*
Primary EPC
*
Ground handling relay
*
Ground service transfer relay
*
Ground service select relay.
The GCUs control the BTBs to connect power between the main ac buses.
Ground Handling Power
When you supply power to the primary external power receptacle, the BPCU energizes the ground handling relay. This connects primary external power to the ground handling ac bus. The ground handling TRU supplies dc power to the ground handling dc bus.
The APU generator can also supply power to the ground handling ac bus, but secondary external power cannot. If both primary external power and the APU generator are available, the BPCU selects primary external power for the ground handling ac bus.
Ground Service Power
The BPCU selects the power source for the ground service bus in this order:
*
Right main ac bus
*
Primary external
*
APU generator.
If the right main ac bus has no power, you can use the ground service switch. When you push the switch, the BPCU connects primary external power or APU generator power directly to the ground service bus.
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EXTERNAL POWER -GENERAL DESCRIPTION
Primary External Power
When you push the primary external power switch, you connect primary external power to the main ac buses. The BPCU closes the primary EPC and the GCUs close the BTBs to connect the power.
Secondary External Power
When you push the secondary external power switch, you connect secondary external power to the main ac buses. The BPCU closes the secondary EPC and the GCUs close the BTBs to connect the power. If primary external power was on the right main ac bus, the right GCU opens the right BTB first to perform a break-power transfer.
No-Break Power Transfers
The BPCU coordinates power transfers between the external power sources and the IDGs or APU generator. The BPCU and GCUs communicate on the systems ARINC 629 buses.
When the ac system changes from an external power source to another power source, it does no-break power transfers. The system momentarily connects the two power sources to one bus so there is no interruption in power. A GCU electronically adjusts an IDG speed to match the power of the external power source to do a no-break power transfer. For transfers between external power and the APU, the APU controller adjusts the APU speed to do the no-break power transfer.
The BPCU always does a break-power transfer between primary external power and secondary external power.
EFFECTIVITY BEJ ALL 24-40-00
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777 AIRCRAFT MAINTENANCE MANUAL
EXTERNAL POWER -GENERAL DESCRIPTION
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EXTERNAL POWER -COMPONENT LOCATIONS
General
The BPCU is on the E1-1 shelf.
These are the components of the external power system:
*
Primary EPC
*
Ground service transfer relay
*
Secondary EPC
*
Ground service select relay
*
Ground handling relay
*
BPCU
*
Ground handling TRU.
Main Equipment Center
Most of the external power components are in the ELMS panels in the main equipment center. See the electrical load management system section for more information on component locations (SECTION 24-09).
These are the components in the P200 right power panel:
*
Primary EPC
*
Ground service transfer relay.
These are the components in the P300 auxiliary power panel:
*
Secondary EPC
*
Ground service select relay
*
Ground handling relay.
The ground handling TRU is in the P320 ground service/ handling power panel.
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777 AIRCRAFT MAINTENANCE MANUAL
EXTERNAL POWER -COMPONENT LOCATIONS
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EXTERNAL POWER -GROUND SERVICE SWITCH
General
The ground service switch arms the BPCU to connect either primary external power or the APU generator directly to the ground service bus. The switch is on the right side of the attendant switch panel at door 2 left.
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