Combination Motor Starters - CB vs MCP

[WoodrowJWeen]

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.

 

[jraef]

The Fuse vs CB issue is an entire design philosphy discussion that could go on for days, be prepared. My boiled-down version: Fuses offer closer protection and higher interrupting capacity, but come with a risk of single phasing the motor, and you must keep spares on-hand to avoid costly downtime. Breakers open all 3 legs and can be reset, but need to be used in a coordinated system so that the motor starter breakers do not blow apart while waiting for some upstream device to clear a major fault. I personally prefer breakers.

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

 

[dpc]

You will find many industrial plants in the US with only fuses in motor starters and many (the majority) with only circuit breakers.

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.

 

[itsmoked]

As for the religiousness of the issue.. I have probably seen three hundred plus refrigeration compressors destroyed by (&^$%$*^##@(^%)^ fuses. We impliment ROAR Rip-Out-And Replace!

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.

 

[dpc]

Back in the day, a lot of refrigeration and HVAC equipment had to be protected by a fuse - that was the basis of their UL approval. Did a lot of designs with a nice MCP starter feeding a fused disconnect at the equipment.

 

[jraef]

Yes dpc, that's because putting a fuse in front of a piece of equipment was a "poor man's current limiter" for equipment manufacturers who did not want to subject their system to a 3 cycle fault energy test at UL. In the earlier days (notice I didn't say OLD days ), breaker ratings were somewhat low, and as available fault levels went up, fuses became the only way to deal with it. In fact if you remember, many of the older HRC (High Rupture Capacity) breakers actually had small current limiting fuses inside! Breakers are much better now than they used to be. Several manufacturers now routinely offer breakers with ratings up to 100kAIC without CL fuses.

"Our virtues and our failings are inseparable, like force and matter. When they separate, man is no more."
Nikola Tesla

 

[ScottyUK]

At our plant (UK) we tend to use thermal-mag MCCBs with phase loss detection on motor starters over, say, 5kW. The MCCB thermal element is usually set to protect the motor cable, with the motor protection relay obviously being set to protect the motor.

The only place we use fuses for motor protection is on DC motors.



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