Thermocapillary mechanism of contact erosion during arcing

The results of theoretical investigations of the interaction of an electrical arc with a contact surface during contact separation are investigated. Two stages of this process, bridging and arcing, are considered. A mathematical model of the first stage, describing the phenomena of appearance, evolution, and rupture of the contact bridge, takes into account heat exchange between the bridge and solid electrode, the motion of the melting zone, the Joule and Thomson effects, the pinch-effect at high current, and surface tension. The variational principle of the minimum of free energy is used to find the unknown shape of the bridge´s surface. The second stage of contact opening, which is accompanied by arcing processes after the bridge´s rupture, has to be considered taking into account the motion of the arc spot, melting, evaporation, convection, ionic bombardment, thermionic emission, and conductive and radiated heat exchange. It is shown that traditional erosion mechanisms, such as an ejection of liquid metal from the melting zone owing to electromagnetic and gas-kinetic pressure, sometimes cannot explain the measured erosion for such materials as tungsten, zirconium, etc