Electrical

Question 1

Name all of the DC Power Sources.

Answer 1

• 3 Batteries (24v)
• 2 DC Starter-Generators (28v)
• 2 TRUs
• 1 APU (with a Starter-Generator)
• 1 DC Ext. Power Receptical

Question 2

Name all of the AC Power Sources.

Answer 2

• 2 Engine driven AC Generators

- 1 AC Ext. Power Receptical

Question 3

Describe the Batteries

Answer 3

• The Batteries are located in the left nose section – Main and Auxillary below, and STBY above.
• They are 24v / 43 amp/hour.
• The Main and the Aux assist with engine starts (STBY, no)
• All three can be used for emergency power (~45 min).
• The Standby provides power to the Hot Bus.

Question 4

What does selecting the Battery Master switch ON do?

Answer 4

It closes three contactors directly connecting all three batteries to the Essential Busses (Main and STBY - RIGHT, AUX - LEFT). This connection is independent of the EPCU.

Question 5

What bus powers the EPCU?

Answer 5

The EPCU is powered by a direct lead from the STBY battery, and indirectly from the essential busses.

Question 6

What does the MAIN BATT switch do?

Answer 6

It directly connects the Main Battery to the RIGHT MAIN BUS allowing a two way power flow.

Question 7

What does the AUX BATT switch do?

Answer 7

It directly connects the Auxillary Battery to the LEFT MAIN BUS allowing a two way power flow.

Question 8

What does the STBY BATT switch do?

Answer 8

It directly connects the STBY Battery to the LEFT MAIN BUS, allowing a one way powerflow for charging only.

Question 9

What do the BATTERY Caution Lights indicate?

Answer 9

They indicate that the respective battery is disconnected from its associated MAIN DC BUS, manually or automatically (under certain conditions, the EPCU will disconnect a battery from its associated Main Bus, regardless of switch possition).

Question 10

What are the different battery temperature indications?

Answer 10

The Electrical Page on the MFD displays battery temps in white (normal), amber (50°C - 65°C), and red (>65°C), depending on temperature. There are also three Warning Lights that will indicate a hot battery (XXX BATT HOT).

Question 11

What does the DC EXT PWR switch do?

Answer 11

It allows the a/c to be powered by a GPU, through the LEFT MAIN BUS, and inhibits APU and Engine DC GENs. The EPCU should automatically close the bus ties, a green DC EXT PWR message should appear in the top left corner of the MFD Electrical Page, and the pilot should ensure adequate voltage (>27.5v per SOP 50.12.2.1 - DC Power-Up Procedures – turn off Batteries to preserve charge; and ~24v per SOP 50.30.1 – Engine Start – Monitor Start Closely ).

Question 12

How does the EPCU provide system protection?

Answer 12

The EPCU will automatically disconnect the external power source if voltage exceeds approx 31v or goes below approx 22v.

Question 13

Describe the DC Starter-Generators.

Answer 13

• 28.5v, 400 Amp
• Driven by the Engine Accessory gearbox
• Air Cooled
• Monitored and Controlled by a GCU
• Toggle switches stay on unless there is a malfunction.
• EPCU will automatically disconnect a Generator from its repective main bus in case of various malfunctions.

Question 14

DC GEN caution lights

Answer 14

1 or #2 DC GEN – the generator is offline because of switch position, or an automatic responce by the EPCU

Question 15

Describe the TRUs

Answer 15

• air cooled
• upper right nose compartment
• convert 115v, variable frequency AC power to 28v DC power.
• 300 Amp
• Feed the SECONDARY BUSSES
• No control switches, only circuit breakers

Question 16

Describe the APU DC GEN

Answer 16

• Located in the tail cone.
• Can supply 28v DC power to all six busses during ground operations only.
• Can provide DC power in parallel with Engine DC GENs.

Question 17

How does the Electrical System account for power source failures?

Answer 17

The system is normally powered by the two engine driven generators and the two TRUs. However, when the EPCU detects a DC power source failure, it automatically reconfigures the electrical system by closing the appropriate bus ties.

Question 18

How many power source failures can the electrical system handle?

Answer 18

The EPCU is able to maintain power to all six DC buses with as many as two source failures. If more than two of four power sources fail, automatic load shedding occurs, and certain buses will be left unpowered.

Question 19

Single DC Starter-Generator Failure

Answer 19

If either DC GEN fails, the MAIN BUS TIE automatically closes to power both main buses with the remaining GEN, and a DC GEN caution light will illuminate.

Question 20

Single TRU Failure

Answer 20

If a TRU fails, the SECONDARY BUS TIE closes to allow both Secondary Buses to be powered by the remaining TRU and a TRU Caution Light illuminates.

Question 21

Dual DC Starter-Generator Failure

Answer 21

If both DC GENs fail, the Main-Secondary buses automatically close allowing the main buses to be powered by the TRUs. The Main-Secondary bus ties always opperate in unison and always close in responce to a dual failure (TRU or DC GENs).

Question 22

Dual TRU and Single DC GEN Failure

Answer 22

The EPCU will only close the Main Bus Tie allowing the remaining GEN to power both MAIN BUSes. However, it will not close the Main-Secondary Ties, leaving both Secondary Buses de-energized.

Question 23

Dual DC GEN failure and Single TRU Failure

Answer 23

Emergency Mode. The EPCU 1) disconnects the Batteries from the Main Buses allowing them to power the Essential Buses (aprox. 45 min), 2) closes the Secondary Bus Tie allowing the one TRU to power both Secondary buses, 3) and will not close the Main-Secondary Bus Tie, leaving the MAIN BUSes unpowered. However, should an engine start be attempted, the EPCU will allow the batteries to be connected momentarily to the Main Buses.

Question 24

Main Bus Fault

Answer 24

1) The EPCU inhibits the Main Bus Tie and the Main-Secondary Bus Ties, isolating the possible fault, and the DC BUS Caution Light Illuminates.
2) The QRH directs the crew to cycle the Bus Fault Reset switch.
3) If an actual fault is then detected, the EPCU will disconect all DC Components (GEN, BATT) associated with that MAIN BUS, leaving it isolated and unpowered.

Question 25

DC BUS / #1 DC GEN / STBY BATTERY / AUX BATTERY

Answer 25

Left Main Bus Fault

Question 26

DC BUS / #2 DC GEN / MAIN BATTERY

Answer 26

Right Main Bus Fault

Question 27

Secondary Bus Fault

Answer 27

1) The TRU CB will blow.
2) The EPCU will close the Secondary Bus Tie.
3) A cross-tie fuse will blow, isolating the failed bus.
4) TRU Caution Light

Question 28

Describe the AC Electrical System

Answer 28

Two oil-cooled 115v variable frequency AC GENs, controlled by designated GCUs and driven by the Prop Reduction Gearbox, provide power. That power is distributed by the Left and Right AC buses. Deicing/Anti-icing Heaters, the STBY HYD Pump, the TRUs, the AUX FUEL Pumps, and Galley Items use AC Power.

Question 29

AC GEN Failure

Answer 29

AC GEN Caution Light illuminates, and the remaining GEN’s cross tie will closes, allowing the remaining AC GEN to power both AC Buses. Galley Loads are shed.

Question 30

AC BUS Fault

Answer 30

If the GCU detects an excessive load, i.e. an AC BUS Fault, it will isolate said bus, and the associated services will be lost, and the associated AC BUS Caution Light will illuminate. The AC GEN will remain online, and the cross-tie will close, allowing the opposite bus to be powered by both GENs.

Question 31

Can one AC GEN power the enitire system?

Answer 31

Yes, provided that both AC Buses and both TRUs remain operational.