Electrical

Question 1

What are the two basic principles of operation for the Boeing 737 electrical system?

Answer 1

No paralleling of AC power sources

The source of power being connected to a transfer bus automatically disconnects an existing source

Question 2

What are the sources of AC power?

Answer 2

Ground Power
IDG 1
IDG 2
APU Generator

Question 3

What could cause the illumination of the generator DRIVE light?

Answer 3

Low oil pressure, caused by:

IDG failure, or
Engine shutdown, or
IDG disconnected manually, or
IDG disconnected due to high oil temperature

Question 4

What happens when positioning the GRD PWR switch ON?

Answer 4

External AC electrical power is connected to both transfer busses and any existing power source is removed

Question 5

What does the BUS TRANSFER switch, in the AUTO (guarded) position, allow?

Answer 5

Allows the bus tie breakers to operation automatically to maintain power to the AC transfer busses from an operating generator or external power

Question 6

Which light(s) indicate(s) the APU is ready-to-load, but not powering a bus?

Answer 6

APU GEN OFF BUS (blue)

Question 7

If the AC Transfer Buses are powered by External Power, momentarily positioning
either APU GEN switch ON will cause the APU to power which bus(es)? What lights
would you see?

Answer 7

Both AC transfer busses

Opposite SOURCE OFF (amber) light is illuminated

Question 8

When the APU is powering a Transfer Bus, how can you power the opposite Transfer
Bus?

Answer 8

APU should be powering both transfer busses.

Could also connect the respective IDG.

Cannot use external power!

Question 9

What buses are shed when the APU is operating in flight as the single electrical
source?

Answer 9

Galley Busses

Main Busses

Question 10

Moving a GEN (Generator) switch to ON powers which bus? What lights would you
see?

Answer 10

Transfer busses

Opposite GEN OFF BUS

Question 11

Describe the Automatic Generator Online Feature.

Answer 11

If you takeoff with the APU powering both transfer busses and the APU is turned off or fails, the engine generators are connected to their respective transfer busses.

Question 12

What does the ELEC light (amber) indicate?

Answer 12

Fault in DC power or STBY power

Question 13

When does the Cross Bus Tie Relay open and isolate DC Bus 1 from DC Bus 2?

Answer 13

When the FD or AP is on and there is ILS GS capture.

Question 14

Which Bus is always connected to the battery?

Answer 14

Hot battery bus

Question 15

When does the BAT DISCHARGE light (amber) illuminate?

Answer 15

When there is excessive battery discharge

Specifically:
5A for 95s, or
15A for 25s, or
100A for 1.2s

Question 16

What is/are the alternate power source(s) for AC standby power?

Answer 16

Batteries via static inverter

Question 17

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

Answer 17

60 minutes

Question 18

With the loss of all engine or APU electrical power, the battery will be powering which buses if the STANDBY POWER switch is in AUTO? Is this the same in-flight and on the ground?

Answer 18

Battery Bus
Hot Battery Bus
Switched Hot Battery Bus
DC Standby
AC Standby