Melting on axial magnetic field contacts during half-cycle drawn arcs in vacuum

High-current arcing operations on electrical contacts in vacuum produce distinct patterns of melting and erosion. Single or multiple half-cycle operations have been performed on Cu-Cr contacts with an axial magnetic field (AMF) to investigate these effects. High-speed movies of drawn arcs inside a vacuum chamber connect the observed melting patterns to the arc behavior. From the initial bridge column, the arc passes through an expanding transition mode, producing a localized region of shallow melting on the anode. The AMF level is sufficient to cause the subsequent formation of a fully diffuse high-current arc throughout the contact gap, generating little or no melting on the anode beyond the transition arc region. The combination of arc visualization and contact erosion experiments demonstrates that the transition arc mode is a critically dominant cause of contact melting. The location of final contact separation moves over multiple operations, causing the region of anode melting to move in turn. This can obscure the melting patterns from the individual half-cycles of current. Therefore, using drawn arcs in both individual half-cycle VI tests and vacuum arc movies provides an improved method for studying the process of AMF contact erosion at high current.