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

The authors show the influence of surface roughness on molten bridge formation at slow separation velocity and attempt to determine the precise occurrence time of the different sequences which finally lead to the arc. An energy balance model that applies below the boiling temperature, just before arc formation, is developed. It enables the calculation of the radius, the temperature, and the voltage across the bridge. The classical approach of the phenomena observed during the separation of contacts leads to the presence of a molten bridge revealed by characteristic voltages. Features based on the interpretation of the observed fast increasing slope voltage rises are presented: (a) between 0.41 and 1 V, an adiabatic melting of roughness takes place whose calculated dimensions agree with Landau´s hydrodynamic stability criterion; and (b) beyond 1 V, an adiabatic heating of a pinched bridge yields unexpected high voltages. These voltages are sufficient for further excitation and ionization of metal vapor resulting from the bridge explosive boiling. Spectroscopical time investigations support this last interpretation of the observed voltage peaks preceding the arc formation. No excited states of the neutral or ionized metal vapor are found before the voltage narrow peak