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747-400 MAINTENANCE MANUAL
 DC ELECTRICAL LOAD DISTRIBUTION - DISCRIPTION AND OPERATION
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General
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 A.  The four main transformer rectifier units (TRUs), two external power TRUs, and the main and APU battery/battery chargers provide the 28-volt dc power for all the essential and non-essential dc loads.
 B.  Thermal circuit breakers (TCBs) and remote control circuit breakers
 (RCCBs) are utilized to protect the dc supplies and the load equipment.28-Volt DC Power Distribution (Ref 24-61-00)
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 A.  The 28-volt dc power for the four main dc buses is supplied by the four main TRUs.
 B.  External power TRUs supply 28-volt dc to the two dc ground handling buses.
 C.  The main battery/battery charger provide power for the main battery hot bus.
 D.  The APU battery/battery charger provide power for the APU battery hot bus.
 E.  Power for the main battery bus is supplied by either dc bus 3 or the main hot battery bus.
 F.  Power for the APU battery bus is supplied by either dc bus 3 or the APU
 hot battery bus.Operation
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 A.  Functional Description
 (1)  
The four main dc buses: dc bus 1, 2, 3, and 4 are connected between their individual TRU and the dc isolation relay (DCIR) of each IDG channel.

 (2)  
Each main dc bus can be powered by its own TRU or by the other main dc buses through the DCIRs, TCBs, and the dc tie bus.

 (3)  
The 28-volt dc power from the four main dc buses is distributed to the load distribution centers, divided into sections, and then sent to the load equipment through TCBs and RCCBs for fault isolation and protection. The four main TRUs are protected by the three phase input TCBs.

 (4)  
The RCCBs are circuit breakers that are controlled from the flight deck by TCBs. When the RCCB senses an overload, it opens, disabling the affected circuit and also tripping its controlling TCB in the flight deck. The RCCB is reset by resetting the TCB on the flight deck panel.

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747-400
MAINTENANCE MANUAL

 (5)  
Two 28-volt dc ground handling buses supply the dc loads required for airplane servicing and maintenance operations on the ground. The external power TRUs, dc buses, and loads are protected by TCBs.

 (6)  
RCCBs are used for protection and control when suppling power to the main and APU battery buses from dc bus 3 and the main and APU hot battery buses.

 (7)  
The main and APU battery chargers input and output wiring is protected by three phase TCBs on the ac input to the units. The battery chargers also have built-in protection.

 (8)  
TCBs are also used to provide protection for the individual loads supplied from the main and APU battery buses and the main and APU hot battery buses.

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747-400 FAULT ISOLATION/MAINT MANUAL
DC ELECTRICAL LOAD DISTRIBUTION
COMPONENT  SHT 102 FIG.  QTY  ACCESS/AREA  REFERENCE 
CIRCUIT BREAKERS - 1  FLT COMPT, P6 
DC 1 DIST P6, C8628  1  6A9  * 
DC 1 DIST P7, C679  1  6A10  * 
DC 2 DIST P6, C8629  1  6A17  * 
DC 2 DIST P7, C680  1  6A18  * 
DC 3 DIST P6, C8630  1  6A27  * 
DC 3 DIST P7, C681  1  6A28  * 
DC 4 DIST P6, C10270  1  6A35  * 
DC 4 DIST P7, C682  1  6A36  * 
CIRCUIT BREAKERS - 117AL, MAIN EQUIP CTR, P180 
　
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