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

The interrupting duty of high-voltage circuit breakers can be reduced by increasing the contact parting time, which is the sum of tripping delay and the circuit breaker opening time. The contact parting time is fixed; however, the tripping delay can be increased. Although this method is not discussed in current technical literature, it is valid according to the ANSI/IEEE standards. Immediate replacement of the circuit breakers or other alternatives to reduce interrupting duties can be avoided, resulting in large cost savings and 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 are retained in service. This paper demonstrates these calculations.