A Study of Conduction Phenomena Near Current Zero for an A-C Arc Adjacent to Refractory Surfaces

As first steps in a general study of arc extinction in magnetic-type air circuit breakers, equipment has been developed for simultaneously measuring both arc voltage and arc current just before and after current zero, and studies have been made of arc path conductance and power input in this critical time region for small-scale tests with arcs of a few hundred amperes either magnetically pressed against or physically confined between refractory insulating plates. Analysis in terms of possible mathematical arc models has aided interpretation of the observations. The general nature of the arc reignition process under these conditions has been revealed clearly and the dynamic properties of the arc and surface leakage paths have been determined quantitatively for these tests in terms of the arc model values: conductance time constant and critical power input or critical applied voltage gradient. Some effects on these values of such controllable variables as plate composition and plate separation have been shown. The leakage conductance along the adjacent plate surfaces just after current zero was found to limit the interrupting ability of arcs under the small-sample test conditions, but the relative importance of this compared to the dielectric recovery of the gas space and other possible factors remains to be determined for very high current interruption in actual circuit breaker arc chutes.