(j)
Tighten the nuts.
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747-400 MAINTENANCE MANUAL
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(3)
For the applicable TRU, remove the DO-NOT-CLOSE tag and close this circuit breaker:
(a)
P414 Power Distribution Center - Left
1) 414B24 EXT POWER TRU A
2) 414B21 EXT POWER TRU B
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747-400 MAINTENANCE MANUAL
AC ELECTRICAL LOAD DISTRIBUTION - DESCRIPTION AND OPERATION
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General
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A. The 115 volt ac 3-phase, the 115 volt ac single phase, and the 28 volt ac single phase power is supplied to the distribution buses and distributed to several electrical loads throughout the airplane.
B. Thermal circuit breakers (TCBs) provide protection for the different power supplies. Also, a short circuit fault condition on any phase of any bus section will be isolated by tripping the appropriate TCB, so that other unfaulted sections of the bus will remain operative.
115-Volt AC Power Distribution (Ref 24-51-00)
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A. The 115 volt 3 phase ac power is supplied to the four main generator buses by the integrated drive generators (IDGs), auxiliary power unit (APU) generators, or by external power.
B. The 115 volt 3 phase ac power for the ground service bus (GSB) is supplied by ac bus 1, APU generator No. 1, or external power No. 1.
C. The 115 volt 3 phase ac power for the ground handling bus is supplied by either external power No. 1 or the APU generator No. 1. This bus is for ground operation only and is not powered in flight.
D. The 115-volt 3-phase ac power for the main deck cargo handling bus is supplied by either external power No. 2 or the APU generator No. 2. This bus is for ground operation only and is not powered in flight.
E. The 115 volt 3 phase ac power for the flight instrument transfer buses is supplied from ac bus 3 for the captain's transfer bus and ac bus 2 for the first officer's transfer bus. If ac bus 2 or 3 looses power, the transfer buses are supplied by ac bus 1.
F. The ac standby bus, which supplies 115 volt single phase ac power to flight critical loads, is powered by ac bus 3 or the static inverter if ac bus 3 is lost (Ref 24-29-00).
28-Volt AC Power Distribution (Ref 24-53-00)
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A. Autotransformers supplied by 115 volt single phase power from ac bus 3 and the ground service bus provide the 28 volt single phase ac power for the main 28 volt ac bus and the 28 volt ac ground service bus respectively.
Operation
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A. Functional Description
(1) The four main ac buses: ac bus 1, 2, 3, and 4 are connected between their individual generator circuit breaker (GCB) and bus tie breaker (BTB) of each IDG channel.
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MAINTENANCE MANUAL
(2)
Each main ac bus can be powered by either its own IDG through the GCB or from the other IDGs, APU generators, or external power through the sync bus, bus tie breakers (BTBs), and the split system breaker (SSB).
(3)
Distribution of power from the four main ac buses is accomplished by routing the power to various load distribution centers, then to individual sections within each distribution center. Protection is provided for the individual sections through electrical load control units (ELCUs) and/or ac distribution TCBs.
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