main generator buses are rated at 325 amps and can only be  replaced on the ground. Most current limiters are designed  for ground replacement only and only after the malfunction  that caused the excess current draw is repaired.  The left and right DC generators are connected to their  respective main generator buses. Each generator feeds its  respective bus, and since the buses are connected under  normal circumstances, the generators operate in parallel.  Both generators feed all loads together. If one generator  fails or a current limiter opens, the generators can operate  independently. This design allows for redundancy in the  event of failure and provides battery backup in the event of  a dual generator failure.  exists. If the circuit breaker fails to open, the current limiter  provides backup protection and disconnects the circuit.  Transport category aircraft typically carry hundreds of  passengers and fly thousands of miles each trip. Therefore,  large aircraft require extremely reliable power distribution  systems that are computer controlled. These aircraft have  multiple power sources (AC generators) and a variety of  distribution buses. A typical airliner contains two or more  main AC generators driven by the aircraft turbine engines,  as well as more than one backup AC generator. DC systems  are also employed on large aircraft and the ship’s battery is  used to supply emergency power in case of a multiple failures.  In the center of  Figure 9-101  are four dual-feed electrical  buses. These buses are considered dual-feed since they receive  power from both the left and right generator buses. If a fault  occurs, either generator bus can power any or all loads on a  dual-feed bus. During the design phase of the aircraft, the  electrical loads must be evenly distributed between each of  the dual-feed buses. It is also important to power redundant  systems from different buses. For example, the pilot’s  windshield heat would be powered by a different bus from  the one that powers the copilot’s windshield heat. If one bus  fails, at least one windshield heat continues to work properly,  and the aircraft can be landed safely in icing conditions.  Notice that the dual-feed buses are connected to the main  generator buses through both a current limiter and a diode.  Remember, a diode allows current flow in only one direction.  [Figure 9-102] The AC generator (sometimes called an alternator) produces  three-phase 115-volt AC at 400 Hz. AC generators were  discussed previously in this chapter. Since most modern  transport category aircraft are designed with two engines,  there are two main AC generators. The APU also drives an  AC generator. This unit is available during flight if one of the  main generators fails. The main and auxiliary generators are  typically similar in output capacity and supply a maximum  of 110 kilovolt amps (KVA). A fourth generator, driven by  an emergency ram air turbine, is also available in the event  the two main generators and one auxiliary generator fail.  The emergency generator is typically smaller and produces  less power. With four AC generators available on modern  aircraft, it is highly unlikely that a complete power failure  occurs. However, if all AC generators are lost, the aircraft  battery will continue to supply DC electrical power to operate  vital systems.  The current can flow from the generator bus to the dual-feed  bus, but the current cannot flow from the dual fed bus to the  main generator bus. The diode is placed in the circuit so the  main bus must be more positive than the sub bus for current  flow. This circuit also contains a current limiter and a circuit  breaker. The circuit breaker is located on the flight deck and  can be reset by the pilot. The current limiter can only be  replaced on the ground by a technician. The circuit breaker is  rated at a slightly lower current value than the current limiter;  therefore, the circuit breaker should open if a current overload  Transport category aircraft use large amounts of electrical  power for a variety of systems. Passenger comfort requires  power for lighting, audio visual systems, and galley power  for food warmers and beverage coolers. A variety of electrical  systems are required to fly the aircraft, such as flight control  systems, electronic engine controls, communication, and  navigation systems. The output capacity of one engine-driven  AC generator can typically power all necessary electrical  AC Power Systems Right main generator bus Circuit breaker Circuit breaker Reverse polarity diode Reverse polarity diode Current limiter Current limiter Main generator bus Dual fed bus # 1 Dual fed bus # 2 Figure 9-102.  Dual-feed bus system.  9-60 Large Multiengine Aircraft