MT SAME AS
W
RESET

FF G-3
W
LV
DIFFERENTIAL
PROTECTION

W
HV
W
DC

G5 APU GENERATORCONTROL UNIT (P6)TRIP GEN 2
K301 FEEDER FAULT
MT

W
FF
INTERNALS
SAME AS
W G-3 LV
RESET
W
HV
UNDER
VOLTAGE
W
DC
G4 GENERATOR CONTROLUNIT 2 (P6)
GEN 1
TRIP

K201 LOW VOLTAGE MT
W
FF
W
RESET
LV

W
OVER

HV
W
VOLTAGE DC
M238 ANNUNCIATOR LIGHTS MODULE (P6)
TRIP

K101 HIGH VOLTAGE
G3 GENERATOR CONTROL UNIT 1 (P6)
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
Annunciator Lights Simplified Schematic
Figure 5 (Sheet 1)

A 737-300/400/500MAINTENANCE MANUAL

P6-4 CIRCUIT BREAKER PANEL M238 ANNUNCIATOR LIGHTS MODULE (P6)
Annunciator Lights Simplified SchematicFigure 5 (Sheet 2)
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
737-300/400/500MAINTENANCE MANUAL
 C.  The above tests are made by using only airplane facilities, but for additional flexibility for a more extensive test of the system, the generator control units (GCUs) and bus protection panel (BPP) are pulled out and a multiple T-connection is made to breakout boxes. When using breakout boxes the two halves of the connector for the GCU or BPP are plugged with the connector halves of the breakout box. Switches and jacks on the breakout boxes provide additional connections to the electrical system which would otherwise be difficult to make, such as a separate source of three-phase power when neither airplane generators nor external power circuits are used. During all such tests indicating lights on the pilots' panel P5 and the electrical power annunciator panel M238 are used to monitor which portions of the system are energized. The annunciator panel indicates four causes which result in the generator control relay tripping for one of the three airplane generators. These are: high or overvoltage (HV), low or undervoltage (LV), generator feeder faults (FF) and a manual trip (MT) due to a manual operation of the generator control switch. The feeder fault may be caused by an overcurrent condition or the annunciator panel may be indicating the operation of the differential protection circuit. The manual generator control relay trip annunciator is reset by momentarily closing the generator control switch. A separate reset switch is used to reset the remaining annunciators. The annunciator panel also has lights to indicate that power is on when any of the several ac and dc buses are energized.
 8.  Master Caution System Inputs____________________________
 A.  The electrical system contains equipment to provide signals to the flight crew when the system functions are abnormal. The system control components which operate due to abnormal conditions in conjunction with generator No. 1 and No.2 causing the master warning system to function are the generator breakers (C801, C802, C889, and C890) and the generator transfer bus relays (R3 and R4).
 9.  Main Power Circuit Breakers___________________________
 A.  Three-phase magnetic-latch type circuit breakers connect the main engine generators, APU generator, and external power to the generator load buses. The main contact rating of the breakers is 125 amps. All of the circuit breakers have auxiliary contacts rated at 7.5 amps for 28 volts dc, and contain an electromagnetic coil which actuates the switching mechanism to the on or off position, and permanent magnets for latching the mechanism in the closed position.
 B.  The transfer bus relays have a main contact rating of 50 amps and are three-phase, double-throw, center-off, electrically held. Each transfer bus relay contains two 28-volt dc coils, and will connect buses in one direction or the other depending on which coil is energized. The main contacts are open if 28 volts dc is not applied to either coil.
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