A Study of the Arc of a Radial Magnetic Field (RMF) Contact Geometry at Currents below the Natural Constriction Level

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A study was carried out to investigate vacuum arcs at low currents on Radial Magnetic Field (RMF) contacts used in commercial Vacuum Interrupters (VI). The RMF contacts operate by moving the naturally constricted arc in a predictable and controlled way in order to have interruption of short circuit currents of 20kA or more. Arcs on RMF contacts at large short circuit currents have been widely studied, but there is little published about how these contact geometries work at low currents. This is of interest as the vast majority of switching operations for VI are actually carried out at load currents, which are typically less than 3150A rms, and are all below the natural constriction level for the arc. The study was to investigate how these naturally diffuse arcs behave under the magnetic fields induced by the RMF contact geometries. The study consisted of a series of high speed films of commercial RMF contacts in a demountable vacuum chamber switching a range of currents from 400A to 3150A rms. The films showed that although the arc moves predictably at high short currents which are in the kA range, for these low currents the diffuse arc behaves quite differently. In fact there is some indication in the films that although parts of the arc try to rotate as they would do at higher currents, much of the arc does not, and in fact the Radial Magnetic Field may in fact inhibit the diffusion of the arc which would normally occur. This is normally not a problem in service, but it may indicate that optimizing the contact geometry for the highest short circuit currents may in fact not give the best performance at lower currents.