Effect of an axial magnetic field upon the development of the vacuum arc between opening electric contacts

Two series of experiments are reported. In the first, a Helmholtz coil provides an independent, constant axial magnetic field B/sub ax/ in the gap between opening Bruce profile, chromium-copper (Cr-Cu) contacts. Peak arc currents up to 40 kA at 50 Hz are studied with B/sub ax/ up to 112 mT. For each peak current, a critical field B/sub crit/ is determined above which anode spots did not form. For B/sub ax/ greater than B/sub crit/, the time from contact parting to the formation of a diffuse vacuum arc is linearly dependent upon separation current, with a change in slope at 7-8 kA. In the second series, the contacts are designed to generate a self B/sub ax/ when current flows. The magnitude of this field is approximately proportional to the current waveform through the contacts, and it is always higher than the B/sub crit/ predicted for the instantaneous current. The time from contact parting to the formation of the diffuse arc is consistent with the first experimental series.<>