Interrupting Capacity of Vacuum Interrupters as a Function of Contact Geometry

Systematic investigations of the interrupting capacity of vacuum circuit breakers with different contact shapes and sizes are reported. For various electrode geometries with current-dependent selfgenerated transverse and axial magnetic fields, the breaking current limit was determined. Of particular interest were the arc appearance (diffuse/ constricted) which is represented in the arc voltage waveform, the erosion behavior, as well as the influence of the arc movement on the extinguishing capability. The experiments for this single-pole interrupting test were carried out with a synthetic WeiI-Dobke test circuit--maximum current 60 kA (rms); voltage conditions corresponding to 20 kV power systems--and a demountable vacuum chamber. The experimental results show that the breaking capacity generally increases with contact diameter. Transverse magnetic fields enhance the performance, provided the conditions for a good arc motion remain throughout the operating life. The best results were obtained with axial magnetic field contacts.