EFFECT OF AN AXIAL MAGNETIC-FIELD UPON THE DEVELOPMENT OF THE VACUUM-ARC BETWEEN OPENING ELECTRIC CONTACTS

Two series of experiments were undertaken. In the first, a Helmholtz c oil provided an independent, constant axial magnetic (AM) field B(ax) in the gap between opening Bruce profile, chromium-copper (Cr-Cu) cont acts. Peak arc currents up to 40 kA at 50 Hz were studied with B(ax) u p to 112 mT. For each peak current, a critical field B(crit) was deter mined above which anode spots did not form. For B(ax) greater than B(c rit), the time from contact parting to the formation of a diffuse vacu um arc was linearly dependent upon separation current, with a change i n slope at 7-8 kA. In the second series, with ac currents up to 123-kA peak, the contacts were designed to generate a self B(ax) when curren t flowed. The magnitude of this field was approximately proportional t o the current waveform through the contacts, and it was always higher than the B(crit) predicted for the instantaneous current. The time fro m contact parting to the formation of the diffuse arc was consistent w ith the first experimental series. Once the diffuse arc had formed, th e contacts showed no distributed melting for currents up to 81 kA. How ever, gross melting occurred at 102-123 kA, even though the arc remain ed diffuse. Formation of diffuse vacuum arcs in AM fields is discussed in terms of the confinement of ions within the contact gap. Anode mel ting under diffuse arcing is explained by the total power input from t he diffuse arc.