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 B.  External power receptacle No. 1 can also supply power to the ground handling and the ground service buses. Each of the XPCs is controlled through their respective bus control unit (BCU).
 C.  The BCUs are microprocessor-based units which provide external power and synch bus protection and coordinate power transfers between auxiliary generator power, external power, and engine-generator power via the BTBs, auxiliary power breakers (APBs), XPCs, and SSB (Ref 24-23-00).
 D.  Groomed power current transformers provide current monitoring of the external power for the BCUs.
 E.  External power transformer rectifier units (TRUs) provide power for the
 28-volt dc ground handling buses.External Power Receptacle (Fig. 1)
_________________________
 A.  The external power receptacles are located in the P36 panel on the right side of the fuselage aft of the nose wheel well doors. External power is supplied to the airplane sync bus through the two receptacles. The 115-volt ac ground handling bus receives external power from the external power receptacle No 1. Also located in the P36 panel are the AC CONNECTED lights that illuminate when the BCU determines that the power being supplied to the airplanes is good, and the POWER NOT IN USE lights that indicate when the external power contactors are open.
External Power Switches (Fig. 1)
_______________________
 A.  The external power switches are momentary push-button switches which incorporate white ON and AVAIL indicator lights. The switches are located on the electrical system control module on the P5 panel and are identified as EXT PWR 1 and EXT PWR 2. The AVAIL light will illuminate when the power quality at the external power receptacle is good and the receptacle is connected properly. The ON light illuminates when the associated external power contactor is closed.
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A
747-400 MAINTENANCE MANUAL
SEE A
SEE  B 
ELECTRICAL SYSTEM CONTROL MODULE 
A 
POWER NOT IN 
USE LIGHTS  AC CONNECTED 
AC CONNECTED  LIGHT 
LIGHT 

FWD
EXTERNAL POWER  EXTERNAL POWER  EXTERNAL POWER 
RECEPTACLE  RECEPTACLE  RECEPTACLE DOOR 
NO. 1  NO. 2 

EXTERNAL POWER RECEPTACLES B
Electrical System Control Module/External Power Receptacle Location
Figure 1

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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.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

 4.  Ground Power Current Transformer (Fig. 2)
________________________________
 A.  There are two ground power current transformers (GPCTs), one for each input of the external power. The three-phase ac feeder lines are routed through the large holes in the GPCT and become the primary winding for each GPCT core. The GPCT provides current monitoring input to the BCUs. The GPCTs are located in the main equipment center in the P714 and P715 panels.
 5.  External Power Contactors (Fig. 2)
_________________________
 A.  There are two external power contactors (XPC) used to connect external power with the airplane sync bus. The contactors are located in the main equipment center. The contactor for external power receptacle No. 1 is located in P714. The contactor for external power receptacle No. 2 is located in P715.
 B.  The XPC is an electrically held three-phase contactor. The XPC main contacts are rated at 115/200 volts, 400 Hz, and 260 amps per phase for continuous operation. The XPC main contacts connect the external power source to the sync bus when closed. The BCUs provide the control for their associated XPC in conjunction with manual operation of the external power switch on the overhead panel.
 C.  The XPC has the capability of interrupting, without damage, a signal phase or multiphase fault current of 2000 amps.
 D.  The XPC has five normally open and nine normally closed auxiliary contacts which operate in unison with the main contacts. The auxiliary contacts are rated for an inductive load of 1 amp at 240 volts ac, and
　
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