A.  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. 1). 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.
Jan 20/83 24-54-0 Page 1
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.

B.  Under normal conditions in flight the standby power switch on the P5 overhead panel should be in the AUTO position. (Fig. 2) A means is thereby provided for automatically transferring the ac and dc standby buses from the 115-volt O/A transfer bus No. 1 and the dc bus No. 1 respectively to the ac supply output from the static inverter and the battery bus through power transfer relays, R36 and R37. (Fig. 3) With the switch in the AUTO position, only the No. 2 standby power transfer relay (R37) is energized by the No. 1 dc bus and a ground generated by the ac voltage sensor when the 115-volt O/A transfer bus No. 1 is energized. When either the dc bus No. 1 or phase A of the No. 1 transfer bus becomes de-energized, transfer relay (R37) relaxes and relay No. 1 (R36) closes, transmitting a dc turn-on signal to the static inverter. Closure of relay No. 1 (R36) connects the inverter output ac through normally closed contacts in relay No. 2 (R37) to the 115-volt ac standby bus and the battery bus dc directly to the dc standby bus. Relay (R36) becomes energized from the battery bus through normally closed contacts in relay (R37) and a ground connection through the landing gear relay (K8). Relay (R36) will drop out on landing. (Fig. 4) To prevent this, the standby power switch may be moved to the BATT position. This supplies an alternate ground for relay (R36), keeping the inverter on as a supply for the 115-volt ac standby bus and the dc standby bus on the battery bus. The standby power switch OFF position provides for de-energizing the system during electrical smoke isolation procedures. (Refer to wiring diagram.) The standby power switch has three connections to ground: two for the coil of the standby power transfer relay (R36) and one for the coil of relay (R37 (Fig. 5) The standby power off light (L5), also on the overhead panel, comes on when both standby power transfer relays (R36 and R37) relax, indicating the loss of power on the standby buses.
500 
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H21710
500  Standby Power System 
Nov 01/74  Figure 1  24-54-0 
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Standby Power System Control Location  501 
24-54-0  Figure 2  Feb 20/84 
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H21719 H21701 H21716
525  Standby Power System Component Location 
Feb 15/71  Figure 3  24-54-0 
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Standby Power Distribution  500 
24-54-0  Figure 4  Jan 15/71 
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504  Standby Bus Circuit Logic 
Jan 15/71  Figure 5  24-54-0 
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BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

STANDBY POWER SYSTEM - TROUBLE SHOOTING
EFFECTIVITY
AT CN-RMI THRU CN-RML CA B2501 THRU B2503 IDO 601 OI G-BGTW THRU G-BJBJ ZD G-AVRL THRU G-BJCV
1.  General

A.  Assume that there are no wiring malfunctions nor more than one problem in the system. If there is more than one malfunction or there are open or shorted wires, further isolation is necessary.
B.  If it is not practical to make the listed checks, replacement of the suspected part(s) may be more efficient for short ground-time maintenance. Refer to wiring diagram 24-34-01 and 24-50-00.
WARNING:  EXERCISE EXTREME CAUTION WHEN WORKING AROUND ENERGIZED PANELS. HIGH VOLTAGES PRESENT CAN BE FATAL.
2.  Prepare for Trouble Shooting
A.  Verify that battery is charged (Ref 24-31-11).
B.  Provide electrical power (Ref 24-22-0).
3.  Standby Power System Trouble Shooting Charts
A.  Place STANDBY POWER switch on P5 panel to BAT.
B.  Open STBY BUS PWR circuit breaker on P6-5 panel and DC STANDBY POWER TR circuit breaker on P6-4 panel.
　
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