Electrical Systems

Question 1

What does the STANDBY PWR switch do and when would you expect it to be used?

Answer 1

Controls Standby Buses:
AUTO - switches to battery power if AC power is lost
BAT - hardwires to battery
OFF - removes power from AC standby and DC standby buses (battery power removed with battery switch)
Expect it to be used in the even of a total AC power loss (engines + APU)

Question 2

How is DC power supplied?

Answer 2

TRs (Transformer Rectifier (TR) Units - 3)

Batteries (main and aux)

Question 3

How is AC power supplied?

Answer 3

IDGs
APU
External power

Question 4

What are the basic principles of operation for the 737 Electrical system?

Answer 4

No paralleling of AC power sources When a source of power is connected, it disconnects an existing source

Question 5

Under normal conditions how is the AC Standby Bus powered by?

Answer 5

AC Transfer Bus 1

Question 6

The DC Standby Bus is powered by?

Answer 6

TR1 OR TR2 or TR3

Question 7

What does the Static Inverter do?

Answer 7

Converts DC to AC

Question 8

What is the Cross Bus Tie Relay and what does it do?

Answer 8

Isolates the DC buses (1&2) when:

• Glide Slope captures during FD or A/P ILS approach.
• Bus transfer switch OFF

Question 9

What protection, if any, is available if the APU is still powering both transfer buses after an aircraft takes off?

Answer 9

Automatic Generator On-line system Would throw both generators on the bus if you took off with the APU running and it fails or gets turned off and IDGs offline

Question 10

What does the BUS TRANSFER switch do?

Answer 10

Allows for the automatic operation of the bus transfer system
AUTO - allows BTB’s to operate automatically to maintain power to the AC transfer buses
OFF - isolates AC buses if one IDG is powering the system

Question 11

Through the Bus Transfer system, how can the transfer buses be powered?

Answer 11

Any combination of sources except external power and APU cannot each power 1 bus

Question 12

What triggers the STANDBY PWR OFF Light?

Answer 12

Turning the STANDBY POWER switch OFF OR 
one more more busses are unpowered: 
AC Standby Bus
DC Standby Bus
Battery BUS

Question 13

Which Display Units are available if all generators are inoperative (Standby Power)?

Answer 13

Captain PFD & ND Upper Display Unit (N1, N2, etc)

Question 14

What does the BAT Switch do?

Answer 14

ON: powers the Battery Bus, Switched Hot Battery Bus & Standby buses (HOT battery bus is always powered)

Question 15

What does an ELEC (amber) light indicate? On the ground and airborne?

Answer 15

a fault exists in the DC Power System or Standby Power System. You won’t get ELEC in the air and it is non-dispatchable

Question 16

What does an illuminated BAT DISCHARGE (amber) light indicate?

Answer 16

main and/or aux battery is discharging

Question 17

When could you expect to see it? (BAT DISCHARGE)

Answer 17

Current draw for either battery is greater than:

• 5 amps for 95 seconds
• 15 amps for 25 seconds, or
• 100 amps for 1.2 seconds (Just know its discharging for a preset amount of time)

Question 18

What is the TR Unit light logic?

Answer 18

Failures in flight:
AMBER = master caution with failure of tru1 or tru 2&3
YELLOW = found with recall only
On ground - any TR failure gives Master Caution

Question 19

What do the Generator DRIVE lights indicate?

Answer 19

Drive is NOT producing power to the system

Disconnect
No oil pressure in idg
Idg failure
High oil temp

Question 20

What happens when an IDG is decoupled?

Answer 20

Physically disconnects 
(IDG must be removed/replaced from the aircraft by maintenance)

Question 21

How is the electrical system designed to work with a single generator in operation?

Answer 21

Single generator will load shed and provide power to essential and some non essential loads (you’ll lose Main and Galley buses unless you supplement with the APU)

Question 22

What does the GEN OFF BUS light indicate?

Answer 22

The generator is OFF THE BUS - not supplying power to the transfer bus

Question 23

What does the APU GEN OFF BUS light indicate?

Answer 23

The APU is OFF THE BUS and operating above 95% RPM

Question 24

What is the load shedding process for the APU?

Answer 24

Galley and Main Buses AUTOMATICALLY shed.

Both IFE buses are also shed if load exceeds design limits.

Question 25

What is the Load Shedding process for the Generators?

Answer 25

APU as sole source? On ground, covers it all. Inflight? Sheds main and galley buses on demand
One IDG as sole source? Load sheds main and galley buses on demand

Question 26

What does the GRD POWER AVAILABLE light indicate?

Answer 26

Ground power is connected and within limits