(c)  
APU power ready (power okay).

 (30)
 When the above conditions are satisfied, the BCU energizes the GSTRs allowing APU generator No. 1 to power the GSB.

 (31)
 Whenever the GSTR is energized, either from external or auxiliary power, the ON indicator light in the ground service pushbutton switch is illuminated. The +28 volt dc is supplied to the switch light from the BCUs GSTR output.

 (32)
 The electrical load control unit (ELCU) is controlled by a remote control input and a power on/reset input. The main three phase contacts will be closed by maintaining nominal 28 volt dc power at the remote control input and the power on/reset input. The main contacts will open when the remote control voltage is reduced below

 1.5 to 10 volts dc, or when the power on/reset voltage drops below
 1.5 to 7 volts dc. The ELCU can be tripped and locked out by either an internal protective trip or by a remote trip signal. Reset is accomplished after a protective trip or a remote trip signal by removing and re-applying the power on/reset input signal if the remote trip signal is no longer present. The UTILITY switches on the electrical system control module provide a means of resetting those ELCUs that trip due to internal ELCU protection.
 (33)
 Differential fault protection is provided by a combination of current sensors in the ELCU and at the load (galleys or utility buses). When a differential fault of 15 +/- 5 amps or more occurs, the contacts inside the ELCU open, removing power from the load and fault. To reset the ELCU, the UTILITY switch must be turned off, then back on.

 (34)
 Differential fault protection is provided by a combination of current sensors in the ELCU and at the load (utility buses). When a differential fault of 15 +/- 5 amps or more occurs, the contacts inside the ELCU open, removing power from the load and fault. To reset the ELCU, the UTILITY switch must be turned off, then back on.

 (35)
 Current transformers inside the ELCU sense total current in the three main feeders. When the current reaches a predetermined level, the contacts inside the ELCU open, removing power from the load. The level that the ELCU opens is set by jumpering pins inside the connector. By jumping these pins, the ELCU may be set to trip for 10, 20, 30, 40, 50, or 60 kva. To reset the ELCU, the UTILITY switch must be turned off, then back on.

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 24-51-00
 ALL  ú ú 08 Page 13 ú Jun 18/00
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
747-400
MAINTENANCE MANUAL

 (36)
 The load management system provides automatic shedding and restoration of the galley and utility loads. This is done by opening and closing the associated ELCUs when an overload condition is sensed (demand exceeds capacity).

 (37)
 Each BCU contains eight normally closed load management relay contacts for eight sheddable loads. These load management relay contacts are open for a shed command and closed for a "load-on" command. The eight load management relay contacts are utilized to control eight blocks of electrical loads via eight ELCUs. Two of these load blocks are connected to each main ac bus as follows:

 ~............................................................ | AC BUS | LOAD BUSES | .............................................................. | 1 | Utility Load No. 1 | .............................................................. | 2 | Utility Load No. 2 | .............................................................. | 3 | Utility Load No. 3 | .............................................................. | 4 | Utility Load No. 4 | 2.............…..............................................1

 (38)
 A closed relay in one of the BCUs provides the 28 volts dc to the associated ELCU while the identical closed relay in the opposite BCU provides the ground. This arrangement between the BCUs and load blocks is as follows for each BCU relay:

 ~................................................................ | BCU RELAY NO. | BCU NO. 1 | BCU NO. 2 | LOAD BUSES | .................................................................. | 1 | Not Used | Not Used | | .................................................................. | 2 | Not Used | Not Used | | .................................................................. | 3 | Not Used | Not Used | | .................................................................. | 4 | Not Used | Not Used | | .................................................................. | 5 | 28 volts dc | Ground | Utility Load No. 1| .................................................................. | 6 | Ground | 28 volts dc | Utility Load No. 4| .................................................................. | 7 | 28 volts dc | Ground | Utility Load No. 2| .................................................................. | 8 | Ground | 28 volts dc | Utility Load No. 3| 2...............….............…..............…...................1
　
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