Motor Contactor Interrupting Rating

[cherry2000]

Can anybody tell me the Interrupting rating of Air Break NEMA magnetic Contactor (Size 1 to 5)?
Is there any value given by NEMA ICS 2? I wanted this value to check coordination with the upstream MCP in case of fault.

 

[waross]

If I understand your question correctly you are not allowed by code to do that. Coordination between breakers and starters is a matter that is determined and verified by the manufacturers in the test lab. Only combinations that the manufacturer has tested and verified may be used in the field.
There have been some discussions on this site on this topic.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[cherry2000]

Bill..The purpose of the check was to see if the contactor still remains within it's rating limits even after setting the MCP at 17 times FLA for premium efficiency motors.Also, for the sake of acedamic interest, I wanted to know what typically are the interrupting ratings.

 

[waross]

Yes. The manufacturer should have tested the combination to verify that it was safe at any setting.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[dpc]

This data is readily available from any US manufacturer - check their website. They will all have the same rating, I believe.

"An 'expert' is someone who has made every possible mistake in a very narrow field of study." -- Edward Teller

 

[cherry2000]

I checked on Eaton's site but couldn't get any value. Possibly I am not searching in the correct location. If you can provide a link, it will be helpful.

 

[dpc]

For a Cutler-Hammer 400 A SJ vacuum contactor, interrupting rating is 7600 A. (maximum of 3).

For an 800 A SJ, it is 13,200 A.

This is what I could grab off my shelf. I'd expect the old air break contactors to be similar.




"An 'expert' is someone who has made every possible mistake in a very narrow field of study." -- Edward Teller

 

[jraef]

I think the problem may be one of semantics. "Interrupt ratings" are for circuit protective devices and has to do with the devioce's ability to safely stop the flow of fault current. The magnetic trip setting of an MCCB has nothing to do with the fault current capabilities of the MCCB, they are related to the point at which the breaker will try to start opening the circuit. The interrupt rating has to do with the point at which the MCCB begins to emit shrapnel or fails to interrupt the fault.

But contactors are not used as interrupt devices*, so they will not show an "interrupt rating". A contactor would have a "breaking capacity" and a "Short Circuit Current Rating" (SCCR) which also used to be referred to as a "Withstand rating". As far as I know, only the breaking capacity might have anything to do with the magnetic current trip settings of a circuit breaker. In other words, the trip point of the breaker being that high means that the contactor may end up being asked to open under conditions up to that setting and the breaking capacity is where that would come in. But the breaking capacity of a NEMA contactor is several times the continuous current rating (I am without my copy of NEMA ICS-2 right now to look it up, maybe someone else has it).

From a fault current standpoint, the SCCR rating is the important issue. The contactor must also be able to avoid becoming shrapnel while it waits for the circuit protective device to clear the fault. But the SCCR rating can be done as part of a "series rating" by the manufacturer, because they can consider the "let-through" energy of the protective device as the maximum the contactor will see. So for instance, you may see that a contactor has an SCCR rating as a stand-alone device at only 5kA or 10kA, but if used in a tested and listed combination with a specific circuit breaker, the combination may be rated by the mfr as 65kA.

*The only time I see contactors have interrupt ratings are on vacuum contactors, because they are often used with protective relays in the same manner as a circuit breaker. Hence dpc's observation above.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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[wolfie1a]

Interesting discussion. Let me add a twist to the convesation. I have seen many of after market ground fault trip devices (AB 1409) used to drop out a contactor in the event of a fault. In these cases the contactor is working as fault clearning device (yes / no). Depending on the fault current, can the contactor clear the fault? I doubt that this was taken into consideration when the after market device were installed?

 

[cherry2000]

A similar situation is seen for Motor locked rotor current. It is seen by the Overload relay and the contactor is asked to "interrupt" the LRC.

 

[cherry2000]

In fact, for a overload relay of Class 20, for 6 times the FLA (which is a good approximation for the motor LRC), the relay will operate after 20 seconds, operating the contactor.

 

[jraef]

There again, the semantics. LRC is considered "normal breaking capacity" of a contactor. The term "interrupt" is used when describing fault current, as in the total available fault current in the circuit being 65,000A for example.


"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

 

[cherry2000]

I asked this question to Eaton who promptly replied back :

Our NEMA line of contactors are designed to NEMA standards and are tested to make and break currents of 10x device ratings. additionally we have withstand ratings As reference in UL SB4-1 of the UL508A as follows

0-50HP 5000kA
51-200HP - 10Ka
201-400HP - 18Ka
401-600HP - 30Ka
601-900HP - 42Ka
901-1500HP - 85Ka

NEMA ICS 2 Sec 8.1 states that for Class A Motor Controllers, the make & break current shall be 10 times that of the rated motor FLC

 

[ScottyUK]

Just to clarify: the withstand rating is the maximum rated through-fault it will survive without melting or disintegrating on the assumption that something else actually clears the fault. It is not an interrupting or breaking rating. Apologies if I'm stating the obvious.


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If we learn from our mistakes I'm getting a great education!

 

[LionelHutz]

Wolfie - it really should not be an issue. If the ground fault causes a high current short then the breaker or fuses will clear anyways.

FYI, the Eaton numbers listed above are the UL minimums. Every device must test to at least those numbers. If there is no marking on the device then you can safely assume those numbers apply.

To be correct, the "withstand rating" was the fault current available at the incoming of a complete package under test which included this component. It is not the current the device will actual withstand. This trips a lot of people.

For practical purposes this can be used to co-ordinate the components. Using the ratings listed above, for a 50hp motor buy at least a 5kA rated circuit breaker and mount them together with an overload on a system with no more than 5kA fault current and you're good to go. You could also find a tested combination where that 5kA rated contactor is good for maybe 50kA with a specific breaker and overload relay.

The fault current ratings are actually fairly complex. The 5kA rating does not mean the device actually saw 5kA of fault current during the test. The test is done as follows. You test a complete starter package (minimum would be a breaker, contactor and overload). This package has incoming wire and a set of outgoing "load" leads something like 18" long connected to a huge shorting contactor. You short the incoming power leads and adjust the test setup until 5kA fault current flows. Then, you connect the device and test. There are 2 tests to perform. The first is you close the output contactor while the device your testing is closed. The second is you close the device you are testing into the fault.

So, you will never see 5kA during the actual test. You add impendance to the test setup when you connect your motor starter and the output contactor. The contactor in the package you are testing will likely begin to open the same time the breaker is opening. All kinds of interactions can occur. The net effect is that the actual current is lower than the 5kA during your test setup.

When testing soft-starters with a bypass, we found that capturing the fault and performing a certain sequence can actually get a higher fault rating. It's not as simple as just keeping the soft-starter on and letting the breaker clear the fault.

Lots of people get confused on the fault ratings. They read 5kA withstand so they think, "Hey, if I install fuses that only allow 5kA of let through current on my 30kA fault capable system then I can call the combination 30kA." It doesn't work like that. However, during testing, we recorded the actual current during the tests so we know the actual current and we can use that data to co-ordinate with different circuit breakers or fuses without actually combination testing the units. We still need UL approval but it's "paper proof" not lab testing.

 

[cherry2000]

We have an interesting case in one of our starters..Size 3.
Here the Contactor cont. amps is 90A. Hence assuming that the conatctor safe breaking current is 10 x In =900A
Now the accompanying MCP has a setting of 500-1250. In other words, there is a possibility that somebody in the field sets the MCP higher than the contactor breaking rating....!!!!

 

[ScottyUK]

Lionel,

IEC 60947-4-1 has section 9.3.4 dedicated to short circuit performance of contactors. The effect of the upstream fuse or breaker is taken in to account when determining the prospective short circuit current used for the test. Type 1 coordination allows the contactor to be damaged beyond repair during the test; Type 2 coordination requires the contactor be fit for service, or able to be made so with minimal intervention.

Cherry2000,

That could apply in any number of situations if unqualified people start messing with protection settings. For example, what if someone changes the dial settings on an ACB or modifies the DIP switches on a relay? Maybe the size of the bang could be bigger but it's the same principle.


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If we learn from our mistakes I'm getting a great education!

 

[LionelHutz]

Scotty - I guess I should have stated that is the testing for UL508. The OP asked about NEMA ratings and NEMA isn't a listing agency but NEMA is typically a North America thing so NEMA rated devices typically get UL tested and listed.

 

[cherry2000]

Agreed Scotty..one thing to be noted..ACB settings, Relay setings etc are much more closely controlled and necessarily has some Project documentation associated with it..comparatively, MCP settings are loosely controlled. Field may adopt the method of setting the MCP at 1100% and then increase the setting till the motor can be successfully started.. now for Premium efficiency motor, with a X/R ratio od 16, worst first cycle inrush (with instant of switching at voltage at maximum), can reach 17.2 times...that effectively means that the contactor has been undersized!!

 

[dpc]

If you are using NEMA-rated contactors, I wouldn't be too worried about the contactor withstand rating as long as the contactor is applied within its horsepower ratings.





"The perfect is the enemy of the good." -- Voltaire