3.
(2)
Two control switches, located on the P5 overhead panel, are associated with the standby electrical system. One switch is a guarded latching push-button switch designated BATTERY. By pushing this switch to the latched-in (ON) position, the main battery bus will be energized from the main battery hot bus and the APU battery bus will be energized from the APU battery hot bus. However, if dc bus 3 power is available, the main and APU battery buses will transfer to that source regardless of battery switch position.
(3)
The second switch is a three-position rotary switch designated STANDBY POWER. The switch positions are OFF, AUTO, and BAT. Normal operation is with the BATTERY switch in the ON position and the STANDBY POWER switch in the AUTO position. In these switch positions, a loss of dc bus 3 power results in the main battery bus transferring automatically to the main battery hot bus, and the APU battery bus transferring to the APU battery hot bus.
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747-400 MAINTENANCE MANUAL
SEE A
SEE B
P6 POWER DISTRIBUTION PANEL
A
ELECTRICAL
CONNECTOR DC VOLTAGE SENSE
RECEPTACLE RELAY
B
DC Voltage Sense Relay Location Figure 1
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747-400 MAINTENANCE MANUAL
SEE A
P180
P180-1
SEE B
P180 DC POWER DISTRIBUTION PANEL
A
SINGLE TRU OPERATION RELAY
B
Single TRU Operation Relay Location Figure 2
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747-400
MAINTENANCE MANUAL
(4) (5) (6) When power is lost on the dc bus 3, the dc voltage sense relay will de-energize causing the battery transfer relay to de-energize.The main battery relay coil will receive a ground through the de-energized contacts of the battery transfer relay when the BATTERY switch is in the ON position. This energizes the main battery relay, which enables the main and APU battery buses to be powered by the main and APU hot battery buses.This bus transfer condition will also occur (be forced) whenever the standby power switch is set to the BAT position or if the BCUs energize the single TRU relay when the BCUs sense a loss of three TRUs.
(7) The APU battery bus transfer relay switches the APU battery bus to the main hot battery bus for 65 seconds during APU starting.
NOTE:____ The APU starter motor is the only component connected to the APU battery during the 65 second start cycle.
DC Standby Power System Simplified Schematic
Figure 3
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747-400 MAINTENANCE MANUAL
DC VOLTAGE SENSE RELAY - REMOVAL/INSTALLATION
_____________________________________________
1. General_______
A. This procedure has two tasks. One task removes the DC voltage sense relay (R7228). The other task installs the relay.
B. The installation task includes an operational check of the relay.
C. The DC voltage sense relay is installed in the P6 (main power distribution) panel. The P6 panel is on the right side of the flight compartment.
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