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 1.  General (Fig. 1)_______
 A.  The 28-volt dc power distribution system consists of dc buses 1 and 2, battery bus, hot battery bus, and switched hot battery bus. Components are T-R units, battery, control relays, and manual control switches.
 2.  Transformer - Rectifier (T-R) Units (Fig. 1)___________________________________
 A.  The three T-R units are in electronic compartment on shelf E3-1. The T-R units convert 115-volt ac power to 28-volt dc power.
 3.  Battery (Fig. 1)_______
 A.  The nickel-cadmium battery is in the electronic compartment near shelf E3.
 4.  Control Relays (Fig. 1)______________
 A.  Control relays R1, R2, R9, R41, R326, and R329 are in panel P6.
 5.  Manual Control (Fig. 1)______________
 A.  Battery switch, bus transfer switch, and standby power switch are in panel P5.
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A 737-300/400/500MAINTENANCE MANUAL

B

DC Power Distribution Component Location
Figure 1

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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A 737-300/400/500MAINTENANCE MANUAL
 6.  Operation (Ref 24-31-00, Fig. 4)
_________
 A.  T-R 1 and 2 receive power from 115-volt ac transfer bus 1 and 2, respectively. T-R 3 receives power from 115-volt ac bus 2 through T-R 3 input relay R329, which is energized when the battery switch is set to ON and bus 2 is energized.
 B.  T-R 1 and 2 provide power directly to dc bus 1 and 2, respectively. T-R 3 provides power to dc bus 1, dc bus 2, and battery bus.
 (1)  
Power from T-R 3 to dc bus 1 and 2 is through diode M1220, which prevents dc bus 1 and 2 from back feeding the battery bus. T-R 3 disconnect relay R9 must be closed for dc bus 1 to receive power from T-R 3. When the bus transfer switch S2 is set to AUTO, R9 is normally energized. Ground for R9 is controlled by the automatic flight control (AFC) accessory unit. During approach, the AFC removes the ground from R9, de-energizing it and separating dc bus 1 from dc bus 2. This prevents a failure on one dc bus from overloading the other dc bus, which would result in loss of power to the dual channel systems necessary during approach and landing. DC bus 1 and 2 may be separated by setting S2 to OFF, which de-energizes R9.

 (2)  
Power from T-R 3 to battery bus is through relaxed contacts of battery bus relay auto R1 and battery bus relay manual R326.

 C.  
The battery is the alternative source of power to the battery bus. Battery bus control is automatic with manual override. Regardless of battery bus source or control, the battery switch must be set to ON to power the battery bus.

 (1)  
If the 115-volt ac bus 2 is energized, and the battery switch is set to ON, T-R 3 input relay R329 and battery transfer relay R2 will be energized. Relay R329 provides a path for input power to T-R 3 and relay R2 disables relay R1. T-R 3 provides power to battery bus.

 (2)  
If T-R 3 should lose output power, relay R2 is de-energized and relay R1 is energized. The battery provides power to battery bus through relay R1.

 (3)  
Manual control of the battery bus is from the standby power switch. Setting the switch to BAT energizes relay R326. The battery provides power to the battery bus through relay R326.

 D.  
The battery provides power to the switched hot battery bus when the battery switch is set to ON. Generator power control relay R41 energizes when the battery switch is set to ON and completes the circuit from the battery to the switched hot battery bus.

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