Power Distribution Component Location
Figure 1
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A 737-300/400/500MAINTENANCE MANUAL
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737-300/400/500MAINTENANCE MANUAL
MAIN 115-VOLT AC POWER DISTRIBUTION -
_____________________________________ DESCRIPTION AND OPERATION
_________________________
1. General_______
A. Engine-driven generator, APU-driven generator or external power can furnish main 115-volt ac power to gen buses 1 and 2. Gen bus 1 supplies power to main bus 1, transfer bus 1 through the normal position of transfer relay 1, transfer bus 2 through the alternate position of transfer relay 2, ground service bus through the ground service relay, elex bus, and standby power bus through the standby power relay. Gen bus 2 supplies power to a similar bus network.
2. Transfer Bus Control (Fig. 1)____________________
A. Each gen bus furnishes power to a transfer bus through normal position of transfer relays. If one gen bus loses power, the other gen bus furnishes power to both transfer buses. To protect the other generator, the galley loads are then disconnected. The 115-volt ac power distribution system, except the gen bus that lost power and its main bus, remains energized.
B. When each gen bus is powered by engine or APU-driven generators or external power and the BUS TRANS switch is in AUTO position, transfer bus control is normal. The normal coils of transfer relays 1 and 2 are energized through a closed contact of transfer control relays R349 and R350 from the 28-volt dc supply in the generator control unit.
C. If power from generator 1 is lost, GB1 will open and the coil of relay R349 will de-energize. The circuit to normal coil of transfer relay 1 will open and the circuit to the alternate coil will close. If generator 2 is still supplying power to gen bus 2, the alternate coil of transfer relay 1 will energize, connecting transfer bus 1 to gen bus 2. A 0.25-second time delay occurs before the alternate coil energizes. This prevents bus transfer during normal power source transfer. The sequence of transfer bus control for other power source combinations is similar.
D. The 1 and 2 power distribution systems may be isolated by setting the BUS TRANS switch to OFF. When the BUS TRANS switch is set to OFF, alternate coils of the transfer relays 1 and 2 and TR-3 disconnect relay are disabled.
E. TRANSFER BUS OFF lights for each transfer bus are on panel P5. The lights are controlled by auxiliary contacts in the transfer relays. When the relay coils are de-energized, the lights illuminate. TRANSFER BUS OFF light for bus 1 extinguishes when normal or alternate coil of transfer relay 1 is energized. TRANSFER BUS OFF light for bus 2 extinguishes when normal or alternate coil of transfer relay 2 is energized.
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A 737-300/400/500MAINTENANCE MANUAL
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737-300/400/500MAINTENANCE MANUAL
28-VOLT AC POWER DISTRIBUTION - DESCRIPTION AND OPERATION
_________________________________________________________
1. General (Fig. 1)_______
A. The 28-volt ac power distribution system consists of 28-volt ac bus 1, 28-volt ac bus 2, 28-volt ac transfer bus 1, 28-volt ac transfer bus 2, and 28-volt ac ground service buses. The 28-volt ac power distribution system receives power from the 115-volt ac power system through autotransformers. There is one autotransformer for each 28-volt ac bus.
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