Combination Motor Starters - CB vs MCP
Combination Motor Starters - CB vs MCP
(OP)
Does anyone have a preference in combination motor starters for using a thermal-magnetic circuit breaker (CB) versus using magnetic only motor circuit protectors (MCP)? Or does anyone like fusible switches with dual-element fuses? This is in regular 208 or 480 volt, 3 phase projects in the states, either in MCCs or freestanding starters.





RE: Combination Motor Starters - CB vs MCP
Among breaker choices, MCPs (technically Magnetic-Only breakers, the term MCP is a brand name owned by Westinghouse and now Eaton) are typically used in motor starters because the Overload Relay that is already in there is relied upon for thermal tripping, leaving the breaker to short-circuit protection duty alone. The magnetic trip settings on MCPs are wider ranging than on regular thermal-mag breakers (typically), so they can help to avoid nuisance tripping on motor inrush current, which can be as high as 2000% FLA. The only problem with MCPs is that they are not UL listed for stand-alone use, they are only UL recognized (UR) for use in motor startes that have been SPECIFICALLY tested and listed as a unit by the manufacturer. So in other words, you cannot go out and buy an MCP and make your own starter unless you don't care about UL listing.
Thermal-mag breakers on the other hand are UL listed for stand-alone use, but because many designs have fixed magnetic trip settings, you may need to oversize the breaker in order to avoid tripping on motor starting. If however you are using a Soft starter or VFD, Thermal-mag breakers are fine since the inrush is limited by the starting device, and besides, you can't use an MCP on them anyway.
"Our virtues and our failings are inseparable, like force and matter. When they separate, man is no more."
Nikola Tesla
RE: Combination Motor Starters - CB vs MCP
As jraef indicates, this argument can take on quasi-religious overtones and there are many purists out there. Personally, I believe the MCP is the best overall choice in most cases. In theory, the fuse will clear a fault more quickly IF there is sufficient fault current, but that's a big IF. If you consider a more typical internal motor fault, the fault current will not approach the maximum bolted fault current and if you look at the time-current curves, you'll see that the MCP will actually operate faster than the fuse over a fairly wide range of fault currents.
Fuses can handle much higher fault currents than breakers, but the gap has closed over the years and fault currents exceeding 65,000 at 480V are not common anyway.
Single-phasing, spare parts and proper fuse replacement are other disadvantages to fuses.
RE: Combination Motor Starters - CB vs MCP
This happens so much, I believe, because these compressors live by themselves out in "the back" burried with others making lots of noise. One goes S-phase? No one ever notices until it goes No-phase. Probably be different in other applications like a production line with a lot of people around.
RE: Combination Motor Starters - CB vs MCP
RE: Combination Motor Starters - CB vs MCP
"Our virtues and our failings are inseparable, like force and matter. When they separate, man is no more."
Nikola Tesla
RE: Combination Motor Starters - CB vs MCP
The only place we use fuses for motor protection is on DC motors.
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