Effect of an axial magnetic field upon the development of the vacuum arc between opening electric currents [currents read contacts]

In one series of experiments, a constant axial magnetic field B_ax was produced in the gap between opening Bruce-profile, chromium-copper contacts. Peak arc currents up to 40 kA at 50 Hz were studied with B_ax u to 112 mT. For each peak current, a critical field B_crit was determined above which anode spots did not form. For B_ax greater than B_crit, the time from contact parting to the formation of a diffuse vacuum arc was linearly dependent on separation current. In a second series, the contacts were designed to generate a self B_ax when AC current up to 123 kA peak flowed. This field was approximately proportional to the current waveform through the contacts, and it was always higher than the B _crit predicted for the instantaneous current. The time from contact parting to the formation of the diffuse arc was consistent with that observed in the first experimental series. Once the diffuse arc had formed, the contacts showed no distributed melting for currents up to 81 kA, but gross melting occurred at 102-123 kA. The formation of diffuse arcs is discussed, and the anode melting is explained