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low voltage induction motor protection
- Thread starter mazen1
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MCP is a specific class of MCCB which has an instantaneous trip setting but does not have an inverse-time curve. An MCP is designed to be used in conjunction with a contactor and thermal overload relay in a 'combination starter'. Normally a MCP will be certified for use with specific contactors and relays to form an overall certified start unit.
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To further that, an MCP can ONLY be used as part of a tested and listed assembly, meaning that its use is effectively limited to motor control equipment manufacturers who can afford the steep cost of 3rd party testing. So why do they do it? 2 reasons:
1) Cost. Since an OL relay is going to be there anyway, the MCP need only have magnetic trips and when using 10's of thousands per year, the small difference in cost adds up.
2) Adjustability. In the past, the lowest cost MCCBs had fixed magnetic trips (Instantaneous Trip or IT), usually at a value between 250 and 400% of rated current, in addition to the fixed Thermal trips (I2t). In some motor starter applications however, this ends up being too low to avoid nuisance tripping and required using unnecessarilly higher Thermal current rated breakers. For example if you have a 75HP motor with 100 FLA, you could use a 150A TM MCCB, but if it had fixed mag trips set for 250% of 150A, that is only 375A, and the starting current of that motor could be as high as 600A. So you would have to increase the TM MCCB size to 250A in order to accommodate it.
Since the NEC will allow the IT settings to be as high as 1300% if necessary, MCPs provide a way of having settings that could be adjusted much higher than would be acceptable in any other application. But many MCCBs are available now with adjustable magnetic trips even if the thermal trips are fixed, so this feature is not as important as it once was.
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1) Cost. Since an OL relay is going to be there anyway, the MCP need only have magnetic trips and when using 10's of thousands per year, the small difference in cost adds up.
2) Adjustability. In the past, the lowest cost MCCBs had fixed magnetic trips (Instantaneous Trip or IT), usually at a value between 250 and 400% of rated current, in addition to the fixed Thermal trips (I2t). In some motor starter applications however, this ends up being too low to avoid nuisance tripping and required using unnecessarilly higher Thermal current rated breakers. For example if you have a 75HP motor with 100 FLA, you could use a 150A TM MCCB, but if it had fixed mag trips set for 250% of 150A, that is only 375A, and the starting current of that motor could be as high as 600A. So you would have to increase the TM MCCB size to 250A in order to accommodate it.
Since the NEC will allow the IT settings to be as high as 1300% if necessary, MCPs provide a way of having settings that could be adjusted much higher than would be acceptable in any other application. But many MCCBs are available now with adjustable magnetic trips even if the thermal trips are fixed, so this feature is not as important as it once was.
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376
Also, IIRC, MCPs are available in trip ratings not normally available in standard MCCB. This can be helpful when dealing with small 3-phase motors.
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"An 'expert' is someone who has made every possible mistake in a very narrow field of study." -- Edward Teller
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