C.  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. (See figure 5.) 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 generators No. 1 and No. 2 causing the master warning system to function are, the generator breakers (C801 and C802) the generator bus tie- breakers (C804 and C805) and the generator transfer bus relays (R3 and R4). Refer to Master Warning and Caution Lights - Description and Operation, Chapter 33.
9.  Main Power Circuit Breakers
A.  Three-phase magnetic latch-type circuit breakers are used to connect the main engine generators to the generator load buses and to connect the APU generator to the common or tie bus. Identical circuit breakers (also known as contactors) connect this central tie bus to the generator load buses. The external power contactor is also this type of circuit breaker, but has a main contact rating of 175 amperes. The main contact rating of the other latch-type breakers or contactors mentioned above is 125 amperes. All of these circuit breakers have auxiliary contacts rated at 7.5 amperes 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 either the open or closed position.
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BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

B.  The transfer bus contactors or transfer relays have a main contact rating of 50 amperes and are three-phase, double throw, center off, electrically held. Each transfer contactor 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 volt dc is not applied to either coil.
C.  Thermal circuit breakers are largely protective devices to limit the current in particular wires or as safety devices for a component. The opening time for such circuit breakers is inversely related to the current flow in the breaker element. Single phase and three-phase thermal circuit breakers are used in the electrical system. An individual phase current which exceeds the breaker trip limit will open all three phases of a thermal circuit breaker. Most of the thermal circuit breakers are located in the left load control center on the P18 panels.
D.  The magnetic latch-type circuit breakers mentioned above are located either in the right-hand load control center P6 or in the right-hand forward and aft compartments outboard of the nose wheel well. This later location contains the external power contactor and the generator No. 1 and No. 2 circuit breakers. This type of breaker is a three-pole electrically-operated, plunger-style solenoid unit. Input and output cables are connected to the A and B terminals respectively. The auxiliary switches and coil circuit connections are made through the multiple wire connector. (See figure 6.)
E.  The magnetic latch-type circuit breaker has one magnet coil which is used for both the close and trip operations. A coil switch in the breaker assembly determines which way the current flows in the coil which in turn determines the resulting close or trip operation of the unit. Two diodes across the coil suppress power spikes generated when the coil circuit is opened following a trip or close signal. Other auxiliary switches are provided for electrical interlocking and circuit monitoring functions
　
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