Role of Metallic Vapor Pressure in Contact Bouncing and Welding at Closure of Electrical Contacts in Vacuum

Phenomena at contact bouncing and welding in vacuum circuit breakers at closure are considered in the dynamics as consecutive stages including the first contact touch, compression, restitution, bridging, arcing and the second touch. The bouncing duration and probability of contact welding are investigated in dependence on the time required for the explosion of micro-asperities and micro-arc duration. The hybrid mathematical model describing the dynamics of temperature in the contact zone and the contact motion due to various forces is elaborated. It is based on the Stefan´s problem for contact temperature with melting, evaporation and freezing, the non-linear equation for contact motion, and the experimental data for current, voltage, contact resistance and contact gap in dynamics. Experiments have been carried out using a standard Cu-Cr vacuum circuit breaker for the current range 0.5-2.0 kA and closing velocity 0.2-0.8 m/s. The relationship between the duration of compression and restitution stages and the time required for freezing of the molten contact zone, which defines the welding radius and welding force, are obtained from the dependence on current, closing velocity and material properties. It was found that the influence of metallic vapor pressure on the contact motion and welding is much more significant for high current (~1 kA) than for low current (~20 A).