(a)  
The APU START SOURCE switch, located on the P5 overhead panel, is in the TR position.

 (b)  
The APU battery overheat relay is not energized.

 (c)  
The 115 volt ac power is supplied to galley number 2.

 (3)  
During the APU start cycle, if the TRU is overheated, one or more of the three thermal switches in the TRU will trip and transfer the APU battery power to the starter motor. This allows continuation of the APU start cycle with minimal power interruption. This source switching will then remain in effect for the remainder of that APU start cycle regardless of the state of the thermal switches thereafter. The next APU start cycle may use TRU power to start only if all three thermal switches are no longer tripped.

 (4)  
If during the APU start cycle, the 115 volts ac power from galley number 2 fails, power to the TRU will be disconnected and the APU battery will be switched to continue the APU starting process.

 (5)  
Anytime during the start cycle, if the APU START SOURCE switch is moved from TR to BAT, power to the APU starter motor will automatically be transferred from the TRU to the APU battery.

 C.
Functional Description of Main Deck Cargo Handling TRU (Fig. 4)

 (1)  
There is one main deck cargo handling transformer rectifier unit (TRU) on combi airplanes. The TRU is a static device which converts the 115-volt ac 3-phase power from the ac ground handling bus to unregulated 28-volt dc power for the dc loads required during ground operation. The TRU is rated for a continuous output load of 20 amps.

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747-400
MAINTENANCE MANUAL

 (2)  
The main deck cargo handling TRUs utilize a wye to wye primary to secondary connection and a 3 phase full-wave silicon rectifier circuit.

 (3)  
The unit output voltage ripple does not exceed 5 percent rms of the dc output voltage over the full load range.

 (4)  
The TRU is designed to deliver an overload current of 30 amps (150 percent of rated load) for 5 minutes.

 (5)  
The TRU and its output connections to the dc cargo handling bus is protected from overloads by thermal circuit breakers on the 3-phase ac input to each TRU. Cooling is provided by natural convection only.

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747-400 MAINTENANCE MANUAL
 MAIN TRANSFORMER RECTIFIER UNIT - REMOVAL/INSTALLATION
______________________________________________________
 1.  General_______
 A.  This procedure has two tasks. One task is the removal of the main transformer-rectifier units (TRU) T1494, T1495, T1496 and T1497. The other task is the installation of the TRUs.
 B.  The tasks are the same for each TRU.
 C.  The installation task includes an operational test of the relay.
 D.  The four main TRUs are installed in the E-30 equipment rack, with the main battery charger and static inverter. The E-30 equipment rack is between the nose gear wheel well and the fuselage, on the right side. Access to the E-30 equipment rack is through an access opening forward of the E/E compartment.
 TASK 24-32-01-004-001
 2.  Main Transformer Rectifier Unit Removal (Fig. 401)_______________________________________
 A.  References
 (1)  
20-11-22/401, Rack-Mounted Electrical/Electronic Module

 (2)  
IPC 24-32-01 Fig. 1

 (3)  
WDM 24-32-11, 24-32-12

 (4)  
SSM 24-32-01

 B.  
Access

 (1)  
Location Zone 116 Area Between Nose Gear Well and Fuselage, RH

 C.  
Procedure

 S 864-002
 (1)  
Open these circuit breakers for the applicable main TRU, and attach DO-NOT-CLOSE tags:

 (a)  
P6 Main Power Distribution Panel
 1) 6A5 TRU 1
　
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