Reducing Interrupting Duties of Medium -Voltage Circuit Breakers by Increasing Contact Parting Time

Interrupting duty of circuit breakers can be reduced by increasing the contact parting time, which is sum of tripping delay considered as one-half cycle in ANSI/IEEE standards and the circuit breaker opening time, which is a function of the breaker interrupting time. The contact parting time is fixed, however the tripping delay can be increased. Though this method is not discussed in current technical literature, it is valid according to ANSI/IEEE standards. Immediate replacement of the circuit breakers or other means to reduce interrupting duties can be avoided, resulting in large cost savings and also process downtime. The problem occurred in a large paper mill distribution system, where the circuit breakers were applied well within their close and latch ratings, but the interrupting duty exceeded by 6%. By introducing an additional tripping delay of one cycle, the interrupting duty is reduced and the existing breakers can be retained in service. The paper demonstrates these calculations.