(1) The simplest protective device is a fuse. All fuses are rated
according to the amount of current that is safely carried by the fuse element
at a rated voltage. The most important fuse characteristic is its current
versustime or "blowing" ability. Currentversustime indicates how quickly an
overloaded fuse will blow: fast, medium, and delayed. Fast may range from five
microseconds through 1/2 second; medium, 1/2 to five seconds; and delayed, five
to 25 seconds. When a fuse blows, it should be replaced with another of the
same rated voltage and current capacity, including the same currentversustime
characteristic. Normally, when the circuit is overloaded, or a fault develops,
the fuse element melts and opens the circuit it is protecting.
(2) A circuit breaker is designed to break the circuit and stop the
current flow when the current exceeds a predetermined value. It is commonly
used in place of a fuse and may sometimes eliminate the need for a switch. A
circuit breaker differs from a fuse in that it "trips" to break the circuit,
and it may be reset, while the fuse melts and must be replaced. Some circuit
breakers must be reset by hand, while others reset themselves automatically.
When the circuit breaker is reset, if the overload condition still exists, the
circuit breaker will trip again to prevent damage to the circuit.
(3) A switch may be described as a device used in an electrical
circuit for making, breaking, or changing connections under conditions for
which the switch is rated. Switches are rated in amperes and volts; the rating
refers to the maximum voltage and current of the circuit in which the switch is
to be used. Because it is placed in series, all the circuit current will pass
through the switch. Switch contacts should be opened and closed quickly to
minimize arcing; therefore, switches normally utilize a snap action.