Electrical Flashcards
What are the basic principles of operation of the 737 Electrical System?
• There is no paralleling of the AC sources of power
• The source of power being connected to a Transfer Bus automatically disconnects an
existing source
What happens when an IDG is decoupled?
The Generator Drive DISCONNECT switch, when placed to DISCONNECT with the Engine Start Lever in IDLE, provides Battery Bus power to a solenoid-operated pawl that mechanically interfaces with a worm-gear. The worm-gear will retract the splined IDG drive shaft out of the gearbox for complete drive decoupling with engine rotation.
NOTE: A decoupled IDG can only be reset on the ground by Maintenance personnel.
What does a generator DRIVE (amber) light indicate?
Indicates the respective IDG has experienced low oil pressure caused by:
- IDG failure
- engine shutdown
- IDG disconnected by activation of the DISCONNECT switch
- IDG auto-decouple due to high oil temperature (182 degrees)
What does the GRD POWER AVAILABLE (blue) light indicate?
When illuminated, it indicates the external power cord is connected to
the aircraft and power (checked within tolerances) is available for connection to the aircraft busses.
Is it possible to power AC Transfer Bus 2 with APU power and AC Transfer Bus 1 with External Power?
It is not possible to power one Transfer Bus with external power and the other Transfer Bus with APU power.
What does positioning the BUS TRANSFER switch OFF do?
Placing the BUS TRANSFER switch to OFF isolates AC Transfer Bus 1 from AC Transfer Bus 2 if one IDG is supplying power to both Transfer Busses.
What is the load shedding logic for single generator operation?
For single generator operation, the system is designed to shed electrical load incrementally based on actual load sensing. The Galleys and Main Bus on Transfer Bus 2 are shed first;
if an overload is still sensed, the Galleys and Main Bus on Transfer Bus 1 are shed; if overload still exists, the In-Flight Entertainment (IFE) Busses are shed.
What does the GEN OFF BUS (blue) light indicate?
It indicates an IDG is not supplying power to the respective Transfer Bus.
Once the IDG Generator Control Breaker closes (connecting IDG power to the Transfer Bus), the respective GEN OFF BUS light will extinguish.
What is the Automatic Generator Online Feature?
The system also incorporates an automatic generator on-line feature in case the aircraft takes off with the APU powering both Transfer Busses. If the APU is either shut down or fails, the engine generators are automatically connected to their related Transfer Busses.
This action occurs only once in flight and only under the circumstances described above.
What are the sources of AC and DC power?
AC Power:
• two Engine-Driven Generators
• one APU Generator
• external power
DC Power:
• three Transformer Rectifier (TR) Units
• Main Battery
• Main Battery Charger
What does an ELEC (amber) light indicate?
The ELEC (amber) light illuminates to indicate that a fault exists in DC Power System or Standby Power System.
NOTE: The ELEC light is inhibited in flight.
What would cause the illumination of a TR UNIT (amber) light?
The TR UNIT (amber) light illuminates:
- On the ground, with a failure of any TR
- In flight, if:
• TR1 fails, or
• TR2 and TR3 have failed
When will the Cross Bus Tie Relay open automatically?
The Cross Bus Tie Relay automatically opens, isolating DC Bus 1 from DC Bus 2, under the following conditions:
- At Glide Slope capture during a Flight Director or Autopilot ILS approach. This isolates the DC Busses during approach to prevent a single failure from affecting both navigation receivers and Flight Control Computers
- Bus Transfer Switch positioned to OFF
Positioning the BAT switch to OFF deenergizes which electrical busses?
When the BAT switch is placed to OFF:
- removes power from Battery Bus and Switched Hot Battery Bus when operating
with normal power sources available
- removes power from Battery Bus, Switched Hot Battery Bus, DC Standby Bus,
Static Inverter, and AC Standby Bus when battery is only power source
What does the BAT DISCHARGE (amber) light indicate?
The BAT DISCHARGE (amber) light illuminates when excessive battery
discharge is detected in the Main Battery and/or Auxiliary Battery
