24-51-00
 ALL  ú ú 04 Page 9 ú Jun 18/00
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
747-400
MAINTENANCE MANUAL

 (8)  
The utility buses are supplied through ELCUs, and these protective devices will operate to isolate utility buses from the other buses supplied from the same source if a short circuit fault condition occurs on the utility load buses.

 (9)  
In addition, each of the bus feeder protective devices is located within the differential protection zone of the applicable channel, main generator channels 1 through 4, so that a failure of the bus feeder protection device will activate backup protection and de-energize the main generator bus.

 (10)
 The ac standby bus supplies single-phase power to flight critical ac equipment. Under normal system operating conditions, this bus is supplied through the P6 power distribution panel, from ac bus No. 3. The standby bus is automatically transferred to the standby system static inverter upon loss of ac bus No. 3, if the standby power switch is in the AUTO position (Ref 24-29-00).

 (11)
 The ground handling bus (GHB) is used for ground operations and can only be powered on the ground.

 (12)
 Two solid state control relays are provided in each BCU to operate an external, 3-position center off ground handling relay (GHR). However, the GHRs are controlled only by BCU No. 1. The GHR contains two coils, one for external power (GHR-XP) and one for auxiliary power (GHR-AP). The separate outputs from BCU No. 1 are each capable of 500 mA continuously at 28 volts dc.

 (13)
 When external power is applied to the No. 1 external power receptacle, the BCU samples the average of all three phases for:

 (a)  
Voltage (104.5 to 125 volts ac)

 (b)  
Frequency (370 to 430 Hz)

 (c)  
Phase rotation (A, B, C)

 (d)  
No faults on the interlock

 (14)
 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 GHB:

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

 (b)  
Airmode: airmode 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.

 (15)
 If all of the conditions are satisfied and the inhibits are not present, the BCU energizes the two GHRs. External power is then applied to the GHB.

 (16)
 The auxiliary generator control unit (AGCU) samples power being supplied by the No. 1 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. 1. If external power is not available, the GHRs have not failed and the airplane is not in airmode, the APU generator No. 1 will power the GHB.

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 24-51-00
 ALL  ú ú 04 Page 10 ú Jun 18/00
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
747-400
MAINTENANCE MANUAL

 (17)
 The main deck cargo handling bus is used for ground operations and can only be powered on the ground.

 (18)
 Two solid state control relays are provided in each BCU to operate two external, 3-position center off main deck cargo ground handling relays. However, the main deck cargo handling relays are controlled only by BCU No. 2. The relays contain two coils, one for external power and one for auxiliary power. The separate outputs from BCU No. 2 are each capable of 500 mA continuously at 28 volts dc.

 (19)
 When external power is applied to the No. 2 external power receptacle, the BCU samples the average of all three phases for:

 (a)  
Voltage (104.5 to 125 volts ac)

 (b)  
　
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