systems. A second engine-driven generator is operated during  flight to share the electrical loads and provide redundancy.  The complexity of multiple generators and a variety of  distribution buses requires several control units to maintain  a constant supply of safe electrical power. The AC electrical  system must maintain a constant output of 115 to 120 volts at  a frequency of 400 Hz (±10 percent). The system must ensure  power limits are not exceeded. AC generators are connected  to the appropriate distribution buses at the appropriate time,  and generators are in phase when needed. There is also the  need to monitor and control any external power supplied to  the aircraft, as well as control of all DC electrical power.  Two electronic line replaceable units are used to control the  electrical power on a typical large aircraft. The generator  control unit (GCU) is used for control of AC generator  functions, such as voltage regulation and frequency control.  The bus power control unit (BPCU) is used to control  the distribution of electrical power between the various  distribution buses on the aircraft. The GCU and BPCU  work together to control electrical power, detect faults, take  corrective actions when needed, and report any defect to the  pilots and the aircraft’s central maintenance system. There  is typically one GCU for each AC generator and at least one  BPCU to control bus connections. These LRUs are located  in the aircraft’s electronics equipment bay and are designed  for easy replacement.  The BPCU is a dedicated computer that controls the electrical  connections between the various distribution buses found on  the aircraft. The BPCU uses contactors (solenoids) called bus  tie breakers (BTB) for connection of various circuits. These  BTBs open/close the connections between the buses as needed  for system operation as called for by the pilots and the BPCU.  This sounds like a simple task, yet to ensure proper operation  under a variety of conditions, the bus system becomes very  complex. There are three common types of distribution bus  systems found on transport category aircraft: split bus, parallel  bus, and split parallel.  BPCU, which allows accurate current monitoring of the  system. A typical aircraft employs several CTs throughout  the electrical system.  When the pilot calls for generator power by activating the  generator control switch on the flight deck, the GCU monitors  the system to ensure correct operation. If all systems are  operating within limits, the GCU energizes the appropriate  generator circuits and provides voltage regulation for the  system. The GCU also monitors AC output to ensure a  constant 400-Hz frequency. If the generator output is within  limits, the GCU then connects the electrical power to the main  generator bus through an electrical contactor (solenoid). These  contactors are often called generator breakers (GB) since  they break (open) or make (close) the main generator circuit.  After generator power is available, the BPCU activates various  contactors to distribute the electrical power. The BPCU  monitors the complete electrical system and communicates  with the GCU to ensure proper operation. The BPCU employs  remote current sensors known as a current transformers (CT)  to monitor the system.  [Figure 9-103] A CT is an inductive unit that surrounds the main power  cables of the electrical distribution system. As AC power  flows through the main cables, the CT receives an induced  voltage. The amount of CT voltage is directly related to the  current flowing through the cable. The CT connects to the  Modern twin-engine aircraft, such as the Boeing 737, 757,  777, Airbus A-300, A-320, and A-310, employ a split-bus  power distribution system. During normal conditions, each  engine-driven AC generator powers only one main AC bus.  The buses are kept split from each other, and two generators  can never power the same bus simultaneously. This is very  important since the generator output current is not phase  regulated. (If two out-of-phase generators were connected  to the same bus, damage to the system would occur.) The  split-bus system does allow both engine-driven generators  to power any given bus, but not at the same time. Generators  must remain isolated from each other to avoid damage. The  GCUs and BPCU ensures proper generator operation and  power distribution. On all modern split bus systems, the APU can be started  and operated during flight. This allows the APU generator  to provide back-up power in the event of a main generator  failure. A fourth emergency generator powered by the ram air  turbine is also available if the other generators fail.  Split-Bus Power Distribution Systems  Figure 9-103.  Current transformer.  BPCU CT output Current transformer Main AC power cable 9-61