This paper presents a comprehensive explanation of the formation of the electric arc between opening contacts in a current carrying electric circuit. As the contacts begin to open a molten metal bridge forms between them. The rupture of this bridge and the initial formation of the electric arc are studied in both atmospheric air and vacuum using experiments to determine the direction of metal transfer between the contacts as a function of time after the rupture of the molten metal bridge. High speed streak photography is also used to show the rupture of the molten metal bridge and the initial formation of the electric arc. Analysis of these data show that a very high-pressure, high-temperature metal vapor zone exists between the contacts after the rupture of the molten metal bridge. Under this condition a pseudo-arc forms where current is carried by metal ions and an anomalous, high net transfer of metal to the cathodic contact occurs. The pressure in this region decreases rapidly and there is a transition to the usual electric arc, which still operates in the metal vapor. In this arc the current is now mostly carried by electrons. The data shows that there is still a net transfer of metal to the cathode, but now its volume is a function of the arcing time.

The showering arc waveforms generated when contact is being separate have poor reproducibility whose causes are not sufficiently clear. This paper describes that the contact surface conditions which change with the number of contact operations are deeply related to the showering arc waveforms. First, it is experimentally shown that the contacts' surface roughness increases with the number of contact operations, and the growth model of contact surface roughness is proposed based on the change of contact resistance for the number of contact operations. Second, the growth model of molten metal bridge is proposed based on the fact that the showering arc waveforms change with the number of contact operations and the evaluation indexes of showering arc are proposed.