Transition to the diffuse mode for high-current drawn arcs in vacuum with an axial magnetic field

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Abstract
The opening of electrical contacts while passing current generates a drawn arc. In vacuum, the arc begins as a bridge of molten metal connecting the contacts, which then ruptures to form a bridge column arc. Previous work observing the development of drawn arcs in vacuum with an imposed axial magnetic field (AMF) measured the time required for the bridge column to evolve into the high-current diffuse mode. Arc visualization experiments on Cu-Cr contacts with an AMF have now determined that the transition to the fully diffuse mode has a more complicated development. With high-speed photography, we characterized the appearance of the arc modes over half-cycles of power frequency short-circuit current. The opening sequence begins with the rupturing of the molten metal bridge, forming the bridge column. This column evolves into the transition mode, and then into the fully diffuse mode. This transition mode in an AMF consists of a region of concentrated cathode spots, similar to the transition mode for butt contacts at lower currents and no AMF. Over a few milliseconds, an increasing number of individual cathode spots begin to appear outside the concentrated region, until a diffuse arc forms. The transition mode produces a transient peak in the arc voltage. Increasing the AMF strength at a particular current can shorten the duration of the transition mode and reduces the arc voltage peak. Single or multiple half-cycle operations have been performed on Cu-Cr contacts to investigate the effect of the transition mode on contact melting. The melting patterns after a single half-cycle of high current are correlated with the behavior observed in the arc movies. Anode melting is confined to one or two regions of shallow melting, while individual cathode spot tracks covered most of the cathode surface. The combination of arc visualization and post-arcing contact examinations demonstrated that the transition arc mode was a significant source of contact melting.

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