C90 Electrical

Question 1

Electrical System:

Answer 1

Triple Bus 28VDC System
Battery - 24V / 42 AMP/Hr
Starter/Generators - 28 VDC / 250 Amp

Question 2

Battery

Answer 2

A 24V, 42 ampere-hour, sealed, lead acid battery located right hand wing root. Available for use during battery starts.

Prior to take off, a BATT ammeter value of 10 or less is recommended. In

Question 3

Generators

Answer 3

2 engine driven, 250AMP self excited starter generators

The generators must be momentarily placed in the GEN RESET position, then released to ON to bring online

In the GEN RESET position, the voltage of the selected generator builds to 28 volts, the line contactor is opened and so that the 2 systems are paralleled

Question 4

Generator Control Unit (GCU)

Answer 4

The GCU provides multiple protection and synchronizing functions

Both relays close allowing power to flow to left engine bypassing the L and R GEN BUS.

With the right engine started, generator 2 recharges battery and helps power left engine start up and also ensures cross-start current is not exceeded.

Over voltage protection is initiated if a voltage > 31.5VDC is sensed. *Check

Reverse current protection , Tie opened if sensed i.e following engine failure

[L or R DC GEN] annunciator indicates that the line contactor is open and the generator is off line

Question 5

GCU’s Load sharing

Answer 5

within +/- 2.5%

Question 6

GCU Engine Start Logic

Answer 6

Both relays close allowing power to flow to running engine by passing the L and R GEN BUS.
Example:
With the right engine started, generator 2 recharges battery and helps power left engine start up and with the R GCU also ensuring cross-start current limit is not exceeded.

Question 7

External Power

Answer 7

28VDC Ground power is connected via the Ground Power receptacle on the Right wing adjacent to the engine nacelle.

Power cart required, capable of between 28VDC & 28.4 V DC, producing 1000 Amp (5 sec) 500 Amp for 2 mins and 300 Amp continuously

The BATTERY must be ‘ON’ and indicating >20 V BEFORE Ground Power with the Ground Power switch engaged

EXT Power disconnected before closing GEN ties

Question 8

Volt and Ammeters

Answer 8

Located on the overhead panel, the Volt/Ammeter displays the Volts and Amps of the selected bus. The selector is located next to the meter and allows selection of all electrical buses, Generators, Battery and Ground power sources.

Question 9

Generator Load Meters

Answer 9

Located on the overhead panel, the generator load meters display the actual load of each generator as a percentage of maximum generator load (28V)

Question 10

Bus Ties

Answer 10

The generator buses are connected to the center bus with left and right bus tie relays
GEN TIES switch controls relays to set bus tie position.

OPEN
Causes relays to open and isolate left and right buses to center bus

NORM
Causes automatic generator closure to the left and right buses with generators on line

MAN CLOSE
Will momentarily close the ties during battery operation to power the generator buses from the battery

Question 11

Circuit Breaker Colour Code

Answer 11

Yellow - Triple Feed bus \ Battery
Blue - Left Gen
Green - Right Gen
Red - STDBY

Question 12

White Circles Around Switch’s means

Answer 12

System Powered By Centre or Triple feed bus
*Will remain powered when battery only OPs - Load Shedding

Question 13

HED’s

Answer 13

Hall Effect Devices
Prevent Excessive High Current
Only sense current flow in a single direction

250 AMP slow fuse in opposite direction can isolate CTR bus

Question 14

Resetting bus tie relay following excessive current flow

Answer 14

BUS SENSE switch ‘RESET’

Question 15

Solid State Bus Tie Trip

Answer 15

275 AMP +/- 5 AMPs

BUS Ties remain open until reset through BUS SENSE switch

Question 16

GEN Tie Trip

Answer 16

Cannot be manually CLOSED, until BUS SENSE HAS BEEN RESET