An interlock circuit prevents the AAPP from being connected to the synchronising busbar when Alternator No.1, No.3. or No.4 are connected. The correct procedure for connecting the AAPP to the synchronising busbar with No.1, No.3 or No.4 Alternator connected, is to isolate the connected alternator, which will automatically connect the No.2 Alternator, allowing you to then connect the AAPP to the synchronising busbar.