Marsh, the reason fuses are required with motors is the short circuit
current or AIC (ampere interrupting capacity). It would not be unusual
for a three phase 40hp motor to fail and have a short circuit of 100,000
amperes. The maximum rating of circuit breakers is 50,000 AIC so the
breaker will explode. Fuses for 40hp motors would be required to have
ratings of 250,000 AIC. Next time you visit Home Depot check their
fuses you will find several price ranges which are related to the AIC.
As for 400hp motors they sometimes use what looks like a circuit breaker
but is only a disconnect and an integral part of the unit is a fuse.
All large motors are protected by fuses.
Best
Art
KA5M@aol.com wrote:
> 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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>
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