuum arc between butt contacts at a fixed gap of 10 mm during the decrease of current to zero were characterised. Three groups of arc modes were featured: (i) constricted-without a significant share of individual cathode spots; (ii) constricted-with a considerable number of cathode spots beyond the concentrated cathode root; and (iii) diffuse modes. It was shown that the most representative from of the high-current vacuum arc before transition to the diffuse phase is the mode in which there is a coexistence of local constricted discharge near the contacts edge and the group of autonomous cathode spots. The threshold current for the transition form constricted to diffuse arc was measured