24-23-00
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
(f)
(g)
A
747-400 MAINTENANCE MANUAL
The BCU provides control of the three-position ground handling relay (GHR). The ground handling bus may be powered by external power or auxiliary power when the airplane is in the ground mode. When the ground handling bus is not powered, the GHR is deenergized (Ref 24-50-00). 1) The BCU commands the GHR to close to external power
position and supply external power to the ground handling
bus when the following conditions are all true:
a) External power is available.
b) Airplane is in ground mode.
2) The BCU commands the GHR to close to the auxiliary power position and supply power to the ground handling bus when the following conditions are all true: a) External power is not available. b) Auxiliary power ready signal is present. c) Airplane is in ground mode.
The SSB is controlled by the BCU thru SSB control switch operation, BCU software, or BCU hardware. 1) The SSB can be closed or tripped through the BCU by the
operation of the SSB control switch. The close operation of the switch is inhibited when both halves of the synchronizing bus are powered by unlike sources. Both the close and trip operations of the switch are inhibited when the airplane is in air mode.
2) The SSB is commanded to trip by the BCU software when any of the following conditions are true: a) Opposite BCU alive signal is not detected. b) Some power transfers to adjacent external power. c) Some power transfers to adjacent auxiliary power. d) Adjacent auxiliary power is available and the following
conditions are all true:
1.
The adjacent synchronizing bus half or both
_ synchronizing bus halves are not powered.
2.
Airplane is in air mode.
_
3.
Actuation of the adjacent APU GEN switch is
_
detected.
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24-23-00
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
747-400 MAINTENANCE MANUAL
4.
A negative sequence voltage (NSV) condition or
_ differential current protection (DP) condition is detected by either the adjacent or opposite BCU.
5.
An opposite BCU SSB trip signal is detected.
_
3) The SSB is commanded to trip by the BCU hardware when any of the following conditions are true: a) The redundant differential fault protection (RNDT DP)
circuitry of the adjacent BCU senses a DP condition not detected by software. b) A NSV condition or a DP condition is detected by the software of the opposite BCU.
c) The redundant differential fault protection (RNDT DP) circuitry of the opposite BCU senses a DP condition not detected by software.
d) A NSV condition or DP condition is detected by the software of the adjacent BCU.
4) The SSB is commanded to close by the BCU software when any of the following conditions are true: a) Both halves of the synchronizing bus are powered by IDG
power and conditions are appropriate for paralleling.
b) The adjacent synchronizing bus is powered by adjacent external power, the opposite synchronizing bus is not powered, the SSB is open, and the SSB control switch is pushed.
c) The adjacent synchronizing bus is powered by adjacent auxiliary power, the opposite synchronizing bus is not powered, the SSB is open, and the SSB control switch is pushed.
d) Either IDG power, external power, or auxiliary power is brought onto an airplane that is not powered and the SSB is open.
e) An opposite BCU SSB close command is detected.
(h) The BCU provides signals to control eight separate loads. When an overload condition exists, the BCU begins shedding loads in sequential order. Load shedding continues until the overload condition is no longer detected. The BCU also provides automatic load restoration of loads that have been previously shed. 1) External Power and Auxiliary Power Load Management
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