AC and DC power distribution

Transformer rectifier units convert AC supply into DC power. The overhead selectors control TRU operation; use ELEC to check the actual bus supply.

Open ELEC

Identify AC source and transformer rectifiers.

Identify the AC source and the displayed transformer rectifier units.

TRU status

FSS describes AUTO for TRU 1 and 2 as connecting each unit to its associated DC bus; OFF isolates that unit.

DC ESS BUS

FSS says TRU ESS in AUTO connects the unit to ESS BUS 3; OFF isolates it. Check the actual ELEC indication.

Review batteries

Check the battery selector positions and battery indications. FSS distinguishes battery 1 ON from battery 2 AUTO in its described normal arrangement.

DC GPU

FSS says DC GPU supplies the APU start bus.

FSS says a DC GPU supplies only the APU start bus and does not charge the batteries. Do not assume that a DC external connection provides the same service as the AC GPU.

Before leaving

Use the current shutdown checklist and wait for the applicable APU shutdown indications before selecting the batteries off. ^[94]