6. Electrical Systems
Page 42 of 139 6. ELECTRICAL SYSTEMS B-777 SYSTEMS
Page 43 of 139 6. ELECTRICAL SYSTEMS B-777 SYSTEMS
Page 44 of 139 6. ELECTRICAL SYSTEMS B-777 SYSTEMS
- For normal in flight operations all required power is provided by 2 engine driven generators.
- A 3rd generator driven by the APU is available in flight and is equal in size and capacity to either engine driven generator.
- Any 2 of these 3 generators meet all electrical requirements.
- Any one generator powers all essential flight loads.
- With only 1 generator available an Electrical Load Management System ELMS, automatically shuts down some low priority items, such as galleys.
• Galleys and galley chillers.
• Left and right utility busses.
- 2 main busses, Left and Right, distribute power to all airplane systems.
- A bus tie and 2 Isolation relays operate automatically to ensure both main busses remain powered at all times.
- The Hot Battery Buss provides power to the:
• Engine Fire Extinguisher Bottles.
• APU Fire Extinguisher Bottles.
• ADIRU if it is switched on.
GROUND HANDLING BUS
- Is powered only on the ground by the Primary External Power or the APU only.
- It is used for cargo door operation, fueling defueling valves, cargo compartment doors, and maintenance access lights.
- Cabin lights can be powered by this bus if the GROUND SERVICE switch at 1L is on.
GROUND SERVICE BUS
- This Bus is for cabin service personnel. It powers loads needed on the ground and in the air. It powers cabin lights, outlets, battery chargers for the Main and APU batteries, the left forward fuel pump (for the APU)
GROUND POWER
- On Ground the Power Source Priority is
• Respective Engine Generator
• APU
• Opposite Engine Generator
• External power is the only AC power source not automatically selected… You must apply it manually.
- Selecting External power to ON will cause both generator breakers to open.
EKIB-34
ELMS Power Panel Damage Due to Arcing Power Contactor
- Use dual electrical sources to power the aircraft during ground operations as much as possible in the following order of priority:
o Primary and secondary external power.
o APU generator power and primary external power.
- If conditions permit, and two independent ground power sources are available, the APU should be switched off to conserve fuel.
BACK UP POWER SYSTEM
- Essential Loads are powered by the L and R Transfer Busses.
- Essential items include:
• Instrument and panel lighting,
• Captain’s and F/O’s navigation and communication radios,
• Center pitot and engine probe heat.
- Transfer busses connect to one of the backup generators when their AC busses are unpowered.
- Backup generator switching is automatic when the switches are in the ON position.
- The Backup power system provides an uninterrupted transfer of power to the Transfer Busses in the event of:
• loss of 1 main AC Bus,
• The loss of both main AC busses,
• When only one main generator is available,
• and during Autoland.
STANDBY POWER SYSTEM
- The Standby System consists of the Battery Busses, the Captain’s and F/O’s Flight Instrument Busses, and an AC Standby Bus.
- Normal power for these busses is from the left and right AC Transfer Busses. Either directly or through TRUs.
- Should the Transfer buss become unpowered, a RAT will provide power through the TRUs and the Standby AC bus will be powered from the battery through a standby inverter.
- The RAT can be deployed in flight to provide electric and hydraulic power simultaneously. It is the primary source of standby electric power if all normal sources are lost.
- The RAT generator only provides 1/10 the power of a regular generator. It will deploy automatically if both Transfer busses are unpowered, or if all 3 hydraulic system pressures are low, or with dual engine failure and the center hydraulic system pressure low.
- The Green Press light in the RAT switch illuminates when the hydraulic pressure is adequate.
- The RAT gives priority to the Hydraulic system and will shed electrics if the hydraulic pressure drops. The Main battery powers the standby system during this time.
- When the RAT is supplying electrics both Flight Instrument busses are powered. When the Battery is supplying power, only the Captains Flight instrument bus is powered.
ABNORMALS
- Low IDG oil pressure will cause an EICAS Message ELEC GEN DRIVE L the drive must be manually Disconnected.
- High generator oil temperature automatically disengages the drive and gives EICAS messages: ELEC GEN OFF L ELEC GEN DRIVE L - Autoland is possible with one main generator and one backup generator operating.
Page 45 of 139 6. ELECTRICAL SYSTEMS EXPANDED QRH B-777 SYSTEMS
ELEC AC BUS L, R ... Caution Message with Beeper CONDITION:
• AC Bus is unpowered
REASON:
CHECKLIST:
NOTES:
ELEC BACKUP GEN L, R CONDITION:
• Backup generator has failed
REASON:
CHECKLIST:
• Only one Reset attempt can be made by cycling the Backup Gen Switch off then on
NOTES:
ELEC BACKUP SYS CONDITION:
• Backup power system has failed
REASON:
CHECKLIST:
NOTES:
ELEC BATTERY OFF CONDITION:
• Battery Switch is off
REASON:
CHECKLIST:
NOTES:
ELEC BUS ISLN L, R CONDITION:
• Bus Tie Breaker is open due to an AC electrical system fault or Bus Tie Isolation Switch is off
REASON:
CHECKLIST:
NOTES:
ELEC GEN DRIVE L, R CONDITION:
• Backup power system has failed
REASON:
CHECKLIST:
NOTES:
ELEC GEN OFF APU CONDITION:
• Backup power system has failed
REASON:
CHECKLIST:
NOTES:
Page 46 of 139 6. ELECTRICAL SYSTEMS EXPANDED QRH B-777 SYSTEMS
ELEC GEN OFF L, R CONDITION:
• Backup power system has failed
REASON:
CHECKLIST:
NOTES:
ELEC GND HDLG BUS CONDITION:
• Ground handling bus relay has failed
REASON:
CHECKLIST:
NOTES:
ELEC STANDBY SYS CONDITION:
• A fault is detected in the standby power system
REASON:
CHECKLIST:
NOTES:
MAIN BATTERY DISCH CONDITION:
• Main battery is discharging or hot battery bus is unpowered