B777 External Power/APU
 

Any idea of the reasoning behind why the Secondary external power drops offline once the APU starts? I assume there is a fuel saving to bring it back online again but always wondered why. There doesn't appear to be any problem operating the APU for Packs only and using ground electric?

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.

Thanks for the detailed reply.
Is there an approximate figure of fuel saving achieved by re-selecting it and not using the APU for electrics, but only bleed air? I haven't been able to find a fuel rate figure on the maintenance pages.
I only ask out of interest as there often seems to be a reluctance to do this and go to APU electrics as soon as packs are needed on the ground even if external power is provided.