Electrical Systems Flashcards
What does the STANDBY PWR switch do and when would you expect it to be used?
Controls Standby Buses:
AUTO - switches to battery power if AC power is lost
BAT - hardwires to battery
OFF - removes power from AC standby and DC standby buses (battery power removed with battery switch)
Expect it to be used in the even of a total AC power loss (engines + APU)
How is DC power supplied?
TRs (Transformer Rectifier (TR) Units - 3)
Batteries (main and aux)
How is AC power supplied?
IDGs
APU
External power
What are the basic principles of operation for the 737 Electrical system?
No paralleling of AC power sources When a source of power is connected, it disconnects an existing source
Under normal conditions how is the AC Standby Bus powered by?
AC Transfer Bus 1
The DC Standby Bus is powered by?
TR1 OR TR2 or TR3
What does the Static Inverter do?
Converts DC to AC
What is the Cross Bus Tie Relay and what does it do?
Isolates the DC buses (1&2) when:
- Glide Slope captures during FD or A/P ILS approach.
- Bus transfer switch OFF
What protection, if any, is available if the APU is still powering both transfer buses after an aircraft takes off?
Automatic Generator On-line system Would throw both generators on the bus if you took off with the APU running and it fails or gets turned off and IDGs offline
What does the BUS TRANSFER switch do?
Allows for the automatic operation of the bus transfer system
AUTO - allows BTB’s to operate automatically to maintain power to the AC transfer buses
OFF - isolates AC buses if one IDG is powering the system
Through the Bus Transfer system, how can the transfer buses be powered?
Any combination of sources except external power and APU cannot each power 1 bus
What triggers the STANDBY PWR OFF Light?
Turning the STANDBY POWER switch OFF OR one more more busses are unpowered: AC Standby Bus DC Standby Bus Battery BUS
Which Display Units are available if all generators are inoperative (Standby Power)?
Captain PFD & ND Upper Display Unit (N1, N2, etc)
What does the BAT Switch do?
ON: powers the Battery Bus, Switched Hot Battery Bus & Standby buses (HOT battery bus is always powered)
What does an ELEC (amber) light indicate? On the ground and airborne?
a fault exists in the DC Power System or Standby Power System. You won’t get ELEC in the air and it is non-dispatchable
What does an illuminated BAT DISCHARGE (amber) light indicate?
main and/or aux battery is discharging
When could you expect to see it? (BAT DISCHARGE)
Current draw for either battery is greater than:
- 5 amps for 95 seconds
- 15 amps for 25 seconds, or
- 100 amps for 1.2 seconds (Just know its discharging for a preset amount of time)
What is the TR Unit light logic?
Failures in flight:
AMBER = master caution with failure of tru1 or tru 2&3
YELLOW = found with recall only
On ground - any TR failure gives Master Caution
What do the Generator DRIVE lights indicate?
Drive is NOT producing power to the system
Disconnect
No oil pressure in idg
Idg failure
High oil temp
What happens when an IDG is decoupled?
Physically disconnects (IDG must be removed/replaced from the aircraft by maintenance)
How is the electrical system designed to work with a single generator in operation?
Single generator will load shed and provide power to essential and some non essential loads (you’ll lose Main and Galley buses unless you supplement with the APU)
What does the GEN OFF BUS light indicate?
The generator is OFF THE BUS - not supplying power to the transfer bus
What does the APU GEN OFF BUS light indicate?
The APU is OFF THE BUS and operating above 95% RPM
What is the load shedding process for the APU?
Galley and Main Buses AUTOMATICALLY shed.
Both IFE buses are also shed if load exceeds design limits.
What is the Load Shedding process for the Generators?
APU as sole source? On ground, covers it all. Inflight? Sheds main and galley buses on demand
One IDG as sole source? Load sheds main and galley buses on demand
What does the GRD POWER AVAILABLE light indicate?
Ground power is connected and within limits
