All large motors are not protected by fuses. Some motor circuits are
protected by fuses, some are protected by circuit breakers, and some are
protected by
magnetic trip only devices known by such names as MCP or HMCP (the MCP stands
for Motor Circuit Protector). I am very aware of AIC and available fault
current. It's unusual, at least in this part of the world, to actually have
100,000 amps of fault current available. A typical 1500kVA power transformer
in a
secondary unit substation with a typical 5.75% impedance can only deliver
about 31,000 amps or so of fault current. Add in motor contribution during a
fault, and you're still a long way from 100,000 amps. A typical
across-the-line
motor starter in series with an HMCP is UL listed at 100,000 AIC symmetrical
at 480 VAC. If you really do have 100,000 amps or more of available fault
current, then you're talking fuses or circuit breakers with current limiter
(fuse)
attachments.
An industrial molded case circuit breaker rated at 65,000 AIC symmetrical at
480 VAC is not uncommon. Typical bus bracing in Motor Control Centers is
65,000 amps.
250,000 AIC fuses? What class of fuse is that? Most fuses used in motor
circuits I've seen are Class R, rated at 200,000 amps.
Home Depot serves mostly the DIY residential market where 10,000 AIC or
22,000 AIC at 240V is more than adequate, unless you live in downtown Chicago
next
to a substation. I'll look at their fuses the next time I'm there.
Non automatic "circuit beakers" with no thermal or magnetic trips are used as
disconnects in front of fuses sometimes to feed large motors. Sometimes
circuit breakers with thermal and magnetic trips have current limiter
attachments
to increase their AIC rating.
Having said all that, this is way, way off topic for the Ten-Tec reflector.
I'm going back to filling out QSL cards.
Have a good evening.
73,
Marsh, KA5M
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