S 714-010

 (2)  
Do these steps at the standby power control panel on the pilots' overhead panel, P5:

 (a)  
Make sure the BAT switch is in the ON position.

 (b)  
Make sure the white light in the switch comes on.

 (c)  
Turn the STBY POWER switch to the BAT position.

 (d)  
GUN 001-010, 101-107, 206;
 Make sure the yellow DISCH light comes on.

 (e)  
GUN 051; Make sure the yellow MAIN BAT DISCH light comes on.

 (f)  
Turn the STBY POWER switch to the AUTO position.

 (g)  
Push the ELEC HYD switch on the EICAS MAINT panel on the right side panel, P61.

 (h)  
Make sure this DC-V value shows on the bottom EICAS display: 1) STBY/BAT 28 +/- 3

 (3)
Remove electrical power if it is not necessary (AMM 24-22-00/201).

 S 864-011
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A

757 MAINTENANCE MANUAL
 TRANSFORMER-RECTIFIER - DESCRIPTION AND OPERATION
_________________________________________________
 1.  General_______
 A.  The transformer-rectifier system supplies primary DC power and APU starting power by converting main AC power. System components include transformer-rectifier units (TRU's), a DC tie control unit, a DC tie relay, an APU start TRU fan control relay, and APU TRU start relay.
 2.  Component Details (Fig. 1)_________________
 A.  Transformer-Rectifier Units
___________________________
 (1)  
Two identical TRU's convert nominal 115-volt, 3-phase AC power into unregulated 28-volt DC power. Each TRU is capable of providing 120 amps continuous current with forced-air cooling. Internal protection circuits prevent TRU damage due to a shorted output. An internal meter shunt allows measurement of TRU output current by the EICAS computers. The main TRU's are located in left forward equipment center rack E1.

 (2)  
A third TRU is used for APU starting. This TRU contains an integral fan and overheat switches for thermal protection. The APU start TRU is located aft of aft equipment rack E6.

 B.  DC Tie Control Unit
___________________
 (1)  
The DC tie control unit controls the operation of the DC tie relay when an autoland signal is not present. The control unit will latch the DC tie relay closed under two conditions. The conditions are:

 (a)  
Voltage on one bus drops below 19 volts for more than 11.5 seconds, or

 (b)  
Buses are not powered up within 5.75 seconds of each other.

 (2)  
The control unit resets (opens the DC tie relay) under three conditions. Condition 1 is if both buses lose power for more than

 11.5 seconds, providing the buses were previously tied. Condition 2 is if both buses lose power within 5.75 seconds of each other, providing the buses were not previously tied. Condition 3 is if the control unit loses power for more than 3 seconds.

 (3)  
The DC tie control unit is located in main power distribution panel P6.

 C.  DC Tie Relay
____________
 (1)  
The DC tie relay is located in P6 panel. It is used to tie the main dc buses together if one loses power. The relay receives 28 volts dc from the 28-volt dc battery bus, and is actuated by the dc tie control unit when the airplane is not in the autoland mode.

 (2)  
When an autoland signal is present, the dc tie relay is actuated by the captain's or first officer's instrument bus voltage sensing unit (IBVSU). When the IBVSU's sense a loss of power on either main ac bus, a ground is applied through the center bus isolation relay to actuate the dc tie relay, which in turn restores the dc power.

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