I'm reluctant to jump into the fray, but I guess I just cannot resist....
I cannot think of an overcurrent protective device I've run across in the
past 35 years working for a manufacturer of electrical controls and power
distribution equipment that was not an inverse time curve device. In other
words,
the higher the overcurrent, the faster the device opens. Typically, fuses are
quicker than circuit breakers because of the reasons Art mentions, no moving
mechanical parts. Of course there are plenty of exceptions, such as time delay
fuses and very fast-acting specialty circuit breakers.
However, I do not understand where Art comes up with fuses being required for
large industrial motor circuits. I can show him lots of large, up to 400HP
on 480V 3-phase, motor applications where a circuit breaker, or more likely a
magnetic trip only circuit breaker, feeds a motor starter that controls the
motor. Sometimes both a circuit breaker and fuses are used in series. An
example is adjustable frequency drive applications, where it is not uncommon to
have
fast-acting fuses on the input side of the drive even when the drive has an
input circuit breaker. The fuses are there because they can clear a fault very
quickly.
One thing is for sure. Somewhere there is a time current curve for the fuse
or circuit breaker you are considering for your application that tells you how
fast the device will open based on the percentage of overload.
The fuse versus circuit breaker debate has been going on for a long, long
time, and nothing I'm going to write here is going to bring it to an end.
73,
Marsh, KA5M
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