(c)  
Phase rotation (A, B, C)

 (d)  
No faults on the interlock

 (20)
 After the above conditions are satisfied, the BCU turns on the AC CONNECTED light on the external power panel and the AVAIL light on the electrical system control module. At this instant, only two conditions may inhibit powering the main deck cargo handling bus.

 (a)  
Main deck cargo handling relay failed: the BCU has internal circuits that monitor the operation of the relays. If either relay has failed, the BCU will not energize the main deck cargo handling bus.

 (b)  
Air mode: Air mode is defined as airplane in the air or any three engines operating above 75 percent N2. The BCU receives the information from air ground logic and IDG speed inputs which indicate engine rpm.

 (21)
 If all of the conditions are satisfied and the inhibits are not present, the BCU energizes the two main deck handling relays. External power is then applied to the main deck cargo handling bus.

 (22)
 The auxiliary generator control unit No. 2 (AGCU) samples power being supplied by the No. 2 APU generator. When the APU output speed is above 7600 RPM and the voltage is between 100 and 130 volts ac, the AGCU sends an APU power ready signal to the BCU. The BCU turns on the AVAIL light for APU generator No. 2. If external power is not available, the main deck cargo handling relays have not failed and the airplane is not in air mode, the APU generator No. 2 will power the main deck cargo handling bus.

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747-400
MAINTENANCE MANUAL

 (23)
 The ground service bus (GSB) is used for both ground and flight operations.

 (24)
 Two solid state control relays are provided in the BCU to operate the external ground service select relays (GSSRs) and the external ground service transfer relays (GSTRs). These control relays operate in conjunction with a ground service momentary action pushbutton switch next to Door 2L. These solid state control relays supply continuous 28 volt dc (250 milliamps) power to the coils of the GSSR and GSTR.

 (25)
 The components associated with the GSB are as follows:

 (a)  
BCU No. 1

 (b)  
Ground service switch

 (c)  
Two ground service transfer relays, one relay is used for the ground service bus and the other for the ground service utility bus. When energized, the relays provide a path for APU or external power to the GSB. When de-energized, the relays provide a path for ac bus 1 power to the GSB.

 (d)  
Two ground service select relays, one relay is used for the ground service bus and the other for the ground service utility bus. When energized, the relays select external power for the GSB through the GSTRs. When de-energized, APU power is selected.

 (26)
 With ac bus 1 powered, the GSTR is de-energized allowing ac bus 1 to power the GSB.

 (27)
 With ac bus 1 not powered and the ground service switch on, and external power No. 1 applied, BCU 1 looks for the following conditions:

 (a)  
GSSR or GSTR failed: the BCU has internal circuits that monitor the operation of these relays. If any of the relays has failed, the BCU will not energize the relays.

 (b)  
External power available (power okay).

 (28)
 When the above conditions are satisfied, the BCU energizes the GSTRs and GSSRs allowing external power No. 1 to power the GSB.

 (29)
 With ac bus 1 not powered and the ground service switch on, and APU generator No. 1 applied, BCU 1 looks for the following conditions:

 (a)  
GSTR failed: the BCU has internal circuits that monitor the operation of the GSTRs. If either relay has failed, the BCU will not energize the relays.

 EFFECTIVITYùùùùùùùùùùùùùùùùùùùùùùùùùùùùùù. ú

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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
747-400 MAINTENANCE MANUAL
 (b)  
　
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