(b)  
Install the four screws that attach the IBVSU to the panel.

 (c)  
Tighten the screws.

 S 434-011
 (2)
Connect the electrical connector DM7507 or DM7508 to the IBVSU.

 S 414-012

 (3)  
Close the access doors on the P6 panel.

 D.
Do a Test of the Instrument Bus Voltage Sense Unit

 (1)  
Supply electrical power (AMM 24-22-00/201).

 S 714-022

 (2)  
Do a test of the IBVSU:

 (a)
Make sure the STANDBY POWER switch is set to AUTO.

 S 864-021
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747-400 MAINTENANCE MANUAL
 (b)  
Open these circuit breakers:

 1)  P6, Main Power Distribution Panel
 a) 6B11, F/O XFR BUS-PRIM
 b) 6B23, CAPT XFR BUS-PRIM
 c) 6B26, APU STBY ALT PWR

 (c)  
Make sure the primary flight display (PFD) and navigation display (ND) operate correctly.

 (d)  
Open these circuit breakers:

 1)  P6, Main Power Distribution Panel
 a) 6B5, CAPT XFR BUS-ALTN
 b) 6B7, F/O XFR BUS-ALTN

 (e)  
Make sure the primary flight display (PFD) and navigation display (ND) do not operate.

 (f)  
Close these circuit breakers:

 1)  P6, Main Power Distribution Panel
 a) 6B11, F/O XFR BUS-PRIM
 b) 6B23, CAPT XFR BUS-PRIM

 (g)  
Make sure the primary flight display (PFD) and navigation display (ND) operate correctly.

 (h)  
Close these circuit breakers:

 1)  P6, Main Power Distribution Panel
 a) 6B5, CAPT XFR BUS-ALTN
 b) 6B7, F/O XFR BUS-ALTN
 c) 6B26, APU STBY ALT PWR

 S 864-014
 (3)  Remove electrical power, if it is not necessary (AMM 24-22-00/201)
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 1.

2.

3.

A
747-400 MAINTENANCE MANUAL
 28-VOLT AC POWER DISTRIBUTION - DESCRIPTION AND OPERATION
_________________________________________________________
General
_______
 A.  Two single phase, 28 volt ac, 400 Hz buses are provided to supply various lights, indicators, and other loads requiring 28 volt ac power. These two 28 volt ac buses are supplied through seperate single phase autotransformers.
28-Volt AC Transformer (Fig. 1)
______________________
 A.  There are two autotransformers, each is an open-frame type cooled by natural air convection. Screw-type terminations are provided for the 115-volt ac input, 28 volt ac output, and the ground connections. Aluminum strip conductors are used in the transformer windings.
 B.  The main 28 volt ac autotransformer, rated at 750 volt-amps, is located in the main power distribution panel P6. The 28 volt ac ground service autotransformer, rated at 2.5 kva, is located in the power distribution center-left panel P414.
Operation
_________
 A.  Functional Description
 (1)  
AC bus three (phase A) supplies 115 volt ac power to the primary of the main 28 volt ac autotransformer. The ground service bus (phase C) supplies 115 volt ac power to the 28 volt ac ground service autotransformer.

 (2)  
Each autotransformer has a single phase thermal circuit breaker that provides protection for the input wiring, autotransformer, and the output wiring to the distribution buses.

 (3)  
The autotransformers are capable of suppling rated load at 28 volts rms, 0.75 lag to unity power factor, continuously. The output voltage regulation limit is within 5.2 percent from no load to full load. The operating efficiency is above 90 percent at normal operating temperature.

 (4)  
The 28 volt ac buses supply power to systems such as pressure and trim indicators, sensors, flight deck lights, and ground service lighting.
　
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