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Old March 5th, 2018, 04:55 PM   #1
LadderLogic
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Machine Main Protection Device

I know this topic has been discussed a few times here but the information is spread between multiple threads... and I still do not have a clear picture in my head.

So a machine needs a main short and overload protection device. Typically I would use an MCCB (molded-case circuit breaker) or even a miniature circuit breaker, as long as it is UL489-compliant.

However, on many European-built machines, I see an MMP (manual motor protector) device being used as the main protection for the whole machine.

Both devices have magnetic trip to protect from shorts and thermal trip to protect from overloads. Many of them are compliant both with UL489 and with IEC/EN 60947. In other words, they both look pretty much equivalent as a main protection device.

Why would one be chosen over another? Typically an MMP would cost two or three times than a circuit breaker of the same amp rating. Does anyone know of any other considerations and if I am missing something big here?

I am attaching screenshots from Allied Electronics web site showing one of each device types, for illustration purposes only.
Attached Images
File Type: jpg Capture.JPG (49.8 KB, 33 views)
File Type: jpg Capture1.JPG (45.6 KB, 30 views)
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Old March 5th, 2018, 05:06 PM   #2
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I have found, even from big manufacturers of stamping presses, that what is used when building a machine first consumes anything left on the shelves from previous & halted builds.

If a machine needs a 30A main breaker and they have a 30A MMP on hand it will be used instead of purchasing a circuit breaker.

As far as 'correct' goes a MMP should not be used as a main breaker.
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Old March 5th, 2018, 05:43 PM   #3
Jeev
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The MMP might cost more than the equivalent CB, but off the top of my head, it would be a lot cheaper than the equivalent MCCB. It also offers a range of adjustment like most MCCBs, but the CB does not. I don't use them for main isolators, but I too have seen this in a few machines coming from EU.
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Old March 6th, 2018, 08:19 PM   #4
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My thanks to all who replied. I would not say that the distinction between the two became clearer to me but I do appreciate the efforts.
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Old March 6th, 2018, 08:55 PM   #5
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Although I may not be able to help with the distinction. I would like to add, I too ask myself most of the time what is the best method MCCB vs for instance a fused disconnect with class J fuses. I see lots of small to midsized cabinets out there with fused disconnects.
I always come to the same answer when I ask myself...and it is "should I know this?"
Anyone could shed some light would be greatly appreciated.
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Old March 6th, 2018, 10:05 PM   #6
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If you are sending a panel outside of North America, the rules are different. Assuming you are shipping inside of North America, then let's discuss OUR rules.

From our standpoint regarding circuit protective devices, you have BRANCH devices, meaning those that are the LAST protective device before the load, and FEEDER devices, meaning they protect ONE OR MORE branch devices below them. The "Main" of a control panel would fall under being a FEEDER device and as such, must be either a UL489 listed circuit breaker, or a UL98 listed fused disconnect switch.

Then under that UL489 listed device category, you must consider the Interrupting Capacity (IC), often abbreviated for thousands of amps as "kAIC". So just because a device passes under UL489, does NOT mean it has a "reasonable" IC rating, nor does it mean the usable VOLTAGE rating is always appropriate. That's where you find the difference between an MCB (miniature Circuit Breaker) and MCCB (Molded Case Circuit Breaker)

kAIC:
MCBs are meant to be small and cheap and snap onto DIN rail, but typically have no MORE than a 14kAIC rating (a few have 18kAIC). You can buy cheaper MCCBs like that as well, but for the most part they start off at 25kAIC and can go up to 65kAIC or even 100kAIC in some cases. MCBs are NOT designed to handle higher interrupt capacity, not only internally, but externally as well; the DIN rail mounting is not strong enough to withstand much physical force. MCCBs bolt down to the panel (for the most part). Why that matters now is because since the 2005 NEC, to connect anything in the field you must now ensure that the SCCR (Short Circuit Current Rating) of anything you want to connect is equal to or greater than the Available Short Circuit Current (aka Available Fault Current) of a system. As an example if your AFC is 23,700A and he SCCR of your panel is 14kA, the contractor installing it cannot legally connect it. So to ATTAIN an SCCR, you must do one of the following:
  • Submit a complete system to UL for expensive short circuit testing,
  • Use a pre-listed series combination of protective devices that the manufacturer has already submitted and listed, using EXACTLY the same devices. So no mixing of manufacturers; Sq. D is not going to pay to test their MCCB with an Eaton motor starter...
  • Make sure EVERY device in the power circuit is rated for the AFC (virtually impossible by the way)
  • Put a "courtesy" listing of 5kA on the assembly.
That last issue of the 5kA SCCR listing is simple for the panel builder, but a NIGHTMARE for the installer, who cannot legally connect it to a system capable of more than 5kA of fault current, which is LESS than a residential load center (typically 10kAIC). Despite rumors and anecdotes of people putting "current limiting fuses" ahead of 5kA SCCR listed panels and getting away with it, it's false. You can't do that unless a PE evaluates it and signs off on it, which means accepting liability for it and they are loathe to do that.

MMPs, aside from the issue described below, are sometimes used in listed assemblies because they are designed as "Current Limiting", so they can often help a mfr attain a higher SCCR than if they used an MCB or even a cheap MCCB; sometimes as high as 65kA. It may come with restrictions (see below) and qualifications as to what the next feeder size is UPSTREAM of it, but often times that is very large, like 1200A. So it works in a lot of cases even though it is more expensive than an MCB or MCCB when used as a BRANCH device, again, the LAST protective device before the load. However it is all but impossible to use an MMP as a MAIN device, because most of them are NOT listed under UL489. Those that are will have to be "fixed trip", not adjustable. They exist, but are generally more expensive than an equivalent MCCB, so they don't get used much.


Votlage:
Voltage rating is an issue as well. MCBs (and their cousins, MMPs) are IEC devices and as such, are NOT designed around voltages we can see in North America. In IEC world, ALL distribution systems are Wye (Star), so the maximum voltage reference to Ground is 58% of the Line-to-Line voltage. That then also means that the maximum asymmetrical fault (Line to Neutral or Ground) is based on that voltage, so the maximum energy in that fault is lower. But here in NA, we have 480V Delta systems and 600V systems (mostly in Canada) where the L-N/G voltage reference will be higher, so the energy in a fault is higher. Those MCBs and MMPs are NOT designed to handle that added energy. So you will see them show what's referred to as a "slash voltage rating" that says "480/277V", and you CANNOT use them on a 480V delta source, or a 600V source (Wye or Delta) in Canada. The problem for a panel builder here is that you usually don't KNOW for CERTAIN what the end user has; if you ask them, they will say "480V" and if you say "Wye or Delta?", they respond "I dunno..., it's 480V..." So it is NOT safe to use MCBs or MMPs in NA unless you know for SURE what the site service voltage configuration is, leaving the only SAFE bet as an MCCB. That said, "slash rated" MCCBs are making their way into the market too, so you have to be on guard.

You are now likely more confused than before, and trust me, it's not something to be taken lightly or all at once, but it's all important.

Last edited by jraef; March 6th, 2018 at 10:11 PM.
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Old March 7th, 2018, 07:28 PM   #7
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Thank you.
I have heard many people say, just throw current limiting fuses ahead of the panel to meet/exceed the AFC of the system.
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Old March 7th, 2018, 09:12 PM   #8
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Jraef, thank you for such detailed response.

I am hesitating to ask if you know how would this all work for a machine that ships to Europe.
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