Electrical System Flashcards
BAT DISCHARGE light (amber)
with BAT switch on, there is excessive battery discharge detected
TR UNIT light (amber)
on ground- any TR has failed
in flight-
- TR 1 failed OR
- TR2 and TR3 failed
ELEC light (Amber)
fault exists in DC power system or standby power system (operates only on ground)
BAT SWITCH (OFF, On)
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
what does the CAB/UTIL switch control?
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
Generator DRIVE lights (amber)
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
Generate drive disconnect switch
disconnects IDG if electrical power is available and engine start lever is IDLE
IDG can’t be reconnected in flight
STANDBY Power OFF light (amber)
illuminates when one or more of the following busses are unpowered:
-AC standby bus
‘
DC standby bus
battery bus
STandby power switch (AUTO, OFF, BAT)
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
GRD POWER AVAILABLE (blue)
ground power available and meets power quality standards
GRD PWR switch
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
TRANSFER BUS OFF lights (amber)
associated transfer bus not powered
SOURCE OFF light (Amber)
If a source is powered the opposite transfer bus, will both transfer busses be powered?
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
Gen Off Bus light (blue)
IDG not supplying power to related transfer bus
GEN switch
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
Bus transfer switch (AUTO, OFF)
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
APU GEN OFF BUS (blue)
APU running and not powering bus
APU GEN switch (OFF, On) with APU powering both AC transfer busses
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
what happens if you select APU GEN off when the APU is powering one AC transfer bus and IDG powering one transfer bus?
the APU gen will disconnect from tie bus and AC transfer bus and the IDG will power both AC transfer busses
What happens when you move a single APU gen switch to ON with no IDGs powering transfer busses?
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
if both AC transfer busses are powered by IDGs, what will moving the APU GEN switch to ON do?
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
What provides the primary electrical power to the Aircraft
TWo integrated drive generators (IDGs) which supply three phase, 115V, 400 cycle AC
What supplies DC power to the Aircraft?
TR units and battery/battery charger
what are the two basic principles of the operation of the 737 electrical system?
- 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
AC power system components
Each system consists of:
transfer bus
main bus
two galley busses
ground service bus
Which transfer bus supplies power to AC standby bus?
Transfer bus 1
If the AC source powering either transfer bus fails or is disconnected, how does the disconnected transfer bus receive power?
it can be powered by any available source through the tie bus with the BTBs
Can you power one transfer bus with external power and one with APU power?
NO
What is the automatic generator on line feature in case the airplane takes off with APU powering both transfer busses?
If the APU is shut down or fails, the engine generators are automatically connected to their related transfer busses.
Load shedding with single generator operations
1.galleys and main bus on transfer bus 2
if overload is still sensed
- galleys and main bus on transfer bus 1 shed
if overload still sensed - IFE buses shed
If APU is only source of power in flight, what is automatically shed?
Galley busses and main buses
DC power system consists of:
three TR units, energized from AC transfer busses.
Battery provides DC power to loads required to be operative with no other source available
What does a TR unit do?
Converts 115V AC to 28 V DC
Where does TR1 normally receive AC power from?
transfer bus 1
Where does TR2 normally receive AC power from?
transfer bus 2
Where does TR3 normally receive AC power from? what is the backup source?
TR3 normally gets AC power from transfer bus 2 but has a backup source of AC power from transfer bus 1.
Are DC bus 1, DC bus 2, and DC standby bus normally connected? if so, how?
they are normally connected through the cross bus tie relay. TR1 and TR2 are normally powering these busses.
TR3 is normally powering which bus?
battery bus
when does cross bus tie relay automatically open to isolate DC bus 1 from DC bus 2?
- 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
For single battery airplanes, what kind of battery is it and how long will it provide standby power?
24V NiCad with a power minimum of 30 minutes
For two battery airplanes, what kind of battery and how long will it provide standby power?
two 24V NiCad with a minimum of 60 minutes
Which DC busses are powered from the battery if you have a loss of all gens?
battery bus
DC standby bus
hot battery bus
switched hot battery bus
STandby power system consists of:
Main battery
Aux battery (as applicable)
Static inverter
DC standby bus
AC standby bus
Battery bus
Switched hot battery bus
Hot battery bus
what does the static inverter do?
converts 24V DC from battery to 115V AC power to supply AC standby bus with loss of normal electrical power
