777 AIRCRAFT MAINTENANCE MANUAL
EXTERNAL POWER -GROUND SERVICE SWITCH
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EXTERNAL POWER -EXTERNAL POWER PANEL
General
You use the external power panel to connect ground power sources to the airplane electrical system. The panel is immediately to the right of the main equipment center access door approximately 10 ft (3 m) above the ground.
Physical Description
The panel contains two power receptacles and their related power cord restraining straps. The forward receptacle is for primary power. The aft receptacle is for secondary power.
Each receptacle has four large pins for ac power and two small ones for the interlock. Above each receptacle are two indication lights; CONNECTED and NOT IN USE.
Above the lights are gatelink connectors. You find details on the gatelink connectors in CHAPTER 23.
The power cord restraining straps are hook-type with adjustable straps. They hold the weight of the power cords to keep the plugs connected to the receptacles. The aft hook, which stows forward, is for the aft receptacle. The forward hook, which stows aft, is for the forward receptacle.
Placards inside the panel access door list specific information about connecting external power.
Training Information Point
When you stow the hooks, make sure the straps are tight.
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777 AIRCRAFT MAINTENANCE MANUAL
EXTERNAL POWER -EXTERNAL POWER PANEL
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777 AIRCRAFT MAINTENANCE MANUAL
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EXTERNAL POWER -FUNCTIONAL DESCRIPTION -BPCU
General
The BPCU controls external power on the airplane. It also controls ground handling and ground service power.
The BPCU sends data to the GCUs and the ELMS across the ARINC 629 systems data buses for power management. It also sends data to the AIMS for indications.
Switch Controls
These three switches supply control signals to the BPCU:
*
Primary external power
*
Secondary external power
*
Ground service.
The BPCU supplies power to the switches and controls the lights in the switches.
Contactor and Relay Control
The BPCU controls these five electrical switching devices:
*
Primary external power contactor (EPC)
*
Secondary EPC
*
Ground handling relay
*
Ground service transfer relay
*
Ground service select relay.
Power Transfer Control
The BPCU controls bus transfers between power sources. A reference frequency circuit in the BPCU permits no-break-power-transfers (NBPT) between external, APU, and IDG power sources on the ground. A NBPT is when two power sources momentarily parallel during a transfer. The power transfer circuit prevents any paralleling in the air and during takeoff and landing.
Protection
The BPCU protects the airplane electrical system from these external power problems:
*
Over/under-voltage
*
Over/under-frequency
*
Over-current
*
Unbalanced current
*
Open phase
*
Sustained parallel source (SPS)
*
BPCU processor failure.
BPCU Power
The BPCU has these three power sources:
*
Primary external power
*
Battery bus #2
*
Right dc bus.
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EXTERNAL POWER -FUNCTIONAL DESCRIPTION -BPCU
A rectifier in the BPCU power supply changes the primary external ac power to dc. Battery bus #2 power connects to the BPCU when you push the battery switch on.
Power Quality
The BPCU checks the quality of the external power sources for:
*
Phase rotation
*
Voltage
*
Frequency
*
Interlock circuit.
If the power is good, the BPCU supplies power to turn on the related CONNECTED and NOT IN USE lights. When you select primary external power or ground service power, the primary NOT IN USE light goes off. When you select secondary external power, the secondary NOT IN USE light goes off.
Training Information Point
The secondary external power source does not supply power to the BPCU. To use secondary external power, you must connect the primary external power or push the battery switch on.
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