Influence of contact distance on the appearance of vacuum arcs with different AMF contacts

Measurements with vacuum interrupters equipped with bipolar axial magnetic field (AMF) contacts have shown that the post-arc current magnitude and its duration fall with shortened arcing time, and decreasing gap distance, respectively. Simulations of post-arc currents point out that this dependency may result either from different arc diameters or different ion densities around current zero. To clarify this, high-speed movies of vacuum arcs up to 60 kA root mean square were taken, using contacts with a bipolar and quadrupolar axial magnetic field structure. The gap distance was varied by delayed contact opening. Moreover, switching experiments with sealed experimental interrupters under 36 kV conditions with a bipolar AMF arrangement in comparison with a quadrupolar arrangement showed that the bipolar contacts yielded higher interruption capability, in correlation with lower post-arc currents. In order to study the reasons for these phenomena, optical investigations of the arc appearance were carried out under variation of the relevant parameters.