voltmeter shows 115 |5 volts ac. 3) Close the STBY BUS PWR circuit breaker on the P6-4 panel.
 S 864-008
 (6) Remove electrical power if it is not necessary (Ref 24-22-00/201).
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
737-300/400/500MAINTENANCE MANUAL
 AC EXTERNAL POWER - DESCRIPTION AND OPERATION
_____________________________________________
 1.  General_______
 A.  AC external power, when provided to the external power receptacle, can power the entire electrical power system or only the ground service buses. The components of the ac external power system are the external power panel, bus protection panel, external power contactors, external power relay, ground service relay, and manual control devices.
 B.  External Power Panel (Fig. 1)
 (1)  
The external power panel P19 is forward and right of the nose wheel well. The external power receptacle and EXTERNAL POWER CONN and NOT IN USE lights are in the external power panel.

 (2)  
The external power receptacle has four long pins, for connecting phases A, B, C, and N, and two short pins, for connecting the dc control interlock. If external power is removed from the receptacle while energized, the interlock is opened and the electrical system is de-energized, preventing arcing at panel P19.

 C.  
Bus Protection Panel (BPP) (Fig. 2)

 (1)  
The bus protection panel (BPP) is in the P6 panel. The BPP provides control and protection for the external power system. The external power T-R, in the BPP, changes 115-volt ac external power to 28-volt dc power.

 (2)  
Control power for the BPP is from the external power T-R. Control power exits the BPP, passes through the external power plug jumper, and re-enters the BPP. Back-up control power is from the hot battery bus.

 (3)  
The BPP protects against overvoltage (130 |3 volts), undervoltage (100 |3 volts), and negative sequence voltage. The BPP trips the external power contactors when a fault is detected. The BPP also has anti-cycling protection, which prevents the external power system from cycling when a fault exists and a manual control device is held to ON.

 D.  
External Power Contactors (EPC) (Fig. 1)

 (1)  
The external power contactors (EPC) are in the right side of the nose wheel well. Power from the external power receptacle energizes the electrical power system when the EPCs are closed.

 (2)  
When the GND PWR switch on P5 panel is set to ON and the BPP senses no faults, external power relay R348 closes. When R348 is closed, EPC 1 and EPC 2 close and GROUND SERVICE switch is deactivated.

 (3)  
The EPCs open under the following conditions.

 (a)  
GND PWR switch set to OFF.

 (b)  
External power faults sensed by BPP.

 (c)  
APU or engine GEN switches set to ON.

 (d)  
External power removed from receptacle while energized.

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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.

A 737-300/400/500MAINTENANCE MANUAL
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A 737-300/400/500MAINTENANCE MANUAL
28V DC
HOT BAT BUS
C1075
EXT PWR CONTROL

EPC 2 TRIP
28V DC

SIGNAL
BAT BUS

MD&T EPC 1 TRIP
B
C26 DC SIGNALBUS INDICATION
B
BATP6-5 CIRCUIT BREAKER R348 EXTERNAL
L1 GND PWR AVAILABLEPANEL P5-4 SYSTEM GEN AND APU PANEL POWER RELAY (P6) TRIP
CLOSE
115V AC
EXT PWR
　
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