Overcurrent Circuit protection would be unnecessary if overloads and
short circuits could be eliminated. Unfortunately, overloads and short circuits
do occur. To protect a circuit against these currents, a protective device must
determine when a fault condition develops and automatically disconnect the
electrical equipment from the voltage source.
Circuit breakers provide a manual means of energizing and de-energizing
a circuit. In addition, circuit breakers provide automatic overcurrent
protection of a circuit. A circuit breaker allows a circuit to be reactivated
quickly after a short circuit or overload is cleared. Unlike fuses which must
be replaced when they open, a simple flip of the breaker’s handle restores the
circuit.
An overcurrent protection device must be able to recognize
the difference between overcurrents and
short circuits and respond in the proper way. Slight
overcurrents can be allowed to continue for some period of time, but as the
current magnitude increases, the protection device must open faster.
Short circuits must be interrupted instantly. Several
devices are available to accomplish this.
Fuse
A fuse is a one-shot device. The heat produced by
overcurrent causes the current carrying element to melt open, disconnecting the
load from the source voltage.
Non time-Delay Fuse Non time-delay fuses provide excellent short
circuit protection. When an overcurrent occurs, heat builds up rapidly in the fuse.
Non time-delay fuses usually hold 500% of their rating for approximately
one-fourth second, after which the current carrying element melts.
This means that these fuses cannot be used in motor circuits
which often have inrush currents greater than 500%.
Time-Delay Fuses Time-delay fuses provide overload and short
circuit protection. Time-delay fuses usually allow five times the rated current
for up to ten seconds to allow motors to start.
Circuit Breaker
The National Electrical Code defines a circuit breaker as a
device designed to open and close a circuit by no automatic means, and to open
the circuit automatically on a predetermined overcurrent without damage to
itself when properly applied within its rating.
Circuit breakers:
• SENSE when an overcurrent occurs.
• MEASURE the amount of overcurrent.
• ACT by tripping the circuit breaker in a time frame necessary
to prevent damage to itself and the associated load cables.
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