Generally, an arc voltage of a vacuum discharge is relatively low, but it grows very rapidly just after the contact separation. Such a rapid increase of the voltage might cause some kinds of transient phenomena in a closed loop circuit with capacitances and inductances. An oscillating current caused by these transient phenomena can create a current zero at the vacuum arc. If the amplitude of the oscillations is larger than the current there, the arc will be quenched. The authors investigated these phenomena, and presented at the previous ISDEIV meeting. Probability of the current interruption by these transient phenomena is limited, and the arc interruption phenomena were observed, where levels were 20 to 40A. After several tens or hundreds milliseconds, however, the current are suddenly interrupted by spontaneous current oscillation. These oscillations are considered to be related to arc instability. We try to clarify these current interruptions caused by arc instability at larger current levels.
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