They also meet or exceed stringent IEEE, NEMA, and ANSI standards. For either the open- or closed-transition configuration, if the primary source is lost, the control initiates an open-transition transfer to the emergency source by tripping the primary source breaker and closing the emergency source breaker. When open-transition retransfer is selected, the switch first verifies that the primary source has returned, opens the emergency source breaker, and then closes the primary source breaker. When closed-transition retransfer is selected, the switch first performs a synch check to ensure that the primary source is in synchronism with the emergency source, then parallels the two sources by closing the primary source breaker, and finally opens the emergency source breaker.
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