Electrical Flashcards
What are the basic principles of operation for the 737 electrical system?
No paralleling of AC power sources, and
A power source connected to a Transfer Bus automatically disconnects an existing source
How is AC power supplied?
Engine IDGs
APU Generator
External Power
How is DC power supplied?
3x TRUs
Main Battery
Main Battery Charger
How is Standby Power supplied?
Normal Operations
–AC Transfer Bus 1
–DC Bus 1
Standby Power
–Main Battery
–Auxiliary Battery
–Battery Chargers
–Static Inverter
What does the STANDBY PWR switch do? When would you expect it to be used?
Provides automatic or manual control of power to the Standby Busses
AUTO – power source switches to batteries if AC power lost
OFF – removes power to Standby Busses
BAT – forces Standby Power to batteries
Used during the STANDBY POWER OFF QRH
How is the AC Standby Bus powered?
Normally – AC Transfer Bus 1
Standby Power – Batteries via Static Inverter
How is the DC Standby Bus powered?
Normally – DC Bus 1
Standby Power – Batteries (also battery chargers)
What is the Standby Power duration with fully charged batteries?
60 minutes minimum
What does the Static Inverter do?
Converts 24 VDC battery power to 115 VAC power for AC Standby Bus
What is the Cross Bus Tie relay and what does it do?
Connects or isolates DC Bus 1 from DC Bus 2
Normally closed (DC Bus 1/2 connected)
Opens if
–BUS TRANS switch OFF, or
–G/S capture on F/D or A/P ILS approach
What is the APU automatic generator online feature?
APU powering both Transfer Busses, and
APU is shut down or fails, then
Each engine IDG is automatically connected to respective AC Transfer Bus
Can only occur once per flight and in-flight
What does the BUS TRANSFER switch do?
Auto
–Allows Bus Tie Breakers (BTBs) to operate automatically
–Maintains power to the AC Transfer Busses
OFF
–Isolates left and right AC and DC
Through the Bus Transfer system, how can the transfer busses be powered?
Powers AC Transfer Busses from any operating generator or external power
APU generator and external power cannot power AC Transfer Busses at the same time!
What does the STANDBY PWR OFF light indicate?
One or more of the following Standby Busses are not powered
–AC Standby Bus
–DC Standby Bus
–Battery Bus
What Display Units are available on Standby Power?
Captain PFD
Captain ND
Upper DU
What does the BAT switch do?
Powers the Battery and Switch Hot Battery Busses
Allows Standby Busses to be powered with loss of normal power
What does an ELEC (amber) light indicate?
Fault in DC Power system or Standby Power system
Inhibited in-flight
What does BAT DISCHARGE (amber) indicate? When might you see it?
Excessive battery discharge in Main or Auxiliary battery
Usually illuminates during APU battery start
May briefly illuminate (rarely) in changes of TR charger mode
What is the TR UNIT (amber) light logic?
Ground
–Failure of ANY TRU
In-Flight
–Failure of TR 1, or
–Failure of both TR 2 AND TR 3
What do the generator DRIVE (amber) lights indicate?
IDG low oil pressure due to:
–Engine shutdown,
–IDG failure,
–IDG manual disconnect,
–IDG auto disconnect due high oil temperature
What happens when an IDG is decoupled?
IDG physically disconnects
Can only be reset on the ground by maintenance
How is the electrical system designed to work with a single generator online?
Maintain power to the AC Transfer Busses
Incremental load shedding as needed based on actual load
What does the GEN OFF BUS (blue) light indicate?
IDG is not powering the respective AC Transfer Bus
What does the APU GEN OFF BUS (blue) light indicate?
APU RPM > 95% RPM and NOT powering an AC Transfer Bus
