Phenomena of Arc Root Immobility in Electrical Contacts

Erosion of electrical contacts in vacuum depends essentially on the time of the initial arc immobility when the temperature on the contact spot rises rapidly up to the boiling point. The mechanism of arc root motion is very complicated and there is no universally recognized model until now which could explain it in general case. Immobility time depends on many factors, and it is very important to understand the predominant cause provoking instability of arc spot on the contact surface, which leads to the arc collapse. 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 the parameters of electrical circuit, properties of contact material, heating, melting and evaporation during interaction of the arc with the electrode surface. It is shown that arc root immobility is conditioned by the 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 can be found from the conditions defining the change of the direction of heat fluxes into the arc for the opposite vector. The time of existence of a cathode spot and arc instability is found for various contact materials and current densities. 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. Comparison shows a good agreement. These results can be applied for elaboration of new electrical contact systems for vacuum circuit breakers with reduced arc duration and erosion.