Electrical

Question 1

What are the sources of DC power?

Answer 1

• Three Transformer Rectifier Units (TRU’s).
• Main Battery.
• Main Battery Charger.

Question 2

What happens when an IDG is decoupled?

Answer 2

When the Generator Drive Disconnect Switch is placed to DISCONNECT, with the Engine Start Lever in Idle, A solenoid operated pawl mechanically interfaces with a worm-gear. The worm gear retracts the splined IDG drive shaft out of the gearbox for a complete de-coupling from engine rotation.

Question 3

What causes the illumination of the DRIVE light?

Answer 3

• The respective IDG has experienced low oil
  pressure caused by:
  o Engine Shutdown.
  o IDG mechanical failure.
  o IDG disconnected by the activation of the
  DISCONNECT switch.
  o IDG auto-decouple due to high oil
  temperature

Question 4

When is the GRD POWER AVAILABLE light illuminated?

Answer 4

A GRD POWER AVAILABLE light on the Ground Power and Bus Switching panels illuminates when the external power cord is connected to the aircraft at 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.

Question 5

What are the basic principles of operation of the 737 Electrical System?

Answer 5

There are two basic principles of the 737
Electrical System:
o There is no paralleling of AC sources of
power.
o The source of power being connected to a
Transfer Bus automatically disconnects an
existing source.

Question 6

How is the AC standby bus normally powered?

Answer 6

The AC standby bus is normally powered by Transfer Bus 1.

Question 7

What is indicated by a GEN OFF BUS (blue) light?

Answer 7

Indicates an IDG is not supplying power to the respective Transfer Bus.

Question 8

Is it possible to power an AC bus with External power and the other AC transfer bus with APU power?

Answer 8

No. It is not possible to power one transfer bus with external power and the other transfer bus with APU power. Reference schematic Figure 7 on 6.010 page 8.

Question 9

What is the load shedding logic when only one source of AC power is available?

Answer 9

• On the Ground, APU Generator Only:
  o The APU incrementally sheds main and
  galley bus power until the load is in limits.
  o Manual restoration of main and galley bus
  power can be attempted by moving he
  CAB/UTIL power switch on the AC and DC
  metering panel to OFF, then back ON.
• In flight, APU generator only:
  o All main and galley busses are
  automatically shed. If the load still exceeds
  limits, both IFE busses are automatically
  shed.
• Single IDG:
  o Load is shed incrementally depending upon
  actual load sensing in the following order:
  § Transfer bus 2 main and galley busses.
  § Transfer bus 1 main and galley busses.
  § IFE busses.
  o When more power sources are added
  (second IDG or APU GEN) automatic load
  restoration occurs.
  o On the ground manual restoration may be
  attempted by moving the CAB/UTIL power
  switch to OFF, then back to ON.

Question 10

What is the automatic generator on-line feature?

Answer 10

• The aircraft 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 shutdown or fails, the engine generators are automatically connected to their related transfer busses.
• This feature only occurs one per flight cycle.

Question 11

How long can two fully-charged batteries provide standby power?

Answer 11

A minimum of 60 minutes.

Question 12

When does the DC cross bus tie relay open?

Answer 12

When the glide slope is active during a Flight Director or Auto Pilot ILS approach.

Question 13

How is the DC standby bus normally powered?

Answer 13

The DC standby bus is normally powered by TR1, TR2, or TR3.

Question 14

The BAT switch controls which bus(ses)?

Answer 14

Positioning the BAT Switch ON will power both the switched hot battery bus and the battery bus.

Question 15

What does the illumination of the BAT DISCHARGE amber light on the AC and DC panel indicate?

Answer 15

Excessive discharge of the main battery and/or auxiliary battery.

Question 16

What it the purpose of the static inverter?

Answer 16

Conversion of DC to AC power.

Question 17

What does illumination of the TR UNIT amber light indicate?

Answer 17

• On the ground:
  o The failure of TR1, TR2, or TR3.
• In flight:
  o The failure of TR1, or the Failure of TR2
  AND TR3.

Question 18

With the loss of all AC power, if automatic switching does not occur, how can the standby busses be powered?

Answer 18

• By positioning the STANDBY POWER switch to BAT.
  o This overrides the automatic relay and
  places the AC standby bus, DC standby
  bus, and battery bus on battery power
  regardless of BAT switch position.

Question 19

What causes the illumination of the STANDBY PWR OFF light?

Answer 19

• One or more of the following busses are un-
  powered:
  o AC Standby bus.
  o DC Standby bus.
  o Battery Bus.

Question 20

What causes the illumination of the ELEC light indicate and when is it armed?

Answer 20

• When a fault is in the Standby Power or DC
  Power System occurs.
• The ELEC light is inhibited in flight

Question 21

If all generators are inoperative, and the main battery and the auxiliary battery are the only sources of power, which Display Units are available?

Answer 21

• CA outboard DU, (display unit) with PFD.
• CA inboard DU, (display unit) with ND
• Captains clock.
• Left EFIS control panel.
• Standby Instruments.