Model of Arc Root Immobility in Vacuum Circuit Breakers

The model of arc evolution and instability in vacuum based on the dynamics of arc root is presented in this paper. It takes into account parameters of electrical circuit, properties of contact material, heating, melting and evaporation during interaction of arc with the electrode surface. It is shown that arc root immobility is conditioned by dynamics of contact evaporation and rise of arc resistivity which depends on the vapor density in the arc and entails arc instability at certain critical current. The time and the critical contact distance of arc root immobility are found from the conditions defining the change of the direction of heat fluxes into arc for the opposite vector. Phenomenon of arc-to glow transition in low current inductive electrical circuits in vacuum due to arc instability is considered as a special case. The conditions required for this transition are obtained in dependence of contact material properties, characteristics of an electrical circuit, opening velocity and gas pressure. Special experiments with arc-to-glow transition have been carried out to verify obtained theoretical results