Scaling Laws for Gas-Blast Circuit-Breaker Arcs during the High Current Phase

A steady state nozzle arc model based on the boundary layer integral method is established and scaling laws are derived. For affinely related nozzles, the solution is uniquely determined by a nozzle coefficient N, which is related to the stagnation conditions, the arc current, and the dimensions of the nozzle. Tests have been performed on nozzle arcs in air using two geometricaly similar nozzles at three stagnation pressures. A good agreement between theory and experiment is obtained which indicates that circuit-breaker arcs can be scaled. To avoid nozzle clogging, the nominal current density at the throat (I/At) should not exceed the highest permissible nominal current density at the throat. For all affinely related nozzles, this upper limit of current density at the throat is proportional to ¿p0¿t, where p0 is the stagnation pressure and Zt the distance of the throat from the nozzle entrance. The overal arc voltage exhibits the precurrent-zero static behavior as indicated by Browne´s composite arc model.