(a)
The AGCU, via the BCU, will provide 28 volt dc close and trip signals to the APB. Momentary actuation of the APU GEN switch when the APB is open and generator power is "available" shall cause a close signal to the APB via the AGCU circuitry. Momentary actuation of the APU GEN switch when the APB is closed will cause a trip signal to be sent to the APB from the AGCU. The BCU will provide the interface between the APU GEN switch, the AGCU, and the APB. The APB trip coil can also be energized directly from the BCU. Any protective function trip of the AGCR will also energize the APB trip coil.
(b)
The APU generator is "available" when the generator output voltage and frequency are within normal limits and a transfer to APU generator power is permitted.
(c)
When the APU generator is "available" the AVAIL indicator in the APU GEN switch is illuminated. In flight, the AVAIL indicator light is inhibited.
(d)
When the APB is closed, the ON indicator in the APU GEN switch is illuminated. In flight, the ON indictor light is inhibited.
(11)
The purpose of the SSB is to allow auto paralleling of the two sync buses for equal load sharing when powered by 115 volt ac, 400 Hz from the IDGs. The SSB also prevents the sync buses from paralleling while being powered by two differenct sources.
(a)
The SSB is operated in conjunction with the momentary contact SSB switch. Each operation of this switch will cause the SSB to alternately close and trip, except that the SSB switch close operation will be inhibited when both halves of the sync bus are energized from unlike sources. This switch control is inoperative when the airplane is in the air mode.
(b)
Either BCU will operate to open the SSB for sync bus negative sequence voltage protection and sync bus differential fault protection.
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747-400 MAINTENANCE MANUAL
(c)
Following a SSB trip due to a sync bus fault, the SSB can be closed by establishing IDG power on both sync bus halves, operating the SSB switch (on the ground only), or bringing the first ac source onto a dead airplane.
(d)
The OPEN indicator light below the SSB switch will be illuminated whenever the SSB is open. When the SSB is closed the indicator light will be extinguished.
B.
Control
(1)
External power, APU power, or engine power can be used to supply electrical power to the airplane. To apply or remove electrical power to/from the airplane, refer to 24-22-00/201.
(2)
When external power is supplied and the airplane is put in the air mode, you must connect a ground to specified connectors and pins on the P415 panel to permit you to have manual control (on/off) of electrical power from the flight compartment. Refer to the task to Prepare the Safety Sensitive Systems for Air Mode (Ref 32-09-02/201) for more information.
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
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747-400 MAINTENANCE MANUAL
MANUAL CONTROL - MAINTENANCE PRACTICES
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1. General
_______
A. This procedure contains the tasks to remove and apply electrical power to these airplane buses:
AC and DC buses
Ground handling bus
Ground service bus.
The electrical power can be supplied by these power sources:
External power
APU generator power
Engine-driven generator power.
B. The tasks are:
Apply External Power to the AC Buses Apply External Power to the Ground Handling Bus Apply External Power to the Ground Service Bus Remove External Power from the AC Buses Remove External Power from Ground Handling Bus Remove External Power from the Ground Service Bus Apply APU Generator Power to AC Buses Apply APU Generator Power to Ground Handling/Ground Service Buses Remove APU Generator Power from AC Buses Remove APU Generator Power from Ground Handling/Ground Service Buses Apply Engine-Driven Generator Power Remove Engine-Driven Generator Power
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