Brady Electrical

Question 1

What are the 3 major components of the 737 electrical system? Of the 3 which is primary?

Answer 1

AC Power (primary)
DC Power
Standby Power

Question 2

What are the 4 sources of AC power?

Answer 2

2 Engine Generators
APU
Ground Power

Question 3

What is an IDG?

Answer 3

Integrated Drive Generator: It is a drive & a generator in one unit unlike the CSD which is a drive and then a separate generator.

Question 4

What will cause the IDG to physically disconnect?

Answer 4

The IDGs will automatically physically disconnect from the drive if the oil temperature exceeds 182 deg C.

Question 5

What will cause the IDG to electrically disconnect?

Answer 5

Over-frequency: 425 Hz for more than 1.5 seconds or 435 Hz for more than 35 milliseconds.

Under-frequency: 375 Hz for more than 1.5 seconds or 355 Hz for more than 150 milliseconds.

Question 6

Does the APU have a Constant Speed Drive Unit?

Answer 6

No, because the APU is a constant speed engine of 48,800 RPM.

Question 7

What side of the engine is the generator placed on?

Answer 7

The lower left side

Question 8

What does the CSD do? Does the NG have them?

Answer 8

It is a Constant Speed Drive that allows for steady AC power from the generator even though the engine is throttled up and down. No, they have an IDG which is a combo CSD & generator.

Question 9

What does a GRD POWER AVAILABLE light on the NG & MAX mean?

Answer 9

That ground power is available AND, AND the quality is sufficiently good to provide the proper AC power.

Question 10

What are the 3 “golden rules” of AC power on the 737?

Answer 10

1) No paralleling of AC Power
2) When a new source of AC Power is connected, the old source is disconnected
3) A source of AC power does not enter the system automatically, it must be selected* (there is an exception with the NG/MAX)

Question 11

The first “golden rule” of AC power on the 737 states, “no paralleling of AC power”. What does this mean?

Answer 11

This means that you cannot put more than one generator on a transfer bus

Question 12

What is so “main” about the main DC bus?

Answer 12

Not much, it’s a misnomer. The “main” DC busses control things like recirc fans, door area heaters, logo lights, etc.

Question 13

Which busses are connected to “big ticket” items like electric hydraulic pumps, required lighting, etc.?

Answer 13

AC XFER BUS 1 and AC XFER BUS 2

Question 14

The CAB/UTIL switch controls what?

Answer 14

It is a bit of a misnomer in that it will switch on & off the galley AND main buses.

Question 15

What is powered by the CAB/UTIL switch on the NG?

Answer 15

Recirc fans
Door area heaters
Drain mast heaters
Lavatory water heaters
All galley buses
Shaver outlets
Logo lights
Potable water comp

Question 16

What do you need to know about the red guarded generator disconnect switches? When would you use them?

Answer 16

Once you use the red guarded generator disconnect switch in flight you can only reconnect the generator ON THE GROUND. The QRH will direct you to use this switch with a DRIVE light above. The drive light comes on for IDG low oil pressure or an under frequency with the engine running.

Question 17

On the generator drive panel, what does it mean when the DRIVE light illuminates?

Answer 17

The DRIVE light comes on for IDG low oil pressure or an under frequency with the engine running. The QRH will direct you to use the red guarded generator disconnect switch.

Question 18

What is the normal source of DC power?

Answer 18

The 3 transformer rectifier units which are powered by the AC Transfer Buses.

Question 19

What is the backup source of DC power?

Answer 19

The Main Battery (and the auxiliary battery if installed, most NG & MAX have it).

Question 20

What does the TRU do?

Answer 20

Transformer Rectifier Units convert 115V AC power to 28V DC power.

Question 21

With the BUS TRANSFER switch in auto, how are the TRs connected?

Answer 21

They are connected in parallel. So energy from TR1 and TR2 is free flowing to DC Bus 1 and DC Bus 2 (TR2 is also getting power from TR3 which then flows along).

Question 22

What happens to the DC electrical system at glideslope capture?

Answer 22

The BUS TIE RELAY opens which isolates TR1 to DC Bus 1 and TR 2 & 3 to DC Bus 2.

Question 23

What is the difference if the TR UNIT light illuminates in flight vs on the ground?

Answer 23

On the ground, if ANY TR unit fails, the light will illuminate. In the air the light will illuminate only if TR1 fails OR if TR2 AND TR3 fail.

Question 24

What is another way to think of the Main Battery Charger?

Answer 24

You can think of it as a 4th TR as it does more than charge the Main Battery. After it charges the Main Battery, it can also provide power to the HOT BAT BUS.