Electrical power sources

A generator can be available while a bus is still being supplied by something else. The ELEC synoptic lets you follow the sources, distribution and bus indications instead of judging the system from the overhead panel alone.

Initial power

With the aircraft secured, check the overhead source controls and note whether ground power is connected. Then open ELEC and EICAS; note which sources are available and which buses are powered.

Connect ground power

Use the ground-service control for the source your setup offers. For an AC GPU, check AVAIL before selecting it with the overhead button. FSS describes the DC GPU separately: it supplies only the APU START BUS and does not charge the batteries. FSS does not specify how to connect or disconnect a DC GPU.

Confirm supply

Read the source and bus indications. FSS shows IN USE on the AC GPU button when AC ground power supplies the aircraft. On ELEC, the DC GPU has a separate icon and IN USE indication when connected and supplying power. Availability alone does not establish which buses are powered.

APU supply

Start the APU when required. Wait for the APU electrical source to become available and confirm the generator selection and source indications.

Engine-source transfer

After an engine start, monitor the relevant generator and buses. The FSS procedure guide recommends waiting after N2 stabilises or watching the electrical synoptic before removing the previous supply.

Remove the AC GPU

This step applies only when an AC GPU is connected. Confirm that a replacement source is supplying the aircraft, press the AC GPU button OUT, and check that IN USE changes to AVAIL while the unit remains connected. Then disconnect the AC unit. The AC GPU button does not control the separate DC GPU; FSS does not document its connection control. ^{[93] [56] [94]}