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A 737-300/400/500MAINTENANCE MANUAL
P6-4 CIRCUIT BREAKER PANEL
28V AC Power Distribution Simplified SchematicFigure 1
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
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737-300/400/500MAINTENANCE MANUAL
STANDBY POWER SYSTEM - DESCRIPTION AND OPERATION
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General (Fig. 1)
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A. The standby system is used to supply power to essential ac and dc systems. Under normal conditions (in flight), the standby power switch is in the AUTO position, the standby ac bus is energized from the 115-volt oA transfer bus No. 1, and the standby dc bus is energized from dc bus No. 1. In the event of losing either the ac or dc normal power, the auto standby power relay (R37) is deactivated, connecting the dc standby bus to the battery bus and connecting the ac standby bus to the static inverter (Fig. 2).
B. During checkout of the system on the ground, or refueling the aircraft on battery power only, the standby system is activated by placing the standby power switch in the BAT position. This activates the manual standby power relay (R328) which connects the dc standby bus to the battery bus and the ac standby bus to the static inverter.
C. During electrical smoke isolation procedures, the standby system is deactivated by placing the standby power switch in the OFF position.
D. The ac standby power bus is a single-phase, 115-volt ac power source which is used to supply 400-Hz power to essential ac systems (Fig. 2). A static inverter, energized by 28 volts dc from the storage battery through the battery bus, is the alternate source of single-phase, 400-Hz, 115-volt power to the ac standby bus. The power is used for essential communication and navigation equipment, so that the pilot's essential flight instruments will not be deactivated in the event all 400-Hz generated power becomes unavailable during flight.
E. Components of the standby power system are the voltage sensor module, static inverter, standby power switch, battery switch, standby power relays, and the standby power off light.
Static Inverter (Fig. 1)
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A. The static inverter converts 28-volt dc power to 115-volt ac power. It can supply 500 watts of power to the ac standby bus. The static inverter is energized whenever the battery switch is set to ON. The static inverter is on electrical equipment shelf E3-1.
Voltage Sensor Module (Fig. 1)
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A. The voltage sensor module receives power from 28-volt dc bus 1 and senses power from 115-volt ac transfer bus 1. When either power source fails, the voltage sensor signals standby power relay auto R37 to de-energize, which connects static inverter output to 115-volt ac standby power bus. The voltage sensor module is in panel P6.
Standby Power Switch (Fig. 1)
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A. The standby power switch on panel P5 arms the standby power system when set to AUTO, activates the standby power system when set to BAT, and deactivates the system when set to OFF.
Battery Switch (Fig. 1)
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A. The battery switch on panel P5, when set to ON, energizes the battery bus, which provides power to the standby power system, and provides power to the standby power off light.
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