Fuses and Circuit Breakers

 Fuses and Circuit Breakers ;

Fuses and circuit breakers are used to protect equipment or wiring against excessive current. For example, in your home, if you connect too many appliances to an outlet, the fuse or circuit breaker in your electrical panel “blows.” This opens the circuit to protect against overloading and possible fire. Fuses and circuit breakers may also be installed in equipment such as your automobile to protect against internal faults. Figure 2–29 shows a variety of fuses and breakers.

Fuses use a metallic element that melts when current exceeds a preset value. Thus, if a fuse is rated at 3 A, it will “blow” if more than 3 amps passes through it. Fuses are made as fast-blow and slow-blow types. Fastblow fuses are very fast; typically, they blow in a fraction of a second. Slowblow fuses, on the other hand, react more slowly so that they do not blow on small, momentary overloads.

Circuit breakers work on a different principle. When the current exceeds the rated value of a breaker, the magnetic field produced by the excessive current operates a mechanism that trips open a switch. After the fault or overload condition has been cleared, the breaker can be reset and used again. Since they are mechanical devices, their operation is slower than that of a fuse; thus, they do not “pop” on momentary overloads as, for example, when a motor is started.