(2)
The APU battery supplies dc power to the APU hot battery bus which is connected directly to the battery output. BCU No. 2 receives inputs from the DCCS which monitors current flow in the APU battery system.
(3)
The main and APU battery chargers are connected to their individual battery and battery hot bus. Each battery charger provides two different functions, one is to provide regulated charging of the battery, the other is to operate as a transformer rectifier to supply bus power if three of the main TRUs fail.
(4)
The ground service bus supplies the 115 volt ac 3 phase power utilized by the main and APU battery chargers.
(5)
The battery charger disable relays remove the input 115 volt ac power to the battery chargers when the battery overheats or the standby power switch is placed to the BAT position. In addition, the APU battery charger disable relay will remove the input to the APU battery charger during APU starting when the APU battery bus transfer relay energizes.
(6)
The main transformer rectifier units 1, 2, 3, and 4 supply 28 volt dc power to their dc load buses. Input 115 volt ac 3 phase power to the TRUs is received from the ac bus in their individual channel.
(7)
DC current sensors (DCCS) in each TRU channel provide operation monitoring for the GCUs. Each GCU processes this information and sends it to the BCUs which then sends it to the EIU for electrical system indication.
(8)
Each dc isolation relay (DCIR) is manually controlled by its respective bus tie switch in the flight deck or automatically by the GCUs when the autoland mode is requested.
(9)
The DCIRs operate to isolate their dc bus when energized or to tie the buses together on the dc tie bus when de-energized.
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24-30-00
ALL ú ú 10 Page 2 ú Jun 18/01
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
747-400
MAINTENANCE MANUAL
DISABLE RELAY APU START
TO BCU
RELAY
ENERGIZES WHEN IN SINGLE TRU OPERATION
ENERGIZES FOR 65 SEC FOR APU START (REF 49-11-00) DC Generation Simplified Schematic Figure 1
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ALL ú
ú 06 Page 3
ú Jun 10/90
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
747-400
MAINTENANCE MANUAL
(10)
With the standby power switch in the OFF or AUTO position, dc bus 3 normally powers the main and APU battery buses through the energized contacts of the battery transfer relay.
(11)
DC standby power is provided by the main and APU hot battery buses, if power is lost on dc bus 3 or the standby power switch is placed to the BAT position. The dc voltage sense relay will de-energize also de-energizing the battery transfer relay.
(12)
With the battery switch positioned to ON, a ground is provided for the coil of the main battery relay. The main battery relay energizes enabling the main and APU battery buses to be powered by the main and APU hot battery buses.
(13)
A loss of dc bus 3 will also cause the battery chargers to enter the TRU mode of operation if 115 volt ac is on the ground service bus. Placing the standby power switch to the BAT position will disable the battery chargers.
(14)
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