The Geometry of Arc Interruption

It is the purpose of this paper to present a rigorous analysis and description of the phenomenon which takes place in an interrupting device during the interval between the parting of the contacts and the current zero at which interruption takes place. It is to be shown that the events which take place during the arcing period prior to interruption play an important part in setting the stage for the drama of circuit interruption. This analysis has a counterpart in the phenomenon which takes place immediately following interruption; namely, circuit recovery characteristics. The knowledge of this latter phenomenon has played a dynamic role during the past ten years. With its aid, oil circuit breaker designs have advanced to the point where their excellent performance has set the high present-day standards of reliable circuit interruption.^2,3 The knowledge of recovery rates^4,5 laid the foundation for the displacement principle;⁶ namely, oil velocity multiplied by the oil dielectric (impulse) strength is equal to the maximum permissible rate of rise of recovery voltage. The stage today, however, is being set for interruptions with devices which employ air as the medium of interruption and insulation.^7,8 New roles are being created. Recovery voltage, although just as important, is being assigned an easier part.