How do the synchronizing lamps indicate the correctness of phase sequence between existing and incoming Alternators?

The correctness of the phase sequence can be checked by looking at the three sets of lamps connected across the 3-pole of the synchronizing switch. If the lamps grow bright and dark in unison it is an indication of the correctness of the phase sequence. If on the other hand, they become bright and dark one after the other, connections to any two machine terminals have to be interchanged after shutting down the machine.