Sizing A Motor Protective Circuit Breaker

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eurekah

Member
Location
BC, Canada
Hello, I have a question regarding sizing a motor protective circuit breaker, the thermal-magnetic type that can provide me with overload and short circuit protection.

I've notice while looking at some Motor SCPD model parameters if I find one that has ideal thermal overload settings for a motor the short circuit parameters will not match up to what is require. I will give an example below. Quick note, I am using the Canadian Electrical Code guidelines but I've noticed it is very similar to what is in the NEC, at least when it comes to motors.

Example.
Motor in question is a 460V, 3PH, squirrel cage motor with a FLA of 5.9A and S.F. of 1.10. The max thermal setting should be 5.9A*115%=6.79A, and for an instantaneous magnetic circuit breaker the release current max should be 5.9A*1300%=76.7A.

Using these parameters I find the motor SCPD. One I found was the Eaton PKZM0-10 which has a overload range of 6.3-10A (great), and a short-circuit release current max of 155A( 2600% of the motors FLA).

Here is where my problem comes in. I often find a thermal characteristic that fits the requirements, but the short-circuit current is always significantly higher that the max 1300%*FLC that is required.

Even the next model size down which has a thermal range of 4-6.3A has a short-circuit setting of 97.7A.


Am I missing something when sizing these protection devices? If I find a device that has the desired short circuit setting the thermal overload settings are always below the FLC of the motor. A bit of background of the project. These motor protection devices will be in a control enclosure which has a main fused disconnect of ~100A, which will then feed into about 5 other motors, each with their own motor protection devices and contactors.

Thanks for the help! I typically only do the design from the power pole on the road and end at the power going into the control enclosures. This is the first time I've had to actually work with control enclosure internals.
 

eurekah

Member
Location
BC, Canada
Typically, motor short-circuit & ground-fault protection devices here in the US are separate devices from motor overload devices. I've never looked for all-in-one devices.

That said, perhaps this publication can answer your question:
http://www.eaton.eu/ecm/groups/public/@pub/@europe/@electrical/documents/content/pct_392009.pdf

Ah, I see. I thought that was what was suppose to be the big selling feature of those devices, having short-circuit and overload protection all-in-one. After reading a through that PDF it does mention in North America additional OCP is often required.

If that is the case I might just go with a fuse with a contactor/overload combo. I was hoping the above would work for the fact that if the short-circuit protection gets tripped I would have an AUX that could be connected to an alarm system, but with a fuse, contactor, overload setup I would only have an overload AUX indicator.

Thanks for the help!
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
In a nutshell however, you are missing one very subtle, but very important detail. You are not alone by the way.

Under 430.52.C
(6) Self?Protected Combination Controller. A listed self protected
combination controller shall be permitted in lieu
of the devices specified in Table 430.52. Adjustable
instantaneous-trip settings shall not exceed 1300 percent of
full-load motor current for other than Design B energy efficient
motors and not more than 1700 percent of full-load
motor current for Design B energy-efficient motors.

See it yet?
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"Adjustable instantaneous-trip settings shall not exceed 1300 percent ..."

The instantaneous mag trip settings in most of these devices are NOT adjustable!

The real deal is, these are UL / CSA listed as a "Self Protected" device. They have been tested to do a very specific job of protecting the motor connected to them. The other rules in the NEC are based on when the INSTALLER can make a decision on something that may affect the performance and protection capabilities, in which case they want to control the limits of those decisions. In this product, you cannot make a decision on the mag trip settings, it was done, tested and listed as is, so it's acceptable.
 

Smart $

Esteemed Member
Location
Ohio
... In this product, you cannot make a decision on the mag trip settings, it was done, tested and listed as is, so it's acceptable.
Acceptable, yes. I think he knows that. His question is regarding the mismatch between SC trip level and overload trip level.
 

eurekah

Member
Location
BC, Canada
It's worded a bit different in the CEC, but looks fairly similar.

"28-210 Instantaneous-trip circuit breakers (see Appendix B)
When used for branch circuit protection, instantaneous-trip circuit breakers shall be
(a) part of a combination motor starter or controller that also provides overload protection; and either
(b) rated or adjusted, for an ac motor, to trip at not more than 1300% of the motor full load current or at not
more than 215% of the motor-locked rotor current, where given, except that ratings or settings for trip
currents need not be less than 15 A; or
(c) rated or adjusted, for a dc motor rated at 50 hp or less, to trip at not more than 250% of the motor full
load current, or for a dc motor rated at more than 50 hp, to trip at not more than 200% of the motor full
load current."


So, what Jraef is suggesting is that since this device is an approved device even though the trip setting might be above what is in the electrical code, it is an exception?
 

Smart $

Esteemed Member
Location
Ohio
...
So, what Jraef is suggesting is that since this device is an approved device even though the trip setting might be above what is in the electrical code, it is an exception?
Based on my understanding, I sure hope not! ...but we'll have to let him reply to know.
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
It's worded a bit different in the CEC, but looks fairly similar.

"28-210 Instantaneous-trip circuit breakers (see Appendix B)
When used for branch circuit protection, instantaneous-trip circuit breakers shall be
(a) part of a combination motor starter or controller that also provides overload protection; and either
(b) rated or adjusted, for an ac motor, to trip at not more than 1300% of the motor full load current or at not
more than 215% of the motor-locked rotor current, where given, except that ratings or settings for trip
currents need not be less than 15 A; or
(c) rated or adjusted, for a dc motor rated at 50 hp or less, to trip at not more than 250% of the motor full
load current, or for a dc motor rated at more than 50 hp, to trip at not more than 200% of the motor full
load current."


So, what Jraef is suggesting is that since this device is an approved device even though the trip setting might be above what is in the electrical code, it is an exception?
Well, it's interesting in that in the case of the CEC, they say "rated or adjusted", which in my mind would be less ambiguous than the NEC. However, this appears to be for an instantaneous trip circuit breaker, better known as an MCP. I don't have a copy of the CEC to see if that applies to the little self-protected starters, but from what was posted, I would read Appendix B. In the NEC they make a distinct differentiation; the rule I posted applies ONLY to those devices in particular.

By the way, side subject: do NOT set the OL at 115% unless the mfr of the OL tells you to! Many (if not most) of them have ALREADY factored that value into the trip settings on the dial. So if YOU set the dial at 115% without checking, you are really setting it at 115% of 115% (132%) of motor nameplate FLA! Having worked at K-M years ago, I can attest to the fact that their OL pick-up point, the point where it BEGINS to move toward tripping, is ALREADY set for 115% of the actual FLA. So in their instruction manual, they tell you to set the dial at EXACTLY the motor FLA from the nameplate. RTFM...

Smart $ said:
Acceptable, yes. I think he knows that. His question is regarding the mismatch between SC trip level and overload trip level.
Well, the "mismatch" is in acceptability; the "not to exceed 1300% (or 1700%) is an issue of acceptability, is it not?

Case in point. If this were a Thermal Mag circuit breaker, aka "Inverse Time" circuit breaker per the NEC, the trip setting is not supposed to be over 250%, but if the smallest breaker available is too large, that's what you use, right? So now see if you can find the magnetic trip setting of a Sq. D FAL34015 molded case basic thermal-mag breaker. Hint: I picked that one specifically because I know it's not easy, so I'll save you the trouble. It is NOT adjustable, it is fixed, and on a 15A breaker it "Must Hold" at 275A and "Must Trip" at 600A, which means the mag trips will trip anywhere BETWEEN 275 and 600A. Remember, this is a 15A circuit breaker. That means the Mag Trips are factory set between 1,800 and 4,000% of the breaker rating! Add to that the fact that this is the breaker you would need to use on a motor with a FLC of 5.9A, and that means the MINIMUM mag trip setting is going to be 4,661% of that motor's FLC! WAY over 1700%, yet PERFECTLY ACCEPTABLE.

That's what I am getting at, you cannot equivocate what they say about ADJUSTMENTS done in the field, and what is part of a tested unit.
 

Smart $

Esteemed Member
Location
Ohio
...

Well, the "mismatch" is in acceptability; the "not to exceed 1300% (or 1700%) is an issue of acceptability, is it not?

Case in point. If this were a Thermal Mag circuit breaker, aka "Inverse Time" circuit breaker per the NEC, the trip setting is not supposed to be over 250%, but if the smallest breaker available is too large, that's what you use, right? So now see if you can find the magnetic trip setting of a Sq. D FAL34015 molded case basic thermal-mag breaker. Hint: I picked that one specifically because I know it's not easy, so I'll save you the trouble. It is NOT adjustable, it is fixed, and on a 15A breaker it "Must Hold" at 275A and "Must Trip" at 600A, which means the mag trips will trip anywhere BETWEEN 275 and 600A. Remember, this is a 15A circuit breaker. That means the Mag Trips are factory set between 1,800 and 4,000% of the breaker rating! Add to that the fact that this is the breaker you would need to use on a motor with a FLC of 5.9A, and that means the MINIMUM mag trip setting is going to be 4,661% of that motor's FLC! WAY over 1700%, yet PERFECTLY ACCEPTABLE.

That's what I am getting at, you cannot equivocate what they say about ADJUSTMENTS done in the field, and what is part of a tested unit.
I'm getting the gist of what you are saying... and it goes back to what you pointed out that the SC trip setting is not adjustable.


Nevertheless, getting back to the device named in the OP, I found the following statement regarding the Eaton PKZM model in the document I linked to earlier ...
The main difference in the evaluation of
conventional European style motor
protective switches, like PKZM or
similar devices, is that they are certified
in North America as ?Manual Motor
Controllers? and fulfill only a motor
overload protective function, even
when they are additionally evaluated
for group installations. Devices of this
construction and certification are not
considered as motor branch short
circuit protective devices by the
electrical codes. (Exceptions to this are
UL 508 Type E and F controllers, to be
covered later.).
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
I'm getting the gist of what you are saying... and it goes back to what you pointed out that the SC trip setting is not adjustable.


Nevertheless, getting back to the device named in the OP, I found the following statement regarding the Eaton PKZM model in the document I linked to earlier ...
Yes, but you can only buy the UL508 Type E and F versions here in North America, at least it was that way when I worked there.

You do need to have another BCP device ahead of it under the rules for the UL508A Type E units, but it can be a MUCH larger circuit breaker, likely what you have as a main in a panel anyway. I don't have an old KM catalog any more, but looking at the A-B version the breaker ahead of a 6-10A Bulletin 140M MPCB must be no larger than 450A... That's not difficult to deal with really.
 

kwired

Electron manager
Location
NE Nebraska
The devices in question must have another overcurrent device ahead of them - and usually can be quite a bit higher trip level as Jraef mentioned.

I believe they also are usually only 5kA interrupt rated. If more then 5kA is available you possibly find it easier to apply them with a particular fuse type ahead of them but nearly impossible to find series ratings for any type of circuit breakers, as they simply would not have been tested with everything that could potentially be installed ahead of them.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
The devices in question must have another overcurrent device ahead of them - and usually can be quite a bit higher trip level as Jraef mentioned.

I believe they also are usually only 5kA interrupt rated. If more then 5kA is available you possibly find it easier to apply them with a particular fuse type ahead of them but nearly impossible to find series ratings for any type of circuit breakers, as they simply would not have been tested with everything that could potentially be installed ahead of them.

If you get a listed combination they are usually 65kA. However, as best I can tell, none of the type E or type F combo starters are good for 480V delta, only for 480/277.
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
If you get a listed combination they are usually 65kA. However, as best I can tell, none of the type E or type F combo starters are good for 480V delta, only for 480/277.
Yes, good point. That's true and we in North America must pay closemattention to that issue so it's good that you brought it up. Elsewhere in the world they don't use Delta power systems, so those devices were / are never designed for the added fault energy capacity you can get with that. That means when they are used by OEMs, the OEM must at the very least inform their end customers of the fact that the control panel can ONLY be connected to a Wye system.

I'd venture to say that issue is ignored and/or misunderstood in a lot of cases unfortunately. Delta is not extremely common, but I've seen a couple of instances where people have installed control panels built with those devices not knowing that they are unsuitable for their 480V Delta system. Because I deal with VFDs a lot, and VFDs have similar issues with Delta, I get dragged into situations where VFDs are failing as a result of people not understanding that their Delta system takes special attention. That's where I have then seen those control panels, especially on pump control systems because a lot of pump suppliers will sell a pre-built "skid" with the panel on it, and the panel uses these devices because they are smaller and cheaper.
 

eurekah

Member
Location
BC, Canada
I checked out the CSA file and class number for the Eaton device. According the the class number for that device covers:

"Manual across-the-line and reduced voltage starters, rotary switches, drum controllers and cam type controllers, open type or enclosed, with horsepower ratings. Starters and controllers may include in their assembly protective devices (e.g. overcurrent and/or overload devices) installed in accordance with the requirements of the Canadian Electrical Code Part I and NFPA 70 National Electric Code (NEC)."

I think this device may be able to be used as both overload and short protection for my branch circuit. The Canadian Electrical Code Part 1 is the code that contained the rule of having the max instantaneous SCPD being no more than 1300%. Sounds like since this has been already certified under that electrical code I might not have to take it into consideration.

Upstream I do have fuses, but they will be protecting more than just this motor.

Here is the link to the full class description:
http://www.csagroup.org/ca/en/servi...ation/certified-product-listing/class/3211-05
 
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