DC Power System

Question 1

DC (Direct Current) system overview

TR (Tie Relay)

Answer 1

• The 737 uses 3 transformer rectifiers to supply 28 volt DC power
• The TRs transfer change 155 volts AC from the transfer busses into 28 volt DC for use by the DC busses
• The TRs change 115 volt AC from the transfer busses into 28 volt DC for use by the DC busses

Question 2

TR (Tie Relay)

Answer 2

-The TRs change 115 volt AC from the transfer busses into 28 volt DC for use by the DC busses

Question 3

TR (Tie Relay) overview

Answer 3

• TR 1 is supplied by transfer bus 1 and is the normal power source for DC bus 1
• TR 2 is supplied by transfer bus 2 and is the normal source for DC bus 2
• Transfer Bus 2 is the normal power source for TR 3 (Tie Relay) but transfer 1 can also be used

Question 4

TR 3 (Tie relay)

Answer 4

• TR 3 is an auxiliary sourced of DC power if TR 1 and 2 fails
• TR 3 is also the normal source of power for the battery bus

Question 5

TR failure (Tie relay)

Answer 5

-If a TR fails, the remaining TRs can supply power for the total DC load

Question 6

DC busses distribution

Answer 6

-DC busses are connected by a cross bus tie relay

Question 7

Relays on an ILS approach

Answer 7

• The relay opens automatically when the glide-slope is captured during the ILS approach
• The relay opens to isolate the navigation receivers and flight computers. This prevents the removal of all approach information because of only 1 electrical failure

Question 8

Relays + bus transfer switches

Answer 8

-The relay also opens when the BUS TRANSFER switch is moved to off

Question 9

Normal sources of power to the DC busses fail

Answer 9

-The emergency source of DC power is the batteries

Question 10

Battery Power

Answer 10

-Battery power:

• 2 standby busses
• Battery Bus
• Hot battery bus
• Switched hit battery bus

Question 11

The Hot Battery

Answer 11

• Hot battery bus is always connected to battery power

- It’s components operate as long as battery voltage is above minimum value

Question 12

Hot Battery Source

Answer 12

• Power to the switched hot battery bus controlled by the battery switch
• The battery switch and battery operations are discussed as part of the standby power system

Question 13

Automatic battery

Answer 13

-The automatic battery charger retorted and maintains full battery voltage

Question 14

Battery charger

Answer 14

-The battery charger is supplied power from AC ground service bus 2 when a source of AC power is available

Question 15

Second battery charger (left side)

Answer 15

-A different battery charger for the auxiliary battery is supplied power AC ground service bus 1

Question 16

AC and DC metering panel

Answer 16

Normal indications:
-Normal voltage 26 volts

• Amps vary with elect
• The maintenance switch is used by maintenance personnel to do electrical system tests