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Reply To: Switchgear Riddle No.18 – Out of phase Breaking Current


    The out of phase current switching capability ordinary is specified in generator circuit breakers and described by IEEE C37.013 clause 5.12. As the machine slows down, it loses synchronism with the power system, such that there can be a significantly higher voltage across the contacts of the interrupter as the circuit breaker opens to interrupt the fault. Theoretically, this out-of-phase condition can be as great as 180 electrical degrees, but as a practical matter, is considered to be no greater than 90 electrical degrees. The out-of-phase condition is limited to 90 electrical degrees to prevent damage to the shaft of the machine if switching occurred with the generator and system more than 90 degrees out-of-phase. The out-of-phase switching application also exists when a machine is being brought up to speed and is being synchronized with the power system. Synchronizing equipment, including a synchronism check function, is almost universally used with large machines to prevent closing of a circuit breaker while the machine is out-of-phase with the power system by more than a nominal amount. Closing under a severe out-of-phase condition would lead to very high transient currents that would cause the protective relays to initiate circuit breaker opening. However, due to the time required for interruption, damage to the machine would most likely occur before the interruption process could be completed. However the out of phase conditions is possible in distribution networks generally and distribution circuit breakers should be capable to overcome these situations. For more information about out of phase capability of distribution CB refer to IEEE C37.06.