Electrical power
On the basis of new for old powers..
When the APU generator is available the APU GCU closes the aux power breaker APB and the Bus Power Contactor Unit BPCU opens the Secondary electrical power contactor EPC. Sec Ext power is removed and the APU generator comes on line to power the left main AC bus and the right main bus via BTB's if the primary ext power is not available.
If both the APU GEN and Primary ext power are available the logic is to use the pri ext power to power the right main AC bus, AC ground handling bus and the ground service bus. the APU GEN will power just the left main AC bus.
Manually selecting the secondary ext power will remove the APU GEN by opening the APB and allow the sec ext power contactor to close and power the left main AC bus. All GCU's and BPCU communicate together on the ARINC 629 bus for correct opening and closing of the breakers and contactors.