Study on Design of Axial Magnetic Field Electrodes depending on Electrode Diameter and Current

The presence of anode spot before current zero limits the performance of practical vacuum interrupters. Using an AMF(axial magnetic field) is one of the methods to prevent anode spot formation and to keep arc plasma in a diffuse mode. A lot of papers have been published concerning the magnitude of an AMF and electrode shapes for diffuse arcs. Big electrode diameter and large area of high AMFs ensure better breaking performance. Therefore, the area where AMF per current is over 4mT/kA, which is called the effective area, has been calculated and used in designing electrodes. However, it is difficult to calculate the effective area and the exact diameter as the area does not change proportionally according to electrode diameter. So a new concept is needed in order to design electrodes irrespective of diameter. In this paper, using a high speed camera, the behavior of vacuum arcs were analyzed depending on arc currents and electrode diameter and compared with the results of magnetic field analysis. A new design criterion for diffuse arcs was established by using concepts such as the effective area and the effective current density when arc currents were assumed to flow only through the effective area.