Electrical

Question 1

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

Answer 1

Auto or man control of power to the Standby Buses. AUTO: batts pwr standby loads if loss of ac xfer
bus 1 or DC bus 1 (loss of AC pwr sources). AC Standby Bus I powered from the batts via the Static Inv,
the DC Standby Bus, Battery Bus, Hot Battery Bus, and Switched Hot Battery directly from batts

Question 2

How is DC power supplied?

Answer 2

3 (TR) Units, Main Battery, Main Battery Charger

Question 3

How is AC power supplied?

Answer 3

Two Engine-Driven Generators, one APU Generator, external power

Question 4

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

Answer 4

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 5

Under normal conditions how is the AC Standby Bus powered by

Answer 5

Powered from AC Transfer Bus 1

Question 6

The DC Standby Bus is powered by?

Answer 6

by TR1, TR2, or TR3

Question 7

What does the Static Inverter do?

Answer 7

24-volt DC power from the battery to 115V, 400 Hz, single phase AC

Question 8

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

Answer 8

Connects or isolates: DC Bus 1, DC Bus 2, and the DC Standby Bus
b. AUTO opens at G/S capture if FD or AP ILS approach. OFF opens the DC Cross Bus Tie Relay to isolate.

Question 9

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

Answer 9

Automatic Generator On-line system,

Question 10

What does the BUS TRANSFER switch do?

Answer 10

Allows BTB’s to operate automatically to maintain power to the AC transfer buses or Off to isolate

Question 11

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

Answer 11

Via BTB’s

Question 12

What triggers the STANDBY PWR OFF Light?

Answer 12

indicates one or more of the following buses are unpowered, AC Stby, DC Stby, Batt BUS.

Question 13

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

Answer 13

Captain OB DU (PFD), Captain IB DU (MFD,) Upper DU (N1, N2, etc)

Question 14

What does the BAT Switch do?

Answer 14

a. ON:
i. Provides power to the Battery Bus and Switched Hot Battery Bus
ii. energizes relays to provide automatic switching of the Standby Electrical System
b. OFF:
i. Removes power from Battery Bus, Switched Hot Battery Bus when normal power sources
available
ii. removes power from Battery Bus, Switched Hot Battery Bus, DC Standby Bus, Static Inverter,
and AC Standby Bus when battery is only power source

Question 15

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

Answer 15

. Fault exists in DC Power System or Standby Power System on ground, ELEC light is inhibited in flight.

Question 16

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

Answer 16

Main or Aux battery is discharging

Question 17

When could you expect to see it?

Answer 17

Excessive discharge (5 amps for 95 s, 15 amps for 25 s, or 100 amps for 1.2)

Question 18

What is the TR Unit light logic?

Answer 18

i. On the ground, with a failure of any TR
ii. In flight, if: TR1 fails, or TR2 and TR3 have failed

Question 19

What do the Generator DRIVE lights indicate?

Answer 19

IDG low oil pressure due: IDG failure, engine shutdown, disconnect

Question 20

What happens when an IDG is decoupled?

Answer 20

IDG must be removed from the aircraft

Question 21

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

Answer 21

One gen can provide essential and non-essential (possibly load shed) loads.

Question 22

What does the GEN OFF BUS light indicate?

Answer 22

IDG is not supplying power to the respective Transfer Bus

Question 23

What does the APU GEN OFF BUS light indicate?

Answer 23

APU is operating above 95% RPM (ready-to-load condition) and not powering a bus.

Question 24

What is the load shedding process for the APU?

Answer 24

a. Gnd: No shed
b. Inflt: Galley and Main Buses AUTO shed.

Question 25

What is the Load Shedding process for the Generators?

Answer 25

a. Incremental based on actual load sensing (Galley and main 2 Then galley/main bus 1 Then IFE)

Question 26

What does the GRD POWER AVAILABLE light indicate?

Answer 26

Connected and the power quality is in limits