Thermodynamic Model of Moving Vacuum Arcs on Contrate Contacts

The behavior of constricted vacuum arcs on radial magnetic field (RMF) contacts in vacuum tubes of medium voltage circuit breakers strongly determines the breaking capacity of these contact systems. The movement of these arcs under the influence of the RMF is investigated with the help of a one-dimensional (1D) thermodynamic model of the arc and its interaction with the contact surface. Heat conduction and evaporation are taken care of analytically, while the size of the arc root and, hence, its current density has been determined experimentally. The energy balance is solved by using empirical values for the respective sheath voltage drops. It is complemented by a momentum balance incorporating neutral vapor loss from the arc boundaries, which is found to be essential for the quantitative understanding of the arc physics. A time-dependent analysis of the arc properties like the arc velocity during a sine half-wave produces a good agreement with experimental values measured for different sizes of contrate contacts. The model is explained and compared to experimental results of arc velocity and experimentally determined switching capacity, respectively