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Flashcards in Electrical Deck (82):

When do emergency lights automatically activate?

When does the "EMERG LTS" caution light illuminate?

Power loss to the left main bus or if selected off.

When the switch is selected off


What are the priority of buses?

1) BAT: Stby power
2)L&R essential buses
3)L&R Main buses
4)L&R Sec buses
5)L&R Var freq AC 115 busses
6)Galley buses


When does emergency mode of operation occur?

What is the indication?

Which buses will be powered if both DC gens and a TRU fails?

Both DC gens not supplying power and 1 TRU or both AC gens offline.

Both DC Gens caution lights come on as well as TRU (or both AC gens).Only ED and #1 MFD and Stby attitude screens work.

*Look for screens and TRU among all caution lights. Take your time!!!

Both SEBs by 1 TRU and L & R EBs by batteries. This is known as emergency mode!!


Will all buses be powered if both TRUs fail?

Yes, through secondary bus tie.


Which battery is HOT battery bus connected to?

Standby battery


Can EPCU disconnect main bus tie even if it is switched on?



What will cause the EPCU to react to a main bus fault?

High DC GEN current


What are the battery ratings?

What are the DC GEN ratings?

What are the AC GEN ratings?

What are the TRU ratings?

24 V

28V 400 amps @ 1.00


300 Amps @ 1.00


Wat is the purpose of TRUs?

What do they power?

To convert 115v AC to 28v DC

Secondary Buses


What are the two main limits on the electrical system?

Battery temperature min of -20°C
Max loading of 0.1 for t/o


What is AC power mainly used for?

Pumps and heaters, galley and deicing


What is the role of the DC GCUs?

What is the role of the AC GCUs?

Protect from over current, reverse current and over voltage.

Protect from over voltage, under voltage, and over current.


What does the battery master power?

What does the main battery connect?

What does aux battery connect to?

What is stby bat used for?

What controls the connection between batteries and respective busses?

Essential buses by connecting batteries to them. MFD 1 and ED. When you turn on the Battery Master, all three batteries are connected to the essential busses.

Located in the nose and connects to right main DC bus (Co-pilot side) MFD 2 and PFD2

Left main DC bus. PFD 1

Hot battery bus. Standby bus. Emergency power (most important battery)

EPCU logic


What are the rules of EPCU?

You can have one horizontal tie closed at a time.

In the event of a dual failure, both vertical ties will be closed.


What are you required to do if a TRU fails?

Pull CB (QRH item)


If you get multiple failures after a DC Bus fail, which do you action first in QRH?

DC Bus


Which buses does DC ext pwr, power?

Which buses does AC ext pwr, power?

All DC buses

All buses


Hoe long do batteries provide emergency power for?

When do gens come online?

30 mins

Nh 50% at startup


What does the Electronic Power Control Unit do?

*Brains of the electrical system!

-It monitors and re-configures the electrical system in an emergency automatically.

-Communicates with ESID.

-Activates caution and warning lights.


What is the role of the Electrical Power Generation and Distribution System (EPGDS)?

(Another name for Electrical System!

To generate, distribute, store and control energy.


What are two lights on the HOT BAT Bus?

Dome and boarding lights


What is the power source of the Integrated Standby System?

Left Bat Bus


Will all buses be powered if both DC Gens fail? How?

Yes. EPCU re configures by closing the MAIN to SEC ties (Vertical ties)


Will all buses be powered if one DC gen fails? How?

Yes. K12 cross tie


With the APU Gen operating, which buses are powered?

What buses are powered by DC external power?

What buses are powered by AC external power?

All DC buses

All DC buses

All AC and DC buses


What is the minimum battery voltage to start with batteries only?



If you select the Main BATT or AUX BATT switches to on with no power to the essential busses, which bus will be powered?

Left Battery Bus


What happens when you turn on the Battery Master?

How does this relate to the ECU?

Closes 3 contacts to connect all 3 batteries to the essential buses. Main and Standby batteries get connected to Right Essential Bus, Aux battery gets connected to Left Essential Bus.

Independent of ECU as essential buses power the ECU


If you get either MAIN BATTERY, AUX BATTERY, OR STBY BATTERY caution lights, what's happening?

The related battery is not connected to its bus.


How many TRUs? Where are they located?

What is their purpose?

CAn the TRUs alone power the DC systems?

2 in the nose

They change 3 phase, 115v variable frequency AC input power into 28VDC at 300 amps.



When does automatic load shedding occur?

Failure of 2 or more DC sources.


What does the select light on the start switch indicate?

It's a CURRENT LIMIT signal. It limits the current of the opposite side GEN for starting and for loads. It stays on for 15 seconds.


Can GENS be online with external power?



Which batteries assist in starting engines?

MAIN and AUX Batteries at all times.


What is the current range of External DC Power protection?

What if the GPU current gets corrected?

>31 VDC or <22 VDC

Recycle the DC Ext power switch.


When do the 115v AC galley buses get shed?

Failure of one AC GEN.


What is AC power required for?

-Standby hydraulic pump
-Dicing/Anti-icing heaters
-Galley loads
-Aux fuel pumps


What controls the 4 bus ties?



Will all buses be powered if both DC GENS fail? How?

Yes EPCU closes the MAIN to SEC ties


What indicates a Secondary Bus Fault Failure

TRU Caution Light


Can you start engines on AC External Power? If you should attempt this, what happens.

No! If you attempt this, a battery start is conducted


What energizes the #3 Hydraulic DCMP?

Where is it located?

Standby DC Contactor Box (SBCB)



When you arrive at a cold dark aircraft, how do you know the GPU is connected?

Engine intake lights on and recirc fan running.


In emergency operation, what other caution lights do you get other than both D.C. gens and a TRU?

Can you start the engines in emergency mode?

All 3 Battery caution lights because the EPCU automatically disconnects the batteries from the main busses.

Yes. Batteries will automatically be connected to main busses even if they are switched off.


Which batteries aid in engine start?

Main and Aux. standby bat is isolated by a diode and is there to power the essential busses during start.


What busses are supplied by AC power and what is there priority?

Left or right AC bus.
Left or right galley bus.


What busses are supplied by AC power and what is there priority?

Left or right AC bus.
Left or right galley bus.


What protects the AC gens during an AC bus fault?

GCUs isolate bus when an excessive load is detected.


Can the AC gens and AC ext power operate all AC and DC Services.



What can cause the emergency lights to activate?

How long do they stay on for?

Failure of left main DC bus.

15 mins


What items are connected to the hot battery bus?

Battery master switch
Main,aux, and standby batteries.
Dome light
Emergency lights
Flight deck dome lights and boarding lights
Clocks one and two
Battery power for refuelling
Fire bottles discharge squibs
Fuel and hydraulic shut offs
Cargo compartment fire switches
APU fire bottle squib-manual discharge


What temperatures are required to trigger a BATT HOT warning light?

On the electrical systems page, what temperatures change the battery temps to amber, then red?

>71C to activate and <63C to extinguish.

50C to amber and >65C to red.


What is DC external power connected to?

When should a new GPU be sourced?

When will the EPCU disconnect external power? How do you reset this EPCU protection logic when you reconnect a new GPU?

Left main bus.

Whenever you get a battery discharge.

Main and Sec busses read less than 27.5 volts.

>31v or <22v.Recycle the DC Ext per switch.


When do the MAIN and SEC bus ties in the EPCU open?

When AC power becomes available.


Where are the GCUs

Behind FOs seat.


What is the max load of the GENs?

What if the load is less then what the gens are designed for?

400 amps

After engine start the GCU always make sure the GEN is supplying 28 VDC and 400 amps regardless of load.


What do GENs provide power to?

GENs provide power to the main busses through the GCUs.


Describe the TRUs

3 phase, 115v, 28 VDC, 300 amps.


What are the four sources of DC power in flight?

2 TRUs and 2 GENs


If one DC gen fails, which bus tie closes?

Main Bus Tie


If one TRU fails, which bus tie closes?

SEC bus tie (horizontal)


If both DC gens fail, which bus tie(s) close?

Both vertical (SEC to MAIN).


If both TRUs fail, which bus ties close?

Both vertical (SEC to MAIN)


If you lose both TRUs and one GEN, what is the result?

EPCU sheds loads and closes main bus tie but the vertical bus ties stay open. You lose both SEC busses and associated services they power.


What typically causes a DC BUS caution light?

Which busses is BUS Fault Protection provided for?

A transient bus voltage

MAIN and SEC busses


Which lights does a transient bus voltage trigger?

How can you verify this?

"DC BUS" and the MASTER CAUTION will be the only lights (not battery caution lights).

Verify on electrical page.


How does the EPCU isolate affected bus in a DC bus failure after main bus fault reset switch fails to reset the system?

1) Disconnects the starter gen
2) Disconnects related batteries
3) Inhibits closure of the Main Bus Tie and SEC to MAIN bus ties


What is the indication of a Secondary Bus Fault?

TRU caution light and associated CB trips. There will also be a loss of services on the related bus.

The EPCU detects the failure and closes the secondary bus tie. The fuse will then blow due to excessive power. The faulty bus will then be isolated. The TRU light will stay illuminated.


When is AC power available?

When the props are unfeathered as AC gens are on prop reduction gear box.


How does an AC gen get cooled?

By pressurized oil from the gearbox


If you have a L or R AC Bus failure, why does the associated gen keep running?

To act as a backup for the other AC gen in case it fails so it can power the opposite bus if required.


Can one AC gen power the entire electrical system? If yes what is required.

Yes both TRUs must be online


What provides control and distribution of DC power sources?

Where is it located?

DC contactor box (DCCB)

Located in the nose


Where does the #3 hydraulic DC motor pump get energized from?

Where is it located?

Standby DC Contactor Box (SBCB)

Located in he nose. Same spot as DCCB.


What does the Bus Fault Reset switch do?

Re-initializes the EPCU after after a bus isolation (bus fault) to enable subsequent bus fault monitoring.


What does the AC Gen 1 or 2 switches do?

Arms related GCU to activate, enabling the GCU to connect its generator to the related variable frequency bus.


Can battery power be applied to the secondary busses?



What do the TRUs power?

Secondary busses


The power sources supply power to the busses in what priority?

Left or right AC busses

Left or right galley busses


Can AC power sources operate in parallel?



If only one AC gen is online, what ensures AC power to all AC variable frequency busses?

An automatic cross tie function controlled by the AC GCU logic circuits. The GCU of the inoperative gen issues a transfer request signal to the operative side. The operational side GCU will issue a close command to the failed side line contractor.


What should you do if the external DC voltage s less than 27.5V?

Start APU. If APU unavailable; MAIN, AUX and STBY BATT- OFF (Remember to shut external power switch to off).

Ref: AOM 3.2.1 Safety check