One of the dominant factor in phase controlled switching is the determination of optimal switching phase

while the optimal switching phase is mainly affected by the pre-strike characteristics of the circuit breaker. According to the pre-strike phase associated with the mechanical scatters

the making factor k is introduced and discussed in classification

and the pre-strike physical process is analyzed in detail. The computing formulas of optimum switching phase are obtained as function of decrease rate of dielectric strength （RDDS） and mechanical scatters.