Thermodynamic Models for RMF- and AMF- Vacuum Arcs

In the medium voltage range the vacuum switching principle is well established (Fink and Renz, 2002). Today vacuum circuit breakers are available up to 52 kV rated voltage and 80 kA short -circuit current. Based on long term experiences and knowledge from basic research thermodynamic models for the RMF- and AMF-arc behaviour were developed and are presented in this paper. In the case of RMF-driven arcs the equation of motion caused by the LORENTZ-force describes various experimental experiences rather well with a minimum of assumptions. The model-approach is that the arc mass is equal to the spot-lake mass. Hence the linear contact diameter-dependence of the interrupting capability and the current-dependence of the arc radius can be described in a rather good agreement to the experimental data. In the case of the diffuse burning AMF-arc the boundary condition for the effective contact area (Renz, 2000) is given by a minimum value of the specific axial-magnetic induction Gamma. This area is related to the molten surface and is a well defined part eta of the geometrical contact area. Treating the arc as a magneto-hydrodynamic medium both eta and Gamma can be estimated in a self-consistent calculation