170 Auto Electricity and Electronics Copyright by Goodheart-Willcox Co., Inc. Open Circuit An open circuit is a complete break or disconnection in the circuit. An open circuit, or “open,” stops current through a section of the circuit. See Figure 9-3. An open circuit can be caused by a: • Broken wire conductor. • Blown fuse. • Burned switch contacts. • Loose connection. • Burned or corroded connector terminals. • Failed electric component. When current is not reaching a component, fi nd out which of these potential problems is present in the circuit. As shown in Figure 9-4, the open can occur before the load, aft er the load, or in the load. If the open occurs in series with a component, the component will not function. However, if the open occurs in a parallel branch, the branches that do not contain the open circuit will still function. Refer to Figure 9-5. High Circuit Resistance High circuit resistance reduces current, but it does not stop all current like an open circuit. A high resistance might make a bulb glow dimly or a motor turn too slowly. See Figure 9-6. High circuit resistance is usually caused by a poor electrical connection. A corroded or burned connector terminal is the most common cause. Faulty components can also have high internal resistance. See Figure 9-7. Short Circuit A short circuit is caused by a conductor accidentally touching ground or another conductor. Shorts are caused by wire insulation damage. Th ere are several classifi cations of shorts, and each causes diff erent symptoms. A hot wire short to ground produces low circuit resis- tance and extremely high current. Th is usually blows the fuse or circuit breaker. Th e fuse or circuit breaker protects the circuit from excess current damage caused by the hot wire short. See Figure 9-8A. A short in an unprotected section of a circuit can cause an electrical fi re when excess current overheats a conductor and melts its insulation. Th e melted insulation then allows other conductors to come into contact. As a result, several wires can begin to burn. Th is can happen in a matter of seconds. Figure 9-3. These are some of the sources of opens or breaks in a circuit. In a series circuit, everything after break is “dead.” Open wire Open fuse Bad ground Open load Poor terminal connection or open Figure 9-4. A—If the open occurs before the load, voltage can- not travel to the bulb socket. B—If the open occurs after the load, the socket does not have a return ground circuit path. C— An open in the bulb itself is a common example of an open inside the load. Power up to here No power Open before load Power up to here Power to bulb terminal Open in load itself Open in ground A B C