Phenomena preceding arc ignition between opening contacts: experimental study and theoretical approach

As the pressure force applied to electrical contacts decreases before the break, the actual cross-section for current passing across the contacts diminishes and the subsequent heating leads to a molten bridge formation. The influence of surface roughness on molten bridge formation at slow separation velocity is shown. The precise occurrence time of the different steps of the sequence which finally leads to the arc are determined. An energy balance model which applies below the boiling temperature, just before arc formation, is developed. It enables tie computation of the radius, the temperature, and the voltage across the bridge. From accurate timing of the sequence of phenomena, the conclusion is that voltages higher than those required for ionization of metal vapor are generated by adiabatic heating of the quickly pinched molten bridge