Ch.6 (Electrical)

Question 1

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

Answer 1

• 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

Question 2

What are all the sources of AC power?

Answer 2

• 2 ENG driven generators
• 1 APU generator
• External power

Question 3

What could cause the illumination of the Generator DRIVE light?

Answer 3

Means that 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 temp

Question 4

4. What happens when positioning the GRD PWR switch ON?

Answer 4

External electric power is connected to both transfer busses

Question 5

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

Answer 5

Allows the Bus Tie Breakers (BTB) to operate automatically to maintain power to the AC transfer busses from any 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 busses are powered by external power, momentarily positioning either APU
gen switch ON will cause the APU to power which Busses? What lights would you see?

Answer 7

Both transfer busses will be powered. The opposite “SOURCE OFF” light illuminates until
the other APU GEN switch is moved to ON

Question 8

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

Answer 8

With the APU or an IDG. Positioning the APU gen switch to off allows for one APU gen to power both transfer busses, moving the other APU gen switch to off, allows the IDG to power both transfer busses

Question 9

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

Answer 9

All galley busses, and main busses are auto shed. If electrical load still exceeds design limits, both IFE busses will be shed

Question 10

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

Answer 10

The respective transfer bus. You shouldn’t see any lights.

Question 11

Describe the automatic Generator Online feature?

Answer 11

If the aircraft takes off with the APU powering both transfer busses, and then the APU is shut off or fails, the engine generators are automatically connected to their respective Transfer busses. This only occurs once in flight.

Question 12

What does the ELEC light (amber) indicate?

Answer 12

That there is a fault in the DC power system or standby power system. This light is inhibited in flight

Question 13

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

Answer 13

• At GS intercept during a FD or AP ILS approach. This is to prevent a single failure from affecting both nav receivers and flight control computers
• Bus transfer switch positioned to OFF

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 excessive battery discharge is detected in the main or Aux battery
Illuminates when discharging at a rate of:
- 5 amps for 95 seconds
- 15 amps for 25 seconds
- 100 amps for 1.2 seconds

Question 16

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

Answer 16

The batteries. AC standby bus is powered through the static inverter.

Question 17

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

Answer 17

A minimum of 60 minutes

Question 18

With the loss of all engines or APU electrical power, the battery will be powering which busses if the STANDBY POWEER switch is in AUTO?
Is this the same inflight and on the ground?

Answer 18

• AC standby bus through the static inverter.
• DC standby bus, battery bus, Hot batt bus, and switched hot batt bus directly from
  the batteries