Originally Posted by
Uplinker
I think you are referring to the ground services bus:
When there is a crew change, the outgoing crew can leave the aircraft without completely shutting it down and ‘putting it to bed’. The ground services bus powers internal lights and power outlets, (and perhaps cargo doors and battery chargers), from the ground power unit (GPU) outside, to allow cleaning and catering staff to turn the aircraft around in the absence of a crew.
To achieve this, a ground service switch, usually located in the forward galley, is held or selected and then the ground power selector in the cockpit can be switched off. All the aircraft electricals, except those I have mentioned above are powered down, leaving the aircraft in a safe state, so not requiring the presence of a pilot in the cockpit.
Same for an Airbus.
We do that when the aircraft is done for the night, not for crew changes. It bears mentioning that the ceiling switch is electromagnetic and will only stay on if ground power is available. At least that's how our birds are (A320 sn 5,000+).