Electrical System

Question 1

BAT DISCHARGE light (amber)

Answer 1

with BAT switch on, there is excessive battery discharge detected

Question 2

TR UNIT light (amber)

Answer 2

on ground- any TR has failed

in flight-

• TR 1 failed OR
• TR2 and TR3 failed

Question 3

ELEC light (Amber)

Answer 3

fault exists in DC power system or standby power system (operates only on ground)

Question 4

BAT SWITCH (OFF, On)

Answer 4

OFF- removes power from battery bus and switched hot battery bus with normal power

• removes power from battery bus, switched hot batt bus, DC standby bus, static inverter and AC standby bus when battery is only source

ON-

• provides power to switched hot battery bus
• energizes relays to provide auto switching of standby electrical system to battery power with loss of normal power

Question 5

what does the CAB/UTIL switch control?

Answer 5

power to:

all AC galley busses

FWD and AFT door area heaters

drain mast heaters

lav water heaters

logo lights

potable water compressor

115V AC shaver outlets

Question 6

Generator DRIVE lights (amber)

Answer 6

IDG low oil pressure caused by:

• IDG failure
• engine shutdown
• IDG auto disconnect due to high oil temp
• IDG disconnect through generate drive DISCONNECT switch

Question 7

Generate drive disconnect switch

Answer 7

disconnects IDG if electrical power is available and engine start lever is IDLE

IDG can’t be reconnected in flight

Question 8

STANDBY Power OFF light (amber)

Answer 8

illuminates when one or more of the following busses are unpowered:

-AC standby bus
‘
DC standby bus

battery bus

Question 9

STandby power switch (AUTO, OFF, BAT)

Answer 9

AUTO:

W/ AC transfer busses powered:
.
-AC standby bus powered by AC transfer bus 1

-DC standby bus powered by TR1, TR2, and TR3

W loss of all AC power:

• AC standby bus powered by battery thru static inverter
• DC standby bus powered by battery
• Battery bus powered by battery

OFF

• standby power OFF light illuminates
• AC standby bus, static inverter, and DC standby bus not powered

BAT:

• AC standby bus powered by battery through static inverter
• DC standby bus and battery bus powered by battery

Question 10

GRD POWER AVAILABLE (blue)

Answer 10

ground power available and meets power quality standards

Question 11

GRD PWR switch

Answer 11

OFF-disconnects ground power from AC transfer busses

ON-

-‘removes previously connected power from AC transfer busses and connects ground power to AC transfer busses if power quality is correct

Question 12

TRANSFER BUS OFF lights (amber)

Answer 12

associated transfer bus not powered

Question 13

SOURCE OFF light (Amber)

If a source is powered the opposite transfer bus, will both transfer busses be powered?

Answer 13

no source has been manually selected to power the related transfer bus or manually selected source was disconnected

Yes– if a source has been selected to power the opposite transfer bus, both transfer busses are powered

Question 14

Gen Off Bus light (blue)

Answer 14

IDG not supplying power to related transfer bus

Question 15

GEN switch

Answer 15

OFF- disconnects IDG from related AC transfer bus by opening gen CB

ON- connects IDG to related AC transfer bus by disconnecting previous source and closing gen CB

Question 16

Bus transfer switch (AUTO, OFF)

Answer 16

AUTO:

• BTBs operate automatically to maintain power to AC transfer busses from any operating generator or external power
• DC cross tie relay provides normal or isolated operation as required

OFF-

• isolates AC transfer bus 1 from AC transfer bus 2 if one IDG is supplying power to both AC transfer busses
• DC cross tie relay opens to isolate DC bus 1 from DC bus 2

Question 17

APU GEN OFF BUS (blue)

Answer 17

APU running and not powering bus

Question 18

APU GEN switch (OFF, On) with APU powering both AC transfer busses

Answer 18

OFF:

• moving a single APU GEN switch to OFF illuminates related SOURCE OFF light but APU continue to power AC transfer busses
• if you also move the other APU gen switch OFF, the APU gen disconnects from tie bus and removes APU power from AC transfer busses

Question 19

what happens if you select APU GEN off when the APU is powering one AC transfer bus and IDG powering one transfer bus?

Answer 19

the APU gen will disconnect from tie bus and AC transfer bus and the IDG will power both AC transfer busses

Question 20

What happens when you move a single APU gen switch to ON with no IDGs powering transfer busses?

Answer 20

both AC transfer busses will connect to APU gen

External power is disconnected

Opposite source off light illuminates until other APU gen switch is moved on

Question 21

if both AC transfer busses are powered by IDGs, what will moving the APU GEN switch to ON do?

Answer 21

it will power the related AC transfer bus from the APU gen and
the other AC transfer bus will continue to receive power from the IDG

Question 22

What provides the primary electrical power to the Aircraft

Answer 22

TWo integrated drive generators (IDGs) which supply three phase, 115V, 400 cycle AC

Question 23

What supplies DC power to the Aircraft?

Answer 23

TR units and battery/battery charger

Question 24

what are the two basic principles of the operation of the 737 electrical system?

Answer 24

• there is no paralleling of AC sources of power (i.e. two generators would not power the same bus at the same time)
• the source of power being connected to a transfer bus automatically disconnects the existing source

Question 25

AC power system components

Answer 25

Each system consists of: transfer bus main bus two galley busses ground service bus

Question 26

Which transfer bus supplies power to AC standby bus?

Answer 26

Transfer bus 1

Question 27

If the AC source powering either transfer bus fails or is disconnected, how does the disconnected transfer bus receive power?

Answer 27

it can be powered by any available source through the tie bus with the BTBs

Question 28

Can you power one transfer bus with external power and one with APU power?

Answer 28

NO

Question 29

What is the automatic generator on line feature in case the airplane takes off with APU powering both transfer busses?

Answer 29

If the APU is shut down or fails, the engine generators are automatically connected to their related transfer busses.

Question 30

Load shedding with single generator operations

Answer 30

1.galleys and main bus on transfer bus 2 if overload is still sensed 2. galleys and main bus on transfer bus 1 shed if overload still sensed 3. IFE buses shed

Question 31

If APU is only source of power in flight, what is automatically shed?

Answer 31

Galley busses and main buses

Question 32

DC power system consists of:

Answer 32

three TR units, energized from AC transfer busses. Battery provides DC power to loads required to be operative with no other source available

Question 33

What does a TR unit do?

Answer 33

Converts 115V AC to 28 V DC

Question 34

Where does TR1 normally receive AC power from?

Answer 34

transfer bus 1

Question 35

Where does TR2 normally receive AC power from?

Answer 35

transfer bus 2

Question 36

Where does TR3 normally receive AC power from? what is the backup source?

Answer 36

TR3 normally gets AC power from transfer bus 2 but has a backup source of AC power from transfer bus 1.

Question 37

Are DC bus 1, DC bus 2, and DC standby bus normally connected? if so, how?

Answer 37

they are normally connected through the cross bus tie relay. TR1 and TR2 are normally powering these busses.

Question 38

TR3 is normally powering which bus?

Answer 38

battery bus

Question 39

when does cross bus tie relay automatically open to isolate DC bus 1 from DC bus 2?

Answer 39

- Glide slope capture during F/D or AP ILS. Isolates the DC busses during approach to prevent a single failure from affected both nav receivers/flight control computers - bus transfer switch positioned to OFF

Question 40

For single battery airplanes, what kind of battery is it and how long will it provide standby power?

Answer 40

24V NiCad with a power minimum of 30 minutes

Question 41

For two battery airplanes, what kind of battery and how long will it provide standby power?

Answer 41

two 24V NiCad with a minimum of 60 minutes

Question 42

Which DC busses are powered from the battery if you have a loss of all gens?

Answer 42

battery bus DC standby bus hot battery bus switched hot battery bus

Question 43

STandby power system consists of:

Answer 43

Main battery Aux battery (as applicable) Static inverter DC standby bus AC standby bus Battery bus Switched hot battery bus Hot battery bus

Question 44

what does the static inverter do?

Answer 44

converts 24V DC from battery to 115V AC power to supply AC standby bus with loss of normal electrical power