Characteristics of a vacuum discharge switching using small contact gaps

Axial magnetic field contacts (AMF) can increase the interruption capability of vacuum circuit breakers. Depending on the design, principles of the local axial field arrangement are different. For unipolar arrangement the direction of the axial magnetic field is equal within the whole contact area. For bipolar arrangement there is one polarity change in the field. Investigations have been carried out to characterise a bipolar AMF contact system and to test its interruption performance. The influence of the bipolar AMF on the arc development and the thermal stress is described by high speed camera and contact surface pictures. In addition, some three-dimensional AMF simulations have been performed by means of a finite-element-program. According to the time-varying AMF excited by a 50 Hz-high-current, the influence of eddy currents is of vital importance. The high interruption performance of the bipolar AMF-contact-system has been demonstrated by using a single phase test circuit up to 36 kV/40 kA. The investigations are completed by means of measuring events during recovery period with respect to the post-arc-current and the shield-potential