Haven't flown in a Mi-8 for a while, so most of this is from memory. :D
Engine generators generate 3-phase 204V AC, that is also:
-transformed to 115V AC single phase (in case of both eng gen failure, the PO-500A inverter is used to power essential equipment)
-transformed to 36V AC 3-phase (in case of both eng gen failure, the PT-200Ts inverter is used to power essential equipment)
-transformed to 36V AC single phase (controlled by instrument inverter switch)
-rectified to 27V DC
Stand by generator (from APU) generates 27V DC that can be connected only to the battery bus.
By using equipment test switch, you're connecting the APU to the rectifier bus, and this is used for equipment testing (:)), but can be also used in flight for emergency situations to power flight essential equipment
AFAIR, the pitch and bank channel of the autopilot system is using attitude data from the co-pilot's attitude indicator, which isn't connected to the essential bus and that's why it could not be turned on. If you had requested external power, you would be able to.
Yes, the eight has a slightly complex electrical system :joystick:
AlphaOneSix, feel free to correct any misinformation. :D