(60)
When loads are shed as a result of an overload condition, the opening of the BCU load management relays occurs such that the ground and the +28 volts to the ELCU are removed almost simultaneously.
(61)
Load restoration, however, requires that the ground to the ELCU be re-applied before the +28 volts. A 50 millisecond time delay is therefore used to delay that relay which supplies the 28 volts to the ELCU. This allows proper coordination for ELCU reclosing.
(62)
When one BCU is inoperative, and loads have been shed, those ELCUs being supplied the 28 volts from the failed BCU cannot be reset upon load restoration since this relay coordination no longer exists.
(63)
The instrument bus voltage sense unit (IBVSU) operates to automatically transfer the captain's or first officer's flight instrument transfer bus from the primary power source to the alternate power source when power to the primary power source is lost. Primary bus sensing circuitry internal to the IBVSU contstantly monitors each phase of the primary power. When the voltage of any phase of the primary source falls below 97 |2 volts for 187 |12 ms, the IBVSU logic circuitry energizes internal relays which tranfers the associated flight instrument transfer bus to the alternate source. The IBVSU will transfer the associated flight instrument transfer bus back to the primary source when the primary source voltage (all three phases) recovers to above 106 |2 volts for
1.2 |0.2 seconds. Power interruption to a flight instrument transfer bus will not exceed 20 ms when transferring from an alternate to a primary source.
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
747-400
MAINTENANCE MANUAL
(64)
The No. 3 main ac bus is the primary source for the captain's flight instrument transfer bus. The No. 2 main ac bus is the primary source for the first officer's flight instrument transfer bus. The No. 1 main ac bus is the alternate source for both the captain's and first officer's flight instrument transfer buses.
(65)
The IBVSU requires a minimum of 20 volts dc for control power. This is supplied by the No. 1 main dc bus. Loss of control power will inhibit bus transfer to the alternate source. If the unit has already transferred to the alternate source, a transfer back to the primary source will occur within 127 |75 ms after loss of control power, regardless of ac voltage sensing levels.
(66)
Auxiliary contacts in each IBVSU are used to provide (via the BCUs) indication on EICAS when a flight instrument transfer bus has transferred erroneously, or when one fails to transfer when required.
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
747-400 MAINTENANCE MANUAL
GROUND HANDLING AND GROUND SERVICE SELECT RELAYS - REMOVAL/INSTALLATION
_______________________________________________________________________
1. General
_______
A. This procedure has two tasks. One task is the removal of the ground handling relays (R117 and R118) and the ground-service select relays (R14, R15, R1168, and R1169). The other task is the installation of the relays.
B. The installation task includes an operational test of the relays.
C. The relays are in the P714 auxiliary/external No. 1 power center, in the E/E compartment.
NOTE: You can get access to the E/E compartment through the access
____ door behind the nose wheel or through the forward cargo compartment.
TASK 24-51-01-004-002
2. Ground Handling or Ground Service Select Relay Removal (Fig. 401)
______________________________________________________
A. References
(1)
24-22-00/201, Manual Control
(2)
IPC 24-22-60 Fig. 1
(3)
IPC 24-51-60 Fig. 3
(4)
WDM 24-22-61, 24-51-61, 24-51-71
(5)
SSM 24-22-06, 24-51-08
B.
Access
(1)
Location Zone 118 Electrical and Electronics Compartment, RH
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