Modern aircrafts have 2 electrical systems, left and right. So is it possible that only one system is.........

Question

...... working while being on the ground (like APU1 or EXT1)? Or do both systems need to have power (from the respective APU or external power)?

Answer 1

If you're talking about twin-engined jet airliner, basically they have 4 sources of electricity: 2 engine generators, 1 apu generator, and the battery. The electrical source than distributed through a series electrical buses, which are AC Buses, Standby Buses, etc, and not a mere left and right system.

It's designed that way so in case of any generator failure, the remaining generator can power the essential aircraft system.

APU generator can normally supply 2 electric bus simultaneously while on the ground, while they always have the option to get the power from external source in case of APU failure.

Answer 2

The answer to your Q is yes, let us say, as your example APU (when you say APU1 it means APU generator 1 & 2 is APU gen 2, only B747 classic has 2generator that is why APU1&APU2) A normal APU w/ 1 gen., powers both the left & the right side of the aircraft through a "Bus Tie Contactor" or  a "SIC" - 747 uses this & i forgot what it denotes.

As you know all this, then you are aware that there's a switch also that would allow only the powering of the Ground SVC Bus which I think you know is for ground crews to do Cargo, cabin cleaning & refueling possible only while cockpit instruments and cabin lightings are not powered. As I have said long ago that in A340 losing one generator in flight, the whole aircraft is powered,  losing two generators half of the aircraft is powered (not necessarily left or right side only), losing 3 generators - aircraft is shedding power and essential instruments for navigation & landing are powered, losing 4 generators you have the battery to power the essential buses + RAT (Ram Air Turbine) gives 4.5KVA powering to essential buses (NAV &LDG)