Determination of optimal arcing interval is one of the problems which must be solved in applying phase-controlled vacuum circuit breaker（VCB）to break short-circuit current.For two major periods of post-arc dielectric strength recovery process of VCB

i.e.sheath expanding period and metal vapor decaying period

mathematical models were established in this paper

and characteristic parameters were used to represent breaking capacity.Influence of arcing time on these characteristic parameters was simulated and analyzed

thus the optimal arcing interval was obtained.By comparing simulation and test results of VCBs used in 10 kV and 35 kV power systems

the validity of simulation was proved.