Protection

The electrical power system uses automatic control to protect the system from source failure or load failure.

These line replaceable units (LRUs) supply the system with protection and control logic:

- Left generator control unit (GCU 1)

- Right generator control unit (GCU 2) APU generator control unit (AGCU)

- Bus power control unit (BPCU)

- Standby power control unit (SPCU)

- Start converter unit (SCU).

The GCUs monitor the system to control and protect the IDGs. The APU GCU and the starter converter unit (SCU) work together to control and protect the APU startergenerator.

The bus power control unit (BPCU) controls and monitors the use of external power. The BPCU protects the airplane from external power with quality out of limits.

Control

These modules on the P5 panel give manual control of the electrical power system:

- Electrical meters, battery and galley power module(P5-13)

- Generator drive and standby power module (P5-5)

- AC systems, generator and APU module (P5-4).

The switches on these modules send signals to the LRUs that control portions of the system. The LRUs then energize or de-energize relays or breakers to control electrical power.

Breakers allow power to the buses. A breaker closes to let power through it. The breakers close when you command the GCU to close it. The GCU closes the breaker only when power is good and no other power source is on the bus. The BPCU monitors the breaker positions. The GCUs and AGCU receive breaker position from the BPCU.

These are the main breakers:

- External power contactor (EPC)

- APU power breaker (APB)

- 2 generator control breakers (GCB)

- 2 bus tie breakers (BTB).

The modules on the P5 also let you monitor the status of the electrical power system. The panels have lights and an alpha-numeric LED display.

737 GENERAL -- ELECTRICAL POWER COMPONENT LOCATIONS

Electrical power components are in these areas of the airplane:

- Flight compartment

- External power panel

- Forward lower fuselage

- EE compartment

- Engine

- APU compartment.

Flight Compartment

Modules on the P5 overhead panel supply these functions for the electrical power system:

- Manual control

- Indication

- DC and standby power system built-in-test equipment (BITE).

The P6 and P18 panels have many electrical power system circuit breakers and relays.

The standby power control unit (SPCU) is in the P6 panel. The SPCU has relays and circuit breakers for the dc and standby power system.