Arc instability and time-constant in air-blast circuit-breakers

A technique for measuring the arc lime-constant in a circuit-breaker is developed. Measurements are carried out on an air-blast circuit­breaker of the axial blast type. Arcs within the current range 18-90 A and the pressure range 6.5-13 atm are investigated, and the measu­rements are treated statistically. 
An equivàlent circuit of the arc for small deviations {rom stationary conditions is developed and combined with a circuit, fairly represen­tative of the interruption of small inductive currents in power networks, to obtain the criteria for arc stability. The stability condition arrived ai includes the criteria developed by several previous authors as special cases and is, moreover, confirmed experimentally. Stability inves­tigations yield a simple method for the determination of the arc time­constant. This is confirmed by the unit-step modulation technique. 
Finally the different forms of current chopping revealed by the expe­riments are briefly discussed.