Theoretically, the breakers should operate immediately on receipt of the 110-kV dc signal, and the three breakers should make or break synchronously. However, due to mechanical imperfection or fatigue, time delays in the operation after the 110-V energization of a coil and nonuniform action of individual circuit breakers might lead to unwanted tripping operations. In addition, a long delay in circuit-breaker operation will cause instability in the 400-kV transmission system during a fault in the system and may damage expensive equipment. The asynchronous action of individual circuit breakers within a group will cause unbalance within the three-phase circuit on a transient basis, resulting in power-system instability. More seriously, either of these two events may be considered a circuit-breaker failure by the protection system, resulting in the tripping action of another circuit breaker.
The normal closing time of each circuit breaker is from 89 ms to 99 ms, the normal opening time is from 33 ms to 37 ms. The synchronization of individual circuit breakers within a group should be within 10 ms for both closing and tripping actions.