Electrical

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 event of a total AC power loss (engines + APU)
SEC 6.010 pg 25

Question 2

How is DC power supplied?

Answer 2

TRs (Transformer Rectifier (TR) Units - 3)
Batteries (main and aux)
SEC 6.010 pg 1

Question 3

How is AC power supplied?

Answer 3

IDGs
APU
External power
SEC 6.010 pg 1

Question 4

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

Answer 4

1. No paralleling of AC power sources
2. When a source of power is connected, it disconnects an existing source
   SEC 6.010 pg 1

Question 5

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

Answer 5

AC Transfer Bus 1

SEC 6.010 pg 24

Question 6

The DC Standby Bus is powered by?

Answer 6

TR1 OR TR2 or TR3

SEC 6.010 pg 24

Question 7

What does the Static Inverter do?

Answer 7

Converts DC to AC
To power the AC Standby Bus in the event of AC failure
SEC 6.010 pg 26

Question 8

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

Answer 8

isolates the DC buses
OPENS when switched turn to OFF
OR
Glide slope capture during a Flight Director or Autopilot ILS approach

Normally CLOSED in cruise flight (allows any TR to power DC buses)
SH 6.010 pg 18

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
SH 6.010 pg 15

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
SH 6.010 pg 11

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

SH 6.010 pg 11 - bleed air app is helpful for understanding this

Question 12

What triggers the STANDBY PWR OFF Light?

Answer 12

Turning the STANDBY POWER switch OFF

OR

One of the following is not powered:
AC Standby Bus, DC Standby Bus, Battery BUS
SH 6.010 pg 25/26

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)
SH 6.010 pg 30

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)
SH 6.010 pg 21/22

Question 15

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

Answer 15

ELEC = a fault exists in the DC Power System or Standby Power System.
You won’t get ELEC in the air and it is NOT dispatchable
SH 6.010 pg 17

Question 16

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

Answer 16

BAT DISCHARGE = main OR aux battery is discharging

SH 6.010 pg 22

Question 17

When could you expect to see an illuminated BAT DISCHARGE indication?

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
SH 6.010 pg 22 

*we do not need to know these numbers - just know its
discharging for a preset amount of time

Anytime on DC power with no AC power connected**
IRS system align also..

Question 18

What is the TR Unit light logic?

Answer 18

Failures in flight:
AMBER TR UNIT light and MASTER CAUTION illuminates if:
TR 1 fails
TR 2 AND TR 3 have failed.

On ground
- any TR failure gives Master Caution (ELEC light will also illuminate)
SH 6.010 pg18

Question 19

What do the Generator DRIVE lights indicate?

Answer 19

Associated IDG is NOT producing power to the system.

SH 6.010 pg 5

Question 20

What happens when an IDG is decoupled?

Answer 20

Physically disconnects
(IDG must be removed/replaced from the aircraft by maintenance)
SH 6010 pg 5

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)
SH 6.010 pg 14

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
SH 6.010 pg 12

Question 23

What does the APU GEN OFF BUS light indicate?

Answer 23

The APU is not powering a bus (i.e. APU is ready-to-load) and operating above 95% RPM
SH 6.010 pg 14

Question 24

What is the load shedding process for the APU?

Answer 24

In flight: Galley and Main Buses AUTOMATICALLY shed.
Both IFE buses are also shed if load exceeds design limits.

On the ground: APU takes full electrical load
SH 6.010 pg 13

Question 25

What is the Load Shedding process for the Generators?

Answer 25

One IDG as sole source? Load sheds main and galley buses on Transfer bus 2, then main and galley buses Transfer bus 1, then IFE, on demand.
SH 6.010 pg 14

Question 26

What does the GRD POWER AVAILABLE light indicate?

Answer 26

Ground power is connected and within limits

SH 6.010 pg 7