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12.5 (ATA 24) ELECTRICAL POWER

12.5.1 Introduction

The Electrical Power Generation and Distribution System (EPGDS) is used to supply the electri-cal energy for all onboard electrical equipment. The EPGDS has DC and AC generating sys-tems. The DC generation system includes a battery system. The EPGDS provides for energyconversion, distribution, storage, control, protection, monitoring, and indication to the flight crew.Provision is made for external connection of DC or AC external power while on the ground.

12.5.2 General

The DC generation system is supplied by three NiCad batteries, two engine driven starter/gener-ators, two Transformer Rectifier Units (TRUs) and an optional Auxiliary Power Unit (APU). TheTRUs supply 28VDC and are powered by two engine driven Alternating Current (AC) generatorsthat supply 115Volts Alternating Current (VAC).

The power is distributed by an electrical bus system. It re configures for individual power sourceand bus failures, by the automatic closing and opening of bus tie contactors.

There are both DC and AC external power receptacles for Ground Power Unit (GPU) connection.

All AC and DC aeroplane services can be operated from the AC generators or the AC externalpower alone.

Dash8 - Q400 - Electrical

Page 1

12.5.3 Controls and Indications - Electrical Power


Page 2

Figure 12.5-1 Engine and System Integrated Displays Control Panel (ESCP)

ESCP CALLOUTS PERTAINING TO ELECTRICAL ITEMS

1. ELEC SYS PUSHBUTTON (momentary action)

PUSH - provides a display of the electrical system page on the MFD (upper area) with MFD 1or MFD 2 set at SYS- there is no action with another push

ENG

SYSNAV

PFD

MFD1

ENG

NAVSYS

PFD

MFD2

ELEC SYS

ENGSYS

FUEL SYS

DOORS SYS

ALL TEST

NORM1 2

EFISATT/HDG

SOURCEADC

OFF

ED BRT

NORM1 2

SOURCE

EFIS

1


Page 3

Figure 12.5-2 DC Control Panel (1 of 3)

DC CONTROL

OFF

STBYBATT

AUXBATT

MAINBATT

BATTERYMASTER

GEN 1 GEN 2 MAIN BUSTIE

BUS FAULTRESET

EXT PWR

OFF OFF OFF

OFF OFF OFF OFF OFF

1 2 3


Page 4

DC CONTROL PANEL CALLOUTS

1. STBY BATT switch (two position)

STBY BATT - left main feeder bus charging circuit connected to standby battery- EPCU logic inhibits this connection under the following conditions:

• with external power (no charge) option• when a DC bus fault is present on the left main feeder bus• during emergency operations

OFF - disconnects standby battery from left main feeder bus- STBY BATTERY caution light turns on any time the standby battery is not connected to

the left main feeder bus

2. AUX BATT SWITCH (two position)

AUX BATT - left main feeder bus charging circuit connected to auxiliary battery- EPCU logic inhibits this connection under the following conditions:

• with external power (no charge) option• when a DC bus fault is present on the left main feeder bus• during emergency operations

OFF - disconnects auxiliary battery from left main feeder bus- AUX BATTERY caution light turns on any time the auxiliary battery is not connected to

the left main feeder bus

3. MAIN BATT SWITCH (two position)

MAIN BATT - right main feeder bus charging circuit connected to main battery- EPCU logic inhibits this connection under the following conditions:

• with external power (no charge) option• when a DC bus fault is present on the right main feeder bus• during emergency operations

OFF - disconnects main battery from right main feeder bus- MAIN BATTERY caution light turns on any time the main battery is not connected to the

right main feeder bus


Page 5


DC CONTROL

OFF

STBYBATT

AUXBATT

MAINBATT

BATTERYMASTER


BUS FAULTRESET

EXT PWR

OFF OFF OFF

OFF OFF OFF OFF OFF

5 6 7

4


Page 6

DC CONTROL PANEL CALLOUTS (cont’d)

4. BATTERY MASTER SWITCH (two position, lever locked at OFF)

BATTERY MASTER- any battery is capable of powering both essential buses- switch control is hard wired and independent of EPCU operation

OFF - disconnects the main, auxiliary, and standby batteries from the associated essentialbuses

NOTE: The battery master switch must be selected to BATTERY MASTER before the main,auxiliary, and standby batteries can power the respective buses.

5. GEN 1 and 2 SWITCHES (two position)

GEN 1 OR 2 - DC GCU permits generator to supply power to the associated feeder bus- DC generators are inhibited with DC EXT PWR ON

OFF - shuts associated generator off- resets GCU for subsequent generator and circuit monitoring following shutdown of a

generator- DC GEN 1 or 2 caution light turns on anytime the respective generator is not supplying

power to its associated feeder bus

6. MAIN BUS TIE SWITCH (two position, lever locked at OFF)

MAIN BUS TIE - manually ties left and right main feeder buses.- EPCU inhibits bus tie operation upon detection of a bus fault

OFF - manually opens connection between left and right main feeder buses

7. BUS FAULT RESET SWITCH (momentary selection)

BUS FAULT RESET - re initializes the EPCU after a bus isolation (following a bus fault) toenable subsequent bus fault monitoring


Page 7


DC CONTROL

OFF

STBYBATT

AUXBATT

MAINBATT

BATTERYMASTER


BUS FAULTRESET

EXT PWR

OFF OFF OFF

OFF OFF OFF OFF OFF

8


Page 8

DC CONTROL PANEL CALLOUTS (cont’d)

8. DC EXT PWR SWITCH

EXT PWR - left and right main feeder buses, essential buses and secondary buses poweredfrom DC external power source- DC and APU generators are inhibited from connection to feeder buses- DC EXT PWR ON vindication (green) is displayed on the MFD Electrical page when DC

external power is connected to the aeroplane- the EPCU incorporates External DC Power Protection from too high or too low supply of

External DC power voltage- external DC power is supplied to the aeroplane via the external DC power receptacle.

The EPCU will enable the connection of the external DC power to the aeroplane if theapplied external voltage is:• of correct polarity, and• its value is within 22 ± 1VDC to 31 + 0.5; 31- 0.75 VDC

- main, auxiliary, and standby batteries are connected to their feeder busesfor the duration of engine start, the standby battery is inhibited from the left main feeder

bus


Page 9

Figure 12.5-5 AC Control Panel

EXTPWR

GEN 2

AC CONTROL+

OFF

GEN 1

OFFOFF

1 2


Page 10

AC CONTROL PANEL

1. EXTERNAL POWER SWITCH (two position)

EXT PWR - connects AC external power to variable frequency buses- is capable of powering all AC and DC buses on the aeroplane- isolates the AC generators from the buses- AC EXT PWR ON indication (green) is displayed on the MFD Electrical page when AC

external power is connected to the aeroplane- an External Power Protection Unit (AC PPU) is installed in the right ACCB to ensure only

AC Power within specification is allowed on the bus- the AC PPU checks for the following parameters:

• voltage level (under voltage trip point 106 VAC ±2%, over voltage trip point 124 VAC ±2%)• frequency (underfrequency trip point 370Hz ±2%, overfrequency trip point 450Hz ±2%)• phase rotation (A-B-C)

OFF - disconnects AC external power from variable frequency buses

2. AC GEN 1 AND 2 SWITCHES (two position)

GEN 1 OR GEN 2 - arms respective GCU to activate, and connect its generator to the asso-ciated variable frequency bus- AC generators are inhibited with AC EXT PWR ON

OFF - shuts respective generator down- resets GCU for subsequent generator and circuit monitoring following shutdown of a

generator- AC GEN 1 or 2 caution light turns on anytime the respective generator is not supplying

power


Page 11

Figure 12.5-6 Copilot’s MFD Electrical Page (1 of 3)

27.728.5

27.728.526.126.1

ELECTRICAL

LOAD

1 1 51 1 51 1 5

. 06

. 06

. 06

1 1 51 1 51 1 5

. 06

. 06

. 06

AB

C

AB

C

+22+22+22

+1. 00- . 34- . 34

+1. 00

+1. 00 . 06

+1. 00 +1. 00

APU GEN

DC GENLOAD1 2

TRULOAD1 2

LOADBATT

°C

VOLTDC BUS

RL

MAIN

AUXSTBY

ESS

MAINSEC

AC GEN 1

AC GEN 2

VOLT LOAD

VOLT LOAD

DC EXTPWR ON

AC EXTPWR ON

HYD PRESS HYD QTYPK

STBYPSI x 1000 % x 100

BRK 1 2 3 1 2 34

2

12

3

4

5

FLAPDEG

35

1050

35

0 0

1


Page 12

MULTI FUNCTION DISPLAY (MFD) ELECTRICAL PAGE CALLOUTS

1. DC EXT POWER ANNUNCIATOR (green)- displayed when a DC external power source is connected to the aeroplane and supply-

ing DC power within voltage tolerances

2. BATTERY LOAD AND TEMPERATURE ANNUNCIATION AREA

BATT (white)LOAD, °C (cyan)MAIN, AUX, STBY (white)

3. DIGITAL DISPLAY OF BATTERY LOAD (white)- displays the load of the battery- + or - sign on the left of the lead digit, whether the battery is in overcharge (“+” displayed)

or in discharge (“-” displayed)- nothing is displayed when the battery is in charge within the expected range- leading zero is suppressed in the lead digit position- digital number gives the rate of the load, with 1.00 equal to 100% of load- examples:

• “+1.30” indicates that the battery is in overcharge at 30% over the maximum rate of the charge• “.60” indicates that the battery is in charge at 60% of its maximum rate of charge• “-0.30 “indicates that the battery is discharging at 30% over the maximum rate of discharge

4. DIGITAL DISPLAY OF BATTERY TEMPERATURE- battery temperature digits are displayed in white with a + or - sign on the left of the lead

digit, when the temperature is within normal limits- when the temperature is in the range (+50°C to +65°C) the digits turn yellow- when the temperature exceeds 65°C the digits are displayed in red

5. DC BUS VOLTAGE ANNUNCIATION AREA

DC BUS (white)L, R (white)VOLT (cyan)ESS, MAIN, SEC (white)


Page 13

Figure 12.5-7 Pilot’s MFD Electrical Page (2 of 3)

27.728.5

27.728.526.126.1

ELECTRICAL

LOAD

1 1 51 1 51 1 5

. 06

. 06

. 06

1 1 51 1 51 1 5

. 06

. 06

. 06

AB

C

AB

C

+22+22+22

+1. 00- . 34- . 34

+1. 00

+1. 00 . 06

+1. 00 +1. 00

APU GEN

DC GENLOAD1 2

TRULOAD1 2

LOADBATT

°C

VOLTDC BUS

RL

MAIN

AUX

STBY

ESS

MAIN

SEC

AC GEN 1

AC GEN 2

VOLT LOAD

VOLT LOAD

DC EXTPWR ON

AC EXTPWR ON

7

6

PFCS

SPOILERS

R1

RELEV

RUDLO L1

LELEV

8

9

10

11

9

10


Page 14

MULTI FUNCTION DISPLAY (MFD) ELECTRICAL PAGE CALLOUTS (cont’d)

6. DIGITAL DISPLAY OF DC BUS VOLTAGE (white)- indicates the voltage on the associated bus

7. DIGITAL DISPLAY OF APU GENERATOR LOAD (white)

APU GEN (white)LOAD (cyan)- is activated only if the APU is installed depending on the aeroplane configuration- the load demand on the APU generator is displayed in the following format:

• “+” sign is displayed on the left of the lead digit to indicate an overload of the DCpower source

• nothing is displayed when the load is in the expected range• leading zero is suppressed in the lead digit position• digital number gives the rate of the load, with 1.00 equal to 100% of load examples:• “.60” indicates that the DC generator or TRU is loaded at 60% of the maximum out-

put• “+1.30” indicates an overload of the DC generator or TRU

8. AC EXTERNAL POWER ANNUNCIATION (green)- displayed when an AC external power source is connected to the aeroplane and supply-

ing AC power

9. AC GEN VOLTAGE AND LOAD ANNUNCIATION AREAS

AC GEN 1 and AC GEN 2 (white)VOLT, LOAD (cyan)A, B, C (white)

10. DIGITAL DISPLAY OF AC VOLTAGE (white)- displays AC bus voltage for each phase


Page 15

Figure 12.5-8 Composite MFD Electrical Page (3 of 3)

27.728.5

27.728.526.126.1

ELECTRICAL

LOAD

1 1 51 1 51 1 5

. 06

. 06

. 06

1 1 51 1 51 1 5

. 06

. 06

. 06

AB

C

AB

C

+22+22+22

+1. 00- . 34- . 34

+1. 00

+1. 00 . 06

+1. 00 +1. 00

APU GEN

DC GENLOAD1 2

TRULOAD1 2

LOADBATT

°C

VOLTDC BUS

RL

MAIN

AUX

STBY

ESS

MAIN

SEC

AC GEN 1

AC GEN 2

VOLT LOAD

VOLT LOAD

DC EXTPWR ON

AC EXTPWR ON

12

13

HYD PRESS PSI HYD QTY %PK BRK STBY 1 2 3 1 2 3

3000 0 30003000 0 100 100 100

FLAPDEG

35

1050

35

SPOILERS

RI

RELEV

RUDLO LI

LELEV

RO

11

11


Page 16

MULTI FUNCTION DISPLAY (MFD) ELECTRICAL PAGE CALLOUTS (cont’d)

11. DIGITAL DISPLAY OF AC GENERATOR LOAD (white)- the load demand on the AC generator is displayed in the following format:

• “+” sign is displayed on the left of the lead digit to indicate an overload of the ACpower source

• nothing is displayed when the load is in the expected range• leading zero is suppressed in the lead digit position• digital number gives the rate of the load, with 1.00 equal to 100% of load

- examples:• “.60” indicates that the AC generator is loaded at 60% of the maximum output• “+1.30” indicates an overload of the AC generator

12. DIGITAL DISPLAY OF DC GENERATOR LOAD (white)

DC GEN (white)1,2 (white)LOAD (cyan)- DC generator load is displayed in the same manner as the APU generator load

13. DIGITAL DISPLAY OF TRU LOAD (white)

TRU (white)1,2 (white)LOAD (cyan)- TRU load is displayed in the same manner as the APU generator load


Page 17

Figure 12.5-9 Pilot’s Lower Circuit Breaker Panel

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Page 18

Figure 12.5-10 Pilot’s Upper Circuit Breaker Panel

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Page 19

Figure 12.5-11 Copilot’s Lower Circuit Breaker Panel

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Page 21

Figure 12.5-13 DC Power Electrical Schematic - Fully Operational

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Page 22

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Page 23

12.5.4 Detailed Description

The Electrical Power Generation and Distribution System (EPGDS) (Figure 12.5-13) has anElectrical Power Control Unit (EPCU) to control, monitor and distribute DC power to theaeroplane’s electrical buses. The EPCU automatically re configures the EPGDS for powersource and bus failures, by the closing and opening of bus ties contactors. Contactor control isdetermined by automatic functions during the operation of the aeroplane. Manual inputs areachieved through the selection of switches in the flight deck that may be vetoed by the EPCU.

12.5.5 AC/DC Power Monitor System

System parameters are monitored in the flight deck on the Electronic System Indicating Displays(ESID). Select the SYS (System) position on the appropriate Multi Function Display (MFD) rotaryswitch on the ESID control panel to give electrical system indications.

12.5.6 DC System

The DC electrical schematic is shown in Figure 12.5-14.

The DC generation system has the following sources:

• Main, Auxiliary, and Standby Batteries• Two Starter/Generators• Two Transformer Rectifiers Units (TRU)• DC External• APU Starter/Generator (Optional)

The power sources supply power to the following buses in order of priority:

• Battery• Left and Right Essential• Left and Right Main• Left and Right Secondary

DC Electrical Medical OutletA 24 VDC, 35 A electrical outlet for medical purposes is installed on the inboard face of the G1galley.


Page 24

12.5.6.1 Batteries

The main and auxiliary 24-volt NiCad batteries are located in the lower left nose compartment.The main and auxiliary batteries have a 40-amp-hour capacity. The standard standby 17-amp-hour battery (optional 40-amp-hour) is located in the tail compartment, when no APU is installed.With an APU installed, the standby battery is located in the nose compartment. The MAIN andAUX batteries are used for engine starting if ground power is not available. All 3 batteries supplybackup power to the aeroplane essential services in flight for 45 minutes or more.

The batteries are connected to the left and right essential buses (Figure 12.5-15) by selecting theBATTERY MASTER switch. Selecting the BATTERY MASTER switch to the ON position closes 3contactors and connects all three batteries to the essential buses. The connection required forthe control of the above contactors is directly hard wired and is independent of the EPCU opera-tion. The EPCU itself is energized from the essential buses.

NOTE: Battery power cannot be applied to the secondary buses.

Battery temperatures, as indicated on the ESID Electrical Page, should be equal to or greaterthan -20°C prior to dispatch.

NOTE: There is no battery temperature limitation for starting Engines or APU.

With the BATTERY MASTER switch on, the batteries are connected to the essential buses at alltimes. In this way powering of flight safety equipment, which is also connected to the essentialbuses, is enabled. In case of emergency operation the batteries are the main energy sources forthe aeroplane.

In addition to the BATTERY MASTER switch, each of the batteries has a dedicated switch.Selecting the MAIN BATT switch to ON, will directly connect the main battery to the right mainbus. Similarly, selecting to ON of the AUX BATT switch will connect the auxiliary battery to the leftmain bus.

The connections of the main and auxiliary batteries to the main buses are controlled by theEPCU logic. Conditions that require battery disconnection from the bus are the following:

• Emergency operation (defined later)• Bus fault detected

Manually selecting the STBY BATT switch to ON will connect the standby battery to the left mainbus. This connection is achieved electronically by the EPCU if the following conditions are met:

• DC external power (battery charge option) connected• No emergency operation • No bus faults detected

MAIN BATTERY, AUX BATTERY and STBY BATTERY caution lights come on whenever therelated battery is not connected to its feeder bus. A battery temperature indicator on the MFDElectrical page, continuously shows Main, Aux and Stby battery temperatures when selected onthe ESID control panel. High battery temperature will turn on the MAIN BAT HOT, AUX BAT HOTor STBY BAT HOT warning light, and cause associated temperature value on the MFD to turnred. The warning light will go out when the temperature drops below the overheat condition.

17 AMP HR Standby Battery-Forward Fuselage

The 17 AMP HR stanby Battery is re-located in the forward fuselage adjacent to the two 40 AMP


Page 25

The charge characteristics of the 40 Ahr batteries are as follows:

Discharging Characteristics - The 40 Ahr batteries have a nominal voltage of 24 VDC (20 cells at1.2 VDC/cell). This is the voltage that would be measured with a fully charged battery under aload equal to its ampere-hour rating, i.e. in this case 40 Ahr. The ‘no-load’ voltage as measuredat the battery terminals approaches 28 VDC. This explains why the batteries will discharge withDC Ground power connected, particularly if the cart voltage is lower than 28 VDC. Batteryreserves will be depleted at a higher rate, as the cart voltage is lower.

Charging Characteristics - In order to restore batteries to a full state of charge, charging voltageshould be at least 1.4 to 1.6 VDC/cell, or 28 to 32 VDC. This is provided with any DC generatoron line. However, if an external ground power cart has an output voltage of less than 28 VDC, thebatteries cannot be recharged to full capacity, regardless of the time that voltage is applied.

When the DC External power cart output voltage is low, the discharge current from the batteriesis not being supplied to the cart but instead supplied to the aircraft loads. When the batteries areonline, the DC External cart and batteries are connected in parallel allowing current to be con-sumed by either source. If a low voltage output cart is connected with properly charged batteriesonline, as the case is after flight since the engine generators output 28.5 VDC to charge the bat-teries, the batteries will provide most of the current to the aircraft loads because charged batter-ies have a low internal impedance. As the batteries discharge, their internal impedanceincreases and most of the current to the aircraft loads is provided by the DC External cart. How-ever by this time a significant portion of the batteries’ reserve can be consumed. Also, it shouldbe noted that even if the DC External cart and the batteries are at the same potential, therewould be a small current flow from the batteries.

Based on the charge/discharge characteristics of NiCad batteries, it is evident that the externalDC Power cart must have a voltage of at least 28 VDC or battery reserves will be depleted andcannot be fully restored.

Operators are advised, that if 28 VDC carts are not available or their voltage is lower than 28 vdc,the batteries should be isolated from the aircraft loads by selecting the BATTERY MASTER,MAIN, AUX and STBY battery switches OFF while DC External power is connected.


Page 26

Figure 12.5-15 DC Power System Battery Power Only

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Page 27

12.5.6.2 Starter/Generators

The starter/generators are located on the accessory gearbox of each engine. Each starter/gener-ator serves as a starter motor and will revert to generator operation if the DC GEN switch is in theON position and the engine speed exceeds 50% NH following successful engine start. Each gen-erator output is monitored and controlled by its Generator Control Unit (GCU). After engine start,the GCU ensures the generator supplies 28.5 VDC (400 Amps max) to its feeder bus regardlessof load.

The EPCU will then monitor and control the DC EPGDS. The EPCU will turn on the #1 DC GENor #2 DC GEN caution lights in the event the DC generator is disconnected from its feeder bus. Ifa generator overheat condition occurs, the appropriate caution light #1 DC GEN HOT or #2 DCGEN HOT will come on. The light will go out when the temperature drops below the overheatcondition.

12.5.6.3 Transformer Rectifier Units

Located in the nose, the two Transformer Rectifier Units (TRU) change three-phase, 115-volt,variable-frequency AC input power into 28 VDC (300 Amps max) nominal output. The TRUs areunregulated but provide DC power in the range of 26 to 29 VDC during operation.

Under normal conditions, each TRU powers its respective secondary bus. The L TRU or R TRUcaution light comes on if either TRU is off line or failed. The L TRU HOT or R TRU HOT cautionlight comes on if the sensor in either detects an overheat condition. The light will go out when thetemperature drops below the overheat condition.

The two TRUs alone are capable of powering the entire DC system.

12.5.6.4 Auxiliary Power Unit (APU) Starter/Generator

The APU Starter/generator is located in the tail cone section of the aeroplane. The APU isdesigned to supply 28VDC to the essential main, and secondary DC buses on the ground.

A Contactor allows the connection of the APU Starter/generator to the right main feeder bus forAPU starting and powering of the aeroplane DC buses. Its control is achieved from the APUGCU.

After the APU is started, the starter/generator is available to supply power in parallel with the bat-teries to start the aeroplane engines.


Page 28

12.5.6.5 Fault Tolerant Operation

The DC Power subsystem is extremely tolerant to power source failures. It has four separatesources of DC power in flight. They are the two generators and two TRUs. When all DC powersources are operational, each source powers its own dedicated bus. When one or more sourcefailures are detected, the EPCU will automatically reconfigure the power flow.

The EPCU monitors the system for failures. Once a failure is detected, the EPCU will cause thebus system to reconfigure and ensure mission completion with minimal effect for the EPGDS andfor the utilization equipment power supply.

Four bus tie contactors are controlled by the EPCU that connect the appropriate feeder busestogether when there are one or more inoperative DC power sources. The EPCU ensures opera-tion of all DC buses with loss of up to two power sources. Automatic load shedding will occur onlyafter more than two DC source failures.

The four contactors connect the following feeder buses:

• L Main to R Main• L Secondary to R Secondary• L Main to L Secondary• R Main to R Secondary

For example, if one DC generator fails, the L to R Main bus tie closes and the other generatorthen powers both main feeder buses. If one TRU fails, the L to R secondary bus tie closes, andthe other TRU then powers both secondary buses. If both DC generators fail, the vertical bus tieclose, and the TRUs supply DC power to both the secondary and main buses. If both TRUs fail,the vertical bus tie close, and the DC generators supply power to both secondary buses.

The EPCU operates in emergency mode when:

• the aeroplane is in air and • both DC Generators are not available and • at least one TRU is not available

When the EPCU operates in emergency mode, it automatically disconnects the batteries fromthe main buses. The main buses are not powered in this mode. In case of emergency operationthe batteries are the main energy sources for the aeroplane. If an engine start attempt is madeduring an emergency condition, the batteries will automatically be reconnected to the main busesfor the duration of the start attempt.

12.5.6.6 Bus Fault Protection

The EPCU and the DC GCUs protection function protects the DC subsystem against short cir-cuits on the main and secondary buses. The selectivity and protection coordination is ensured bythe logic, detection and by reaction timing in these units.


Page 29

12.5.6.7 Main Bus Failures

If a main bus fault occurs, the EPCU prevents the upper horizontal and two vertical bus ties fromclosing, isolating the bus. The DC BUS caution light comes on to warn of the fault impendingcondition.

If the fault persists after approximately 5 seconds, the EPCU sends a TRIP signal to the GCU,isolating the affected generator. The EPCU will also open and lock-out the contactors connectingthe batteries to the affected main bus. The MAIN BATTERY or AUX and STBY BATTERY cautionlight(s) and related DC GEN caution light will come on as a result. The EPCU continues to moni-tor the operating buses. All main DC services on the faulted bus side will not function.

NOTE: Manual operation of the main bus tie through the MAIN BUS TIE switch is not possi-ble once the EPCU has reacted to a fault.

If the fault subsequently clears, power may be restored with the BUS FAULT RESET switch.

12.5.6.8 Secondary Bus Failures

In the case of a secondary bus short, the over current condition will immediately trip the associ-ated TRU primary circuit breaker. Seven seconds later, with the EPCU declaring TRU failed thecontactor L to R Secondary bus tie will close, transferring the short circuit to the opposite sideTRU. At that moment, the cross tie fuse is blown isolating the fault. This is indicated by a L TRUor R TRU caution light and loss of services on the associated secondary bus.


Page 30

12.5.6.9 Engine Starting

The starting process is initiated by selecting an engine position using the SELECT switch fromthe Engine Start control panel. The starting process is initiated by depressing the START button.The power source (which feeds the main buses for the supply of the starter/generator) for thestarting process can be different. Therefore, the availability of different power sources results inthe following starting modes:

• Starting from batteries• Generator assisted starting • Starting from DC external power• APU assisted starting

In all cases the BATTERY MASTER, MAIN BATT, AUX BATT, STBY BATT and MAIN BUS TIEswitches are selected on prior to start.

a) Starting from Batteries

Only the main and auxiliary batteries, in parallel, participate in the starting process. The standbybattery is diode isolated from to the left main, to ensure an acceptable level of voltage on theessential buses during engine start.

NOTE: Observe starter cranking limits.

NOTE: After an aeroplane NiCad battery start, the charge rate for fully serviceable batterieswill decrease after approximately three minutes.

NOTE: Battery temperatures and charging rates must be continuously monitored on theMFD Electrical page, while charging.

b) Generator Assisted Starting

The generator assisted starting is usually performed, after the start up of an engine is finished (inany mode of starting). After the right DC generator is connected to the right main bus, together,in parallel with the batteries, will perform starting of the other engine.

During the start process and 15 seconds following it, the DC GCUs will be supplied with a ‘CUR-RENT LIMIT’ signal. This will limit the current the opposite side generator will contribute to thestarting and also to limit the current the starter/generator (in generating mode) will supply to theloads.


Page 31

Figure 12.5-16 DC External Power Receptacle

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Page 32

c) Starting from DC External Power

A Ground Power Unit (GPU) can be connected to the DC external power receptacle (Figure12.5-16) on the left side of the forward fuselage. The GPU can supply DC power for theaeroplane EPGDS and for engine starting (Figure 12.5-17). Contractors connecting the main andaux batteries to the main buses are closed automatically upon selection of engine start, as themain and auxiliary batteries assist in the start. The standby battery is diode isolated from the leftmain, to ensure an acceptable level of voltage on the ESS Busses during the starting process.

When the starting process is terminated the power source is still the external power. While DCexternal power is connected to the aeroplane, the generators connections to the main buses isinhibited by the EPCU. The main and auxiliary batteries stay connected to the main buses andthe standby battery is reconnected.

Once the DC EXT PWR switch is turned off after engine start, the generators will come on line ifthe GEN switches are in the ON position. Both vertical bus ties connecting the main to the sec-ondary buses will remain closed until DC power is available from the TRUs.

The EPCU incorporates external DC power protection from too high or too low supply of externalDC power voltage. If the external voltage is more than 31+0.5/-0.75 VDC, or less than 22 ± 1VDC, an over/under voltage condition will cause the external ground power to stop supplyingelectrical power to the aeroplane. If the external power over/under voltage is rectified, the exter-nal power source can be reselected by moving the DC EXT PWR switch to OFF and then to EXTPWR.


Page 33

Figure 12.5-17 External DC Power Schematic

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Page 34

12.5.7 AC System

The AC electrical schematic is shown in Figure 12.5-18.

Electrical power sources for the AC portion of the EPGDS include:

• Two alternating current variable frequency generators• AC External

The power sources supply power to the following buses in order of priority:

• Left or Right AC bus• Left or Right Galley bus

During normal mode of operation, one AC source supplies its dedicated bus. AC power isrequired for the following systems:

• De-icing/Anti-icing heaters• Standby hydraulic pump• Galley loads• Transformer Rectifier Units (supplements DC power)• Auxiliary fuel pumps

AC Electrical Outlet

A 115 V AC, 15A, 400 Hz electrical outlet for vacuum cleaning requirements is installed on theinboard face of the G1 galley.

12.5.7.1 Variable-Frequency AC Power

Two 115 V, 45 KVA AC generators (mounted on the propeller reduction gearbox) supply variablefrequency (340 to 560 Hz) AC power. The AC power is supplied to the left and right AC buses.AC power sources are prevented from being operated in parallel.

AC power is available once the condition levers are out of START & FEATHER in the MIN/850 toMAX/1020 range, and the GEN 1 and GEN 2 switches on the AC CONTROL panel are on.

If one AC generator fails, the #1 AC GEN or #2 AC GEN caution light comes and the remaininggenerator is capable of carrying the aeroplane’s AC electrical load except galley power. An auto-matic cross tie function, controlled by the AC GCU logic circuits, ensures that all variable-fre-quency buses are powered when only one AC generator is on line. Whenever a fault conditionexists, the GCU of the inoperative generator issues a transfer request signal to the operationalside AC GCU. The operational side AC GCU will issue a CLOSE command to the failed side linecontactor. In this configuration, the remaining generator will power both AC buses. In this situa-tion the load shedding relays will not allow power to the galley buses.

The AC generators are protected from bus faults by the GCUs that detect any excessive loadthat might result from a short circuit on a bus. Once a heavy load is detected, the GCU isolatesthe bus, and turns on the appropriate L AC BUS or R AC BUS caution lights.

The # 1 AC GEN HOT or # 2 AC GEN HOT caution lights come on whenever an AC generatoroverheats. The AC generator must be switched off.

NOTE: All AC and DC aeroplane services can be operated from the AC generators or theAC external power alone.


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Figure 12.5-19 AC External Power Receptable

GROUND POWER

A.C. 115V

A


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12.5.7.2 AC External Power

Location of the AC Ground Power Connection

A GPU can be connected to the AC external power receptacle on the forward right hand fuselagenear the nose cone (Figure 12.5-19), to supply AC power to the aeroplane.

An external power switch on the overhead AC CONTROL panel (Figure 12.5-5) connects powerdirectly to the left and right variable-frequency buses, supplying power to all AC and DC buses.

The External Power Protection Unit monitors the qualities of the incoming power. The externalpower must be within acceptable limits, otherwise the external power is rejected.

12.5.7.3 External Power Protection

An External Power Protection Unit (AC PPU) is installed on the right ACCB to make sure theexternal ground power is within specified limits before supplying power to the aeroplane busses.The AC PPU monitors the following parameters:

• Undervoltage• Overvoltage• Underfrequency• Overfrequency• Phase Rotation (A-B-C)

12.5.8 Circuit Breakers

The EPGDS utilizes passive protections in the form of circuit breakers and current limiters. Cir-cuit breakers are protection devices that open the circuit in case of excess current flow. Circuitbreakers protect electrical system power sources, component control circuits and bus distribu-tion.

There are circuit breakers on:

• Four flight deck circuit breaker panels• DC Contactor box in the nose compartment• AC contactor boxes in the left and right main landing gear wheel wells

Two wardrobe circuit breaker panels

Each circuit breaker is identified by the:

• Circuit breaker panel• Electrical bus• Identification label• Alphanumeric location.


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