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 A.  The main TRUs are static devices which convert 3-phase nominal 115/200 volt, 400 Hz ac input power into unregulated 28 volts dc output power for the airplane main dc system.
 B.  Each TRU employs a transformer with a wye connected primary and paralleled wye-delta secondary connections. The 6-phase output from the secondaries is connected to a 6-phase full wave rectifier bridge and filtered to produce the desired output. Each TRU has an output voltage of 29 volts at no load and approximately 27 volts at rated load. The four main TRUs are rated for a continuous output load of 75 amps.
 C.  Forced air cooling is required when maximum output current is needed. However, the units may be operated at 25 to 30 percent of their rating with convection cooling only.
 D.  The TRUs are located on the E-30 equipment shelf in the forward-right main equipment rack.
 3.  DC Bus Isolation Relay (Fig. 2)
______________________
 A.  The dc isolation relay parallels the four main dc buses when de-energized during normal operation. When energized, they isolate their associated main buses. The four DCIRs operate in conjunction with the BTBs and the bus tie switches.
 4.  DC Current Sensor (DCCS) (Fig. 3)
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 A.  The DCCS is used in six locations in the airplane dc system to operate in conjunction with the batteries, TRUs, BCUs, and GCUs to provide the required current sensing. They are rated for continuous operation at 180 amps dc.
 B.  The DCCSs are used for the following functions:
 (1)  
Provide TRU current sensing to the GCUs and BCUs for indication on the EICAS maintenance page.

 (2)  
Provide monitoring of TRUs for proper operation and relaying this information to the GCUs and BCUs which provide failed TRU indication on EICAS and for triple channel autoland operation.

 (3)  
Provide battery current indication on the EICAS maintenance page and battery discharging indication at an advisory level on EICAS via the BCUs.

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A
747-400
MAINTENANCE MANUAL

SEE A
TRU 4 
SEE B TRU 1  SEE B TRU 2  SEE B TRU 3  SEE  B 
E-30 EQUIPMENT RACK 
A 

MAIN TRU (TYPICAL)
B
Transformer Rectifier Unit (TRU) Location Figure 1 (Sheet 1)
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A 747-400 MAINTENANCE MANUAL
SEE C
SEE  D 
MAIN DECK 
CARGO  TERMINAL LEAD 
TRANSFORMER RECTIFIER  C 
UNIT 
BULKHEAD 
TERMINAL 
BLOCK COVER 

FWD
D
Transformer Rectifier Unit (TRU) Location Figure 1 (Sheet 2)
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A

747-400 MAINTENANCE MANUAL
SEE A
DC BUS  DC BUS 
ISOLATION  ISOLATION 
RELAY  RELAY 
DC BUS  SEE  B  SEE  B 
ISOLATION  DC BUS 
RELAY  ISOLATION 
SEE  B  RELAY 
SEE  B 
P180 DC POWER DISTRIBUTION PANEL
DC BUS ISOLATION RELAY (EXAMPLE)  A 
TERMINAL NUT 
AND WASHER 
COVER 

B
DC Bus Isolation Relay Location Figure 2
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