electrical system

Electrical faults are automatically detected and ___ isolated.

Automatically — System operation is automatic. Electrical faults are automatically detected and isolated. [Ref: Chapter 6 — Electrical — 6.20.1]

The main electrical power sources are:

4 IDGs, 2 APU Gen, 2 Ext Power — The main AC electrical power sources are: • four IDGs • two auxiliary power sources (APU generators) • two external power sources [Ref: Chapter 6 — Electrical — 6.20.1]

The ground service bus is powered whenever which bus is powered?

AC Bus 1 — The ground service bus is powered on the ground and in flight whenever AC bus 1 is powered. [Ref: Chapter 6 — Electrical — 6.20.4]

The First Officer transfer bus is normally powered by which source, with which backup?

AC Bus 2, with AC Bus 1 as backup — The first officer transfer bus is normally powered by AC bus 2. If AC bus 2 is unpowered, AC bus 1 powers the first officer transfer bus. [Ref: Chapter 6 — Electrical — 6.20.5]

During automatic ILS approaches, AC and DC buses 1, 2, and 3 remain connected to the synchronous bus for redundant electrical backup to the autopilots. True or False?

False — During automatic ILS approach, AC and DC busses 1, 2, and 3 are isolated from the synchronous bus to provide independent power sources for the three autopilots. [Ref: Chapter 6 — Electrical — 6.20.7]

With the standby power selector switch in BAT, approximately how many minutes of emergency power are provided?

30 minutes — The main and APU batteries can provide power to the standby bus and APU alternate power system for at least 30 minutes. [Ref: Chapter 6 — Electrical — 6.20.10]

If AC Bus 1 is unpowered with all engines operating, what will be observed?

EICAS message ELEC AC BUS 1 — The system description identifies the relevant source/actuation: “ELEC AC BUS 1, 2, 3, 4 Caution Beeper AC bus unpowered.” That is why the correct source is EICAS message ELEC AC BUS 1. [Ref: FCOM 6.30.1]

When is the EICAS advisory message DRIVE DISC displayed?

When the IDG is disconnected — The FCOM describes the triggering logic as “>DRIVE DISC 1, 2, 3, 4 Advisory IDG disconnected.” Those are the conditions that lead to When the IDG is disconnected. [Ref: FCOM 6.30.1]

AC bus 1, 2, and 3 isolation lights and the EICAS advisory message ELEC BUS ISLN may be displayed during autoland. True or False?

False — During autoland: • AC bus isolation lights 1, 2, and 3 remain extinguished, • the EICAS message ELEC BUS ISLN is not displayed. [Ref: Chapter 6 — Electrical — 6.20.7]

What does the ground handling bus supply power to?

Cargo handling equipment and compartment lights, the fueling system, and auxiliary hydraulic pump 4 — The ground handling bus powers individual equipment items such as: • lower cargo handling equipment and compartment lights • fueling system • auxiliary hydraulic pump 4 [Ref: Chapter 6 — Electrical — 6.20.5]

When the EICAS advisory message ELEC BUS ISLN is displayed, what has occurred?

BUS tie breaker open (BTB), effective bus is disconnected from synchronous bus, ISLN displayed on synoptic — The ELEC BUS ISLN advisory indicates the applicable bus tie breaker is open; the affected bus is isolated from the synchronous bus. [Ref: FCOM 6.30.1]

The EICAS advisory message BATTERY OFF is displayed when the main battery is discharging. True or False?

False — >BATTERY OFF — Advisory — Battery switch OFF. [Ref: Chapter 6 — Electrical — 6.30.1]

When is the EICAS advisory message BAT DISCH APU displayed?

When the APU battery is discharging or during the standby power test — The FCOM describes the triggering logic as “EICAS Alert Messages 4X-IAB, 4X-ICK 4X-ICA 4X-ICA 4X-IAB, 4X-ICK Message Level Aural Message Logic >BAT DISCH APU Advisory APU battery.” Those are the conditions that lead to When the APU battery is discharging or during the standby power test. [Ref: FCOM 6.30.1]

With both APU generators connected, after engine 4 has been started, the respective IDG automatically powers its side of the synchronous bus. True or False?

True [Ref: FCOM Chapter 6 — Electrical]

What occurs if available generator power is less than electrical system demand?

Automatic load shedding occurs without EICAS messages or lights — The system description identifies the relevant source/actuation: “EICAS alert messages may display in the order shown depending upon fuel system configuration and the extent of load shedding.” That is why the correct source is Automatic load shedding occurs without EICAS messages or lights. [Ref: FCOM 6.20.1]

When autoland is engaged, system logic provides independent power sources for each of the three autopilots. Which statement is true?

AC and DC buses are isolated as required, while AC Bus 4 continues to power the synchronous bus — During automatic ILS approach, AC and DC busses 1, 2, and 3 are isolated from the synchronous bus to provide independent power sources for the three autopilots. AC bus 4 continues to power the synchronous bus. [Ref: Chapter 6 — Electrical — 6.20.7]

What does placing the BUS TIE switch in AUTO do?

Closes the related BTB and DC isolation relay and enables automatic AC bus tie breaker operation — Pushing the Bus Tie switch to AUTO closes the related BTB and DCIR. [Ref: Chapter 6 — Electrical — 6.20.11]

What does the EICAS message ELEC GEN OFF 3 mean?

Generator control breaker 3 is open and AC Bus 3 will be unpowered [Ref: FCOM 6.30.1]

During preflight, selecting AUTO on the standby power selector with the battery switch ON activates the primary EICAS display and enables the required caution and warning functions. True or False?

True [Ref: FCOM Chapter 6 — Electrical]

The four main DC buses remain interconnected through the DC isolation relays and DC tie bus if there is an AC bus or transformer rectifier failure. True or False?

True — The four DC busses are connected through DC isolation relays (DCIRs) to the DC tie bus, which allows each DC bus to remain powered if its related AC bus is unpowered or TRU fails. [Ref: Chapter 6 — Electrical — 6.20.11]

The Captain's transfer bus is normally powered by:

AC Bus 3 — The captain transfer bus is normally powered by AC bus 3. [Ref: Chapter 6 — Electrical — 6.20.5]

Pushing a BUS TIE switch OFF opens the respective DC isolation relay. The associated DC bus remains powered by its respective AC bus and transformer rectifier. True or False?

True — Pushing the Bus Tie switch OFF opens the BTB and DCIR. This isolates the DC bus from the DC tie bus, leaving it powered by its AC bus and TRU. [Ref: Chapter 6 — Electrical — 6.20.11]

If AC Bus 1 is unpowered with all engines operating, which EICAS message will be observed?

ELEC AC BUS 1 — The system description identifies the relevant source/actuation: “ELEC AC BUS 1, 2, 3, 4 Caution Beeper AC bus unpowered.” That is why the correct source is ELEC AC BUS 1. [Ref: FCOM 6.30.1]

Which AC bus serves as the primary backup power source for the Captain's and First Officer's transfer buses?

AC Bus 1 — The system description identifies the relevant source/actuation: “6.20.5 First Officer Transfer Bus .” That is why the correct source is AC Bus 1. [Ref: FCOM 6.10.1]

The ground handling bus supplies power to:

Cargo handling equipment and compartment lights, the fueling system, and auxiliary hydraulic pump 4 — The system description identifies the relevant source/actuation: “FWD • FUEL OVRD 3 FWD • FUEL PRESS CTR L • FUEL PUMP 2 FWD AC Electrical System Power.” That is why the correct source is Cargo handling equipment and compartment lights, the fueling system, and auxiliary hydraulic pump 4. [Ref: FCOM 6.20.1]

With the standby power selector in BAT and the battery switch ON, what is the electrical system configuration?

Main bat supplies Stby bat via the main hot bat bus, main Stby inverter and apu charges are disabled — In BAT, the battery chargers are disabled and standby power is supplied from the batteries through their hot battery busses/inverters. [Ref: FCOM 6.20.11]