Transient thermophysical phenomena at the pre-arcing period during opening of electrical contacts

A mathematical model describing interdependent transient temperature and electromagnetic fields in electrical contacts at three stages of pre-arcing of opening of contact (closed, initial opening, and bridging) is discussed. The dynamics of temperature change and its dependence on the contact films, and Thomson, Kohler, and Peltier effects are described. It is shown that nonuniform current density in the contact spot over the frequency range of 0-10 Hz is enhanced by current constriction rather than by the skin-effect. The relationships between the rate of appearance of the extra forces in contacts during the opening period are established. The model of thermal explosion is invoked to explain the mechanism of spontaneous separation of contact resulting from the combined action of electrodynamic and thermal forces generated by the explosive evaporation of material at high current. The model was used to describe the bridging stage of contact opening. It is established that the shape of the bridge, its temperature field, and the lifetime mechanism of rupture are determined by the dynamics of contact opening and also forces resulting from the surface tension, the pinch effect, and Joule, Thomson, and tunnel heating.<>