Phase Loss

[jim dungar]

Will the breaker trip if the continuous current reaches a level of 35 or 40A? What will cause the breaker to trip in this case since it is an instantaneous breaker?

No the breaker will not trip, but it will probably eventually fail due to overheating. As I said earlier think of its continuous rating like it was a piece of wire not like a trip rating. The breaker trips instantly when it reaches its set point (which is a multiple of its continuous rating).

Typical small "MCP" breakers continuous current ratings are 3, 7, 15, and 30A with 300-1100% trip settings.

 

[rcwilson]

Mull - the 30A Motor Circuit Protector (MCP) is rated to carry up to 30A continuously when used as a part of a listed Combination Motor Starter. It might look like a 30 amp circuit breaker, but it is not. It does not have any overload protection, just short circuit protection. According to Code, an MCP should only be used as the short circuit protection in a combination motor starter where the overload relay provides the overcurrent protection. The motor overload protects the motor, the cable and the MCP from overloads. The MCP protects for short circuits. Together they protect each other, the motor and the wires.

Why do that? The 30A MCP's magnetic trip unit (short circuit) usually has more adjustment available than a 30A MCB. It can be set to trip at a current just above the motor's starting inrush current. Any current above that on a motor circuit is due to a fault, so the MCP trips instantaneously. A standard circuit breaker may not have the wide range of adjustment to match the motor characteristics and its overcurrent element might over protect the motor. The combination of the MCP and overload relay can be set to match the motor characteristics, providing the best protection and preventing unnecessary trips.

But back to your problem. The MCP should trip only if it sees a very high current, (unless it is defective). I'm out of ideas on what might be causing a high current without doing any damage, unless something upstream (loose connection, chattering contact) is momentarily opening a phase and reclosing. A recording meter would help isolate the problem.

 

[micromind]

You'll never get anyone to admit to this, but if the trip adjustment screws were turned past their limits, (even just once), it'll mess up the springs inside, and usually cause the breaker to trip erratically at way less than the setting.

There's no way I'd connect anything to an MCP except a motor starter. As has been stated before, you have only short circuit and ground fault protection, not overcurrent. The overload part of the starter provides overcurrent protection. In the event that part of the xfmr primary winding shorts to an adjacent winding (on the same core), you'd have an overcurrent, but not a short, and the MCP wouldn't see a problem. The wire now becomes a fuse, but only after the insulation has melted. After the fire, you'd have a short, then the MCP would trip. I know this sounds dramatic, but it's certainly possible.

 

[nakulak]

seems like a good time to install a 24 hour recorder and find out what your spikes actually are so you can maybe find the source of the problem.

 

[mull982]

So in the future when picking an instantaneous breaker as part of a motor starter combination are there certain guidelines for selecting the size for the rated continuous duty of the breaker. Does the continuous duty rating of the breaker have to be at least 125% of FLA of the motor?

In an example where a motor has a FLA of 6.7A looking at an instantaneous breaker chart I see that I can choose a 7A, 15A, or 30A breaker to allow the adjustable settings needed to overcome the inrush of the motor (6*FLA). Which one is best to choose?

Actually looking at the code and table 430.52 it states the the maximum rating or setting of an instaneous breaker is 800% of the motor FLA. So in my example above 6.7A * 8 = 53.6A, as being the maximum setting allowed for an instantaneous breaker. That would then eliminate the 30A breaker because its lowest setting is 90A which is much higher then the maximum 53.6A calculated for the motor. What would then be the best choice for selecting between the 7A and 15A rated breaker? Does the 1.25% FLA make the 15A breaker the only choice? Am I going about the idea of selecting an instantaneous breaker correctly?

 

[jim dungar]

So in the future when picking an instantaneous breaker as part of a motor starter combination are there certain guidelines for selecting the size for the rated continuous duty of the breaker. Does the continuous duty rating of the breaker have to be at least 125% of FLA of the motor?

No you do not need to take 125% of the FLA.
For a 2.8A motor Square D says to use their 3A breaker.

 

[Jraef]

...snipped

There's no way I'd connect anything to an MCP except a motor starter. As has been stated before, you have only short circuit and ground fault protection, not overcurrent. The overload part of the starter provides overcurrent protection. In the event that part of the xfmr primary winding shorts to an adjacent winding (on the same core), you'd have an overcurrent, but not a short, and the MCP wouldn't see a problem. The wire now becomes a fuse, but only after the insulation has melted. After the fire, you'd have a short, then the MCP would trip. I know this sounds dramatic, but it's certainly possible.

He is absolutely correct, but just to clarify;
It is ILLEGAL to use an MCP (magnetic-only breakers) for anything other than a motor starter, and even then, only if you are the manufacturer of the completed motor starter and have tested and listed the entire assembly with UL!

I know people do this all the time, but they are NOT supposed to. MCPs are NOT listed by UL, they are only UR (UL Recognized components) which means they can only be used in combination systems as described above. UL panel shops can technically use them when building motor starters, but only if they go through the expense of having the final entire assembly tested for short-circuit withstand, and at roughly $20k / line item, they have to be selling a LOT of starters to make it worthwhile.

What you can do is to buy a Thermal mag. circuit breaker with adjustable instantaneous Mag. trips (adjustable thermals are also verbotten). That is why you must start with the NEC rules for 125% of the FLA; they are speaking of the Thermal Trip setting, not the current rating of the Mag-only MCP breakers. There is technically nothing confusing about this if you read and follow ALL of the details.

 

[coulter]

...(adjustable thermals are also verbotten). ...

Hummm - Are you sure?

NEC 240.6.B, 240.6.C

carl

 

[Jraef]

Hummm - Are you sure?

NEC 240.6.B, 240.6.C

carl

Off topic now from the original post, but let me clarify.
Permissible by NEC, not "listable" (not really a word) by UL. Exception; breakers with Electronic Trip Units where the accuracy and repeatability of settings done in the field by potentially unqualified personnel can be shown to be adequate. In other words, the NEC makes allowances for something the UL will accept under specific circumstances, but it means going with a more expensive breaker. If you shown me a standard thermal-mag breaker with with adjustable trips and a UL 489 label, I'll eat my words. There are some TM breakers out there with adjustable trips, but if you look up the UL file numbers you will find they are UL508 listed as "manual motor starters", not as UL489 circuit breakers.

 

[jim dungar]

If you shown me a standard thermal-mag breaker with with adjustable trips and a UL 489 label, I'll eat my words. There are some TM breakers out there with adjustable trips, but if you look up the UL file numbers you will find they are UL508 listed as "manual motor starters", not as UL489 circuit breakers.

I assume you are talking about adjustable THERMAL trips. Adjustable magnetic trips are standard on almost all thermal-magnetic UL489 breakers larger than 200A.

 

[mull982]

Thanks for all the responses guys, I've learned alot about these MCP breakers, and breakers in general from these posts.

I have been talking to the engineers who specified this breaker for this application and asked them why they used an instantaneous magnetic breaker to feed a transformer. Their argument was that since this transformer was powered from a starter (we use our plant control system to control the starter to send power to the transformer thus turning on the magnet) that this breaker was acceptable. Although the connected load is a transformer they are arguing that this breaker is part of a starter combination with an electronic overload relay providing the overload or "thermal" protection. Based on the information that I have gained from these posts I have tried to argue with them that the instantaneous breaker should only be used on a motor application, but they continue to argue that because the breaker is part of a combination starter that it does not matter that it feeds a transformer and is alright.

I'm going to start pointing out some code reference and some other amo (from all these informative posts) to pleed my case!

 

[Jraef]

I assume you are talking about adjustable THERMAL trips. Adjustable magnetic trips are standard on almost all thermal-magnetic UL489 breakers larger than 200A.

Yes, and I said that earlier.

One thing I think happens a lot is that people see the breakers with ETUs (Electronic Trip Units) that have adjustability of all parameters and think that, because the "L" (Long Time) trip functions are designed to operate in the same range as Thermal Trip Elements, they are the same thing. They are not. A Thermal Trip Element measure current indirectly by measuring the heat generated by current as it passes through a bimetal strip. This is an indirect method and although refinements over the years have made them very accurate, there are calibration and repeatability issues involved in their use that make them unacceptable for the US market unless they are preset and sealed at a factory or by a test technician with inspected calibration tools.

ETUs are directly sensing the current and the trip curve, although programmed in most cases to emulate the Thermal Element's response, is nevertheless much more accurate, repeatable and finite as far as making adjustments in the fields without special calibration tools. They are also considerably more expensive (until you get to the largest frame sizes) so most people don't think of them unless they have a specific need for adjustability if the L and/or S functions.

 

[jim dungar]

I'm going to start pointing out some code reference and some other amo (from all these informative posts) to pleed my case!

A primary reference should be 240.9, which says the starter overload relay can not protect conductors except as allowed in 430.40.

jraef
Do you consider a circuit breaker with an interchangeable thermal trip unit to be "adjustable"?

 

[Jraef]

A primary reference should be 240.9, which says the starter overload relay can not protect conductors except as allowed in 430.40.

Yes, that's the main point of reference.

If you need more, then go to 240.21.3.5 where is says the primary of a transformer MUST be protected by a CIRCUIT BREAKER (or fuses), not a motor starter with an OL relay.

240.4.G permits the use of "other" protective devices in specific references to other sections, such as Article 430, but there is NO reference to transformers being special.

430.1, where it says "This article covers motors, motor branch circuit and feeder conductors and their protection, motor overload protection, motor control circuits, motor controllers and motor control centers." Notice that the word "motor" is in every segment of that declaration. Transformer is not.

430.51 (motors, motors, motors again; no transformers).

430.52.3, where they allow Instantaneous Trip circuit breakers only when used in MOTOR controllers.

Your buddies are dead wrong.

jraef
Do you consider a circuit breaker with an interchangeable thermal trip unit to be "adjustable"?

No, the thermal trip elements are still calibrated and sealed at the factory. Read 240.82

 

[coulter]

...No, the thermal trip elements are still calibrated and sealed at the factory. Read 240.82

I read it. Did you miss the part that says. "...for other than intended adjustments"

The article you quoted didn't lock any thing up that the mfg intends to be adjustable.

carl