Effect of Vapour Force at the Blow-Open Process in Double-Break Contacts

This paper presents results of a theoretical and experimental investigation of the phenomena of electrical contact repulsion and its associated blow-off characteristics at high current. The voltage and current waveforms as well as contact's displacement oscillograms are recorded for analysis. The mathematical model of the phenomena based on differential equations for the arc, forces, contact heating and evaporation and displacement of the movable contact piece is elaborated to describe repulsion dynamics. All stages of contact separation including opening start, Joule explosion of the constriction zone and arcing are considered in series. It is found that side by side with electromagnetic force, which is responsible for the initiation of contact opening, thermal force due to vapour pressure, which develops further contact separation, is also very important and should be taken into consideration. Dependence of thermal force on voltage, current, circuit parameters and properties of contact material are calculated theoretically and examined against data obtained experimentally for the asymmetric contact pair AgC-AgNi.