These systems in general use today are unsupervised by phase angle relays and can subject the motors to severe transient torques due to an out of phase condition. In addition, should the tripping breaker be abnormally slow, the sources can be briefly paralleled introducing the problems of Parallel Transfer. Supervision of the Fast Transfer requires a high speed sync-check relay to ensure that the phase angle between the motor bus voltage and the station service source voltage is within acceptable limits prior to closing the start-up source breaker. Due to the dynamic nature of the phase during the open circuit period, the sync-check relay must be able to accurately determine the phase in the fastest possible time. A discussion of the phase characteristic of the motor bus is inciudea in Section III of this paper.