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.