ELECTRICAL Flashcards
What are the sources of DC power?
The DC Power System receives power from:
• three Transformer Rectifier (TR) units
• main battery
• main battery charger
What happens when an IDG is decoupled?
Positioning a generator drive DISCONNECT switch to DISCONNECT with the engine start lever in IDLE will retract the splined IDG drive shaft out of the gearbox for complete drive decoupling from engine rotation.
NOTE: Decoupling the IDG is irreversible and can only be reset on the ground by Maintenance personnel.
What causes the illumination of the DRIVE light?
Generator DRIVE lights illuminate when an IDG is experiencing low oil pressure caused by:
• engine shutdown
• IDG mechanical failure
• IDG disconnected by activation of the DISCONNECT switch
• IDG auto-decouple due to high oil temperature
When is the GRD POWER AVAILABLE light illuminated?
When the external power cord is connected to the aircraft and the power is checked to be within tolerances and available for connection to the AC buses. This light remains illuminated so long as those conditions are met, regardless of whether external power is powering the AC System.
What are the basic principles of operation of the 737 electrical system?
• There is no paralleling of AC power sources
• The source of power being connected to a transfer bus automatically disconnects an existing source
How is the AC standby bus normally powered?
The AC standby bus is powered by transfer bus 1
What is indicated by a GEN OFF BUS light?
GEN OFF BUS lights illuminate any time an IDG is not supplying power to the transfer bus.
Is it possible to power one AC transfer bus with external power and the other AC transfer bus with APU power?
APU generator power and external power cannot be applied to the transfer buses at the same time. Selection of external power with APU generator power will remove the APU power and supply external power to the transfer buses. Conversely, applying APU generator power will remove external power from the transfer buses and supply APU generator power to the transfer buses.
What is the load shedding logic when only one source of AC power is available?
On the ground, the APU incrementally sheds main and galley bus power until the load is within limits.
In flight, all main and galley buses are automatically shed. If electrical load still exceeds limits, both IFE buses are also automatically shed.
For a single IDG, the system sheds electrical load incrementally in the following order:
• transfer bus 2 main and galley buses
• transfer bus 1 main and galley buses
• IFE buses
What is the automatic generator on-line feature?
In the event an aircraft takes off with the APU powering both transfer buses and the APU is subsequently shut down or fails, the IDGs are automatically connected to their related transfer buses.
How long can two fully-charged batteries provide standby power?
Two fully-charged batteries are capable of providing standby power for a minimum of 60 minutes.
When does the DC cross bus tie relay open?
When Glide Slope is active during a Flight Director or Autopilot ILS approach.
How is the DC standby bus normally powered?
The DC standby bus is powered by TR1, TR2, or TR3
The BAT switch controls which bus(ses)?
Closing the switch guard or positioning the switch ON will power both the switched hot battery bus and battery bus.
What does the illumination of the BAT DISCHARGE light on the AC and DC Metering Panel indicate?
Indicates an excessive discharge of the main battery and/or auxiliary battery.
