9-47 The GCU is also used to turn the AC alternator on or off.  When the pilot selects the operation of an AC alternator, the  GCU monitors the alternator’s output to ensure voltage and  frequency are within limits. If the GCU is satisfied with the  alternator’s output, the GCU sends a signal to an electrical  contactor that connects the alternator to the appropriate  AC distribution bus. The contactor, often call the generator  breaker, is basically an electromagnetic solenoid that controls  a set of large contact points. The large contact points are  necessary in order to handle the large amounts of current  produced by most AC alternators. This same contactor is  activated in the event the GCU detects a fault in the alternator  output; however, in this case the contactor would disconnect  the alternator from the bus.  Virtually all aircraft contain some form of an electrical  system. The most basic aircraft must produce electricity for  operation of the engine’s ignition system. Modern aircraft  have complex electrical systems that control almost every  aspect of flight. In general, electrical systems can be divided  into different categories according to the function of the  system. Common systems include lighting, engine starting,  and power generation.  Light aircraft typically have a relatively simple electrical  system because simple aircraft generally require less  redundancy and less complexity than larger transport  category aircraft. On most light aircraft, there is only one  electrical system powered by the engine-driven alternator or  generator. The aircraft battery is used for emergency power  and engine starting. Electrical power is typically distributed  through one or more common points known as an electrical  bus (or bus bar).  Almost all electrical circuits must be protected from faults  that can occur in the system. Faults are commonly known  as opens or shorts. An open circuit is an electrical fault that  occurs when a circuit becomes disconnected. A short circuit is  an electrical fault that occurs when one or more circuits create  an unwanted connection. The most dangerous short circuit  occurs when a positive wire creates an unwanted connection  to a negative connection or ground. This is typically called  a short to ground.  There are two ways to protect electrical systems from faults:  mechanically and electrically. Mechanically, wires and  components are protected from abrasion and excess wear  through proper installation and by adding protective covers  and shields. Electrically, wires can be protected using circuit  breakers and fuses. The circuit breakers protect each system  in the event of a short circuit. It should be noted that fuses  can be used instead of circuit breakers. Fuses are typically  found on older aircraft. A circuit breaker panel from a light  aircraft is shown in  Figure 9-83 . The aircraft battery and battery circuit is used to supply  power for engine starting and to provide a secondary power  supply in the event of an alternator (or generator) failure. A  schematic of a typical battery circuit is shown in  Figure 9-84 .  This diagram shows the relationship of the starter and external  power circuits that are discussed later in this chapter. The  bold lines found on the diagram represent large wire (see the  wire leaving the battery positive connection), which is used in  the battery circuit due to the heavy current provided through  these wires. Because batteries can supply large current flows,  a battery is typically connected to the system through an  electrical solenoid. At the start/end of each flight, the battery  is connected/disconnected from the electrical distribution bus  through the solenoid contacts. A battery master switch on the  flight deck is used to control the solenoid. Although they are very similar, there is often confusion  between the terms “solenoid” and “relay.” A solenoid is  typically used for switching high current circuits and relays  used to control lower current circuits. To help illuminate the  confusion, the term “contactor” is often used when describing  a magnetically operated switch. For general purposes, an  aircraft technician may consider the terms relay, solenoid,  and contactor synonymous. Each of these three terms may  be used on diagrams and schematics to describe electrical  switches controlled by an electromagnet. Here it can be seen that the battery positive wire is connected  to the electrical bus when the battery master switch is active.  A battery solenoid is shown in  Figure 9-85 . The battery  switch is often referred to as the master switch since it turns  Figure 9-83.  Light aircraft circuit breaker panel. Aircraft Electrical Systems Small Single-Engine Aircraft Battery Circuit