Page 1 D633W101-BEJ Jan 05/2002
BOEING PROPRIETARY -Copyright # -Unpublished Work -See title page for details.

TRU -GENERAL DESCRIPTION

General
Four TRUs change 115v ac power into 28v dc power to supply the main dc buses. The TRUs normally get power from the transfer buses. If the transfer buses have no power, the RAT generator supplies power to the two center TRUs.
The dc subsystem card controls all the relays. The card is in the EEU in the P310 standby power management panel.
General Description -Left and Right DC Buses
During normal operation, the left TRU supplies power to the left dc bus, and the right TRU supplies power to the right dc bus. If a TRU fails, the dc bus tie relay closes to let the other TRU supply power to the two buses.
General Description -Flight Instrument DC Buses
During normal operation, TRU C1 supplies power to the captain’s flight instrument bus, and TRU C2 supplies power to the first officer’s flight instrument bus. If a TRU fails, the capt-F/ O bus tie relay closes to let the other TRU supply power to the two buses.
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777 AIRCRAFT MAINTENANCE MANUAL

TRU -GENERAL DESCRIPTION

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TRU -FUNCTIONAL DESCRIPTION -LEFT AND RIGHT DC BUS

General
Normally the right TRU supplies power to the right dc bus, and the left TRU supplies power to the left dc bus. If one TRU does not operate, the other TRU can supply power to both buses through the dc tie relay.
The EEU in the standby power management panel has a dc subsystem card that controls the dc tie relay. The card gets voltage inputs from the left and right dc buses. The card also monitors the output current from each TRU.
DC Tie Relay -Normal Control
The dc tie relay is normally open to isolate the left and right dc buses. If one of the two dc buses has low voltage, the dc subsystem card closes the dc tie relay. This connects the left and right dc buses. The dc tie relay stays closed until the card senses output current from both TRUs. If the tie relay closes and the card senses low voltage on the two dc buses, the card opens the relay. This is typical of a bus fault.
DC Tie Relay -Autoland Control
When the dc tie relay is open during autoland, the dc subsystem card keeps it open. This continues to isolate the left and right dc buses. If the relay is closed before autoland, it stays closed. ELMS rejects the autoland request to isolate the buses.
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TRU -FUNCTIONAL DESCRIPTION -LEFT AND RIGHT DC BUS

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TRU -FUNCTIONAL DESCRIPTION -CAPT & F/O FLIGHT INSTRUMENT DC BUSES

General
Normally the TRU C1 supplies power to the captain’s flight instrument bus, and the TRU C2 supplies power to the first officer’s flight instrument bus. If TRU C1 does not operate, TRU C2 supplies power to the two buses through the captain-F/O bus tie relay. If TRU C2 does not operate, TRU C1 supplies power to the two buses through the captain-F/O bus tie relay.
During standby power operation, the RAT generator supplies power to the two flight instrument buses through the TRU C1 and C2 relays.
The EEU in the standby power management panel has a dc subsystem card that controls the captain-F/O bus tie and TRU C2 relays. The card and the RAT GCU control the TRU C1 relay.
Standby Power Control
When the RAT deploys, the RAT GCU sends a RAT power ready signal to the dc subsystem card. The RAT GCU also energizes the TRU C1 relay. This permits the RAT generator to supply power to the captain’s flight instrument bus through the TRU C1. The dc subsystem card supplies a backup signal to energize the TRU C1 relay.
If the right transfer bus has no power the card energizes the TRU C2 relay. This permits the RAT generator to supply power to the F/O flight instrument bus through the TRU C2. Energizing the TRU C2 relay also causes the dc subsystem card to energize the captain-F/O bus tie relay.
Captain-F/O Bus Tie Relay Control
The captain-F/O bus tie relay is normally open to isolate the captain’s and first officer’s flight instrument buses. If one of the flight instrument buses has no power and the main battery relay is not energized, the dc subsystem card energizes the captain-F/O bus tie relay. If one of the flight instrument buses has no power, the card also energizes the relay during autoland when the battery/captain isolation relay energizes. The relay connects the two flight instrument buses.
The control outputs of these two conditions latch to keep the relay energized. The latch releases if any of these occur:
　
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