50 or 17 MS transfer switch WCR

[electrofelon]

I am working on incorporating a 600 amp ATS into our electrical system. The spec sheet shows 2 different WCR/SCCR ratings, 50 and 17 millisecond. The 50 ms is 35k and the 17 Ms is 85k. AFC is about 50k. How do I determine which column to use? Do I have to look at the circuit breakers trip curve that is ahead of the ATS to determine if I can use the 17 millisecond rating?

 

[jim dungar]

Yes, you need to use the trip curves.

 

[electrofelon]

Yes, you need to use the trip curves.

Is interpolation allowed?

 

[jim dungar]

Is interpolation allowed?

Most curves require some amount of interpolation, or they would be very cluttered.
However you can not interpolate beyond the boundaries of the curve axises.
For example: most curves are cut off at .01 sec, so you can not assume any type of performance beyond that point.

 

[electrofelon]

Most curves require some amount of interpolation, or they would be very cluttered.
However you can not interpolate beyond the boundaries of the curve axises.
For example: most curves are cut off at .01 sec, so you can not assume any type of performance beyond that point.

Thinking about this some more, I guess I am confused here. If I have a bolted fault, I will have 50k of fault current. So what purpose does providing 1 cycle and 3 cycle ratings provide? If the 1 cycle rating is 35k, how can I EVER use the ATS where there is more than 35k of fault current?

 

[electrofelon]

Ok, maybe I've got it:.

So I take my 50k AFC and use the trip curve to find the clearing time. Then I interpolate the 1 and 3 cycle times for the ATS to see if my clearing time is less than the interpolated value?

 

[jim dungar]

Thinking about this some more, I guess I am confused here. If I have a bolted fault, I will have 50k of fault current. So what purpose does providing 1 cycle and 3 cycle ratings provide? If the 1 cycle rating is 35k, how can I EVER use the ATS where there is more than 35k of fault current?

Your are correct, especially if your protective devices are molded case circuit breakers.

 

[jim dungar]

Ok, maybe I've got it:.

So I take my 50k AFC and use the trip curve to find the clearing time. Then I interpolate the 1 and 3 cycle times for the ATS to see if my clearing time is less than the interpolated value?

Yes.

 

[electrofelon]

Yes.

Jim, It looks like this will be ok. Would you mind checking me? Looks like at 50k the breaker will trip in under 1 cycle. Is that possible? Find the documents below. The desired breaker is a LXD6 600 amp, trip curve found on page 21. 600A transfer switch also.

BTW, I made a mistake in post #5: 35K was the 3 cycle rating, not the 1 cycle.

https://assets.new.siemens.com/siemens/assets/api/uuid:1030ab65-5c56-4902-a5da-d18a1e623374/sie-im-jd-ld-info-instruction-guide.pdf

 

[jim dungar]

Yes it is possible from breakers to clear in less than 1cycle, but most TCCs do not show this. That data might come from the breaker manufacturer for use with series testing by other equipment manufacturers, I don't think I have ever seen it generally published.

 

[electrofelon]

Yes it is possible from breakers to clear in less than 1cycle, but most TCCs do not show this. That data might come from the breaker manufacturer for use with series testing by other equipment manufacturers, I don't think I have ever seen it generally published.

For this breaker, the TCC does go down to about 1/2 cycle for 240V, but just a little under 1 for 600V and 480V

I can get the withstand up to a whopping 150KA using a CLD breaker, but that is a very expensive breaker so there is an incentive to use a much cheaper LXD6. Looks like even the LXD6 will trip in .025 ms at 35 KA, which is the 50 ms rating.

 

[steve66]

ASCO gives instructions for using their ratings. I don't think the link will work: just search for "Asco Withstand Ratings":

https://irp-cdn.multiscreensite.com/d9391e9e/files/uploaded/1128_R12_2011-5-21.pdf#:~:text=ASCO%20Power%20Technologies%20provides%20withstand%20current%20ratings%20on,to%20the%20WCR%20of%20the%20phase%20switching%20poles.

Back to your application. The max. time to clear a fault is going to depend on the setting for the instantaneous, correct?

Highest setting gives a max. clearing time of about 0.025 seconds, which is more than one cycle. But at the minimum setting, it would clear in less than 0.001 seconds. It would be nice if they gave one more curve in between min and max.

If we assume the instantaneous is set to minimum, it looks like the breaker is OK. It will always trip in much less than a cycle.

If we look at the trip time for our maximum of 50K (83x600A), it is just slightly less than 17 ms. So it looks like we meet the 17ms withstand rating of 85K.

But now I'm confused. How do we know exactly what current to use to determine the maximum breaker opening time? Do we assume the maximum fault current available - 50KA for this example? I think that's how I've always done it.

But that doesn't seem quite fair, since the calculations are always conservative (the actual fault current will always be somewhat less), and the breaker will always take longer to open.

 

[electrofelon]

But now I'm confused. How do we know exactly what current to use to determine the maximum breaker opening time? Do we assume the maximum fault current available - 50KA for this example? I think that's how I've always done it.

But that doesn't seem quite fair, since the calculations are always conservative (the actual fault current will always be somewhat less), and the breaker will always take longer to open.

That is what I was a bit unsure of. We could look at multiple cases and see if it works for all of those cases, I think. For example, at 50K, we clear at about .0125 which is under one cycle, and the ATC can handle 85K for 1 cycle, so we are definitely ok there. So then lets look at the 3 cycle rating: the ATS can handle 35K for 3 cycles. At 35K of fault current, looks like the breaker will trip at .015 which is still under a cycle, so good there. But what if we go much lower? Seems like they should provide a "long time" or indefinite rating? For example at 5500A we are getting into the range of 20 seconds (that doesnt seem right does it??).

 

[jim dungar]

This is why I have always chosen compatible breakers and transfer switches. If the switch manufacturer doesn't say the breaker will protect it why would I want that responsibility.

Switch manufacturers often have many unpublished combinations, but it takes some work to find them.

I think the switch has some time current points in the 'seconds' range, again you might have to ask for them.

 

[electrofelon]

This is why I have always chosen compatible breakers and transfer switches. If the switch manufacturer doesn't say the breaker will protect it why would I want that responsibility.

Switch manufacturers often have many unpublished combinations, but it takes some work to find them.

I think the switch has some time current points in the 'seconds' range, again you might have to ask for them.

I understand where you are coming from, but in this case we are talking a $500 breaker vs a $5000 breaker.

I have been through designing around several different transfer switches (client changed manufacturers several times for various reasons) and the Thomson is the only one that has provided different time ratings. For example, the Cummins, for the general circuit breaker, was just 25KA must clear in .05 seconds. They did however say, "This transfer switch does not include short time current ratings", which I think is what I was getting at in post #13. What exactly does that mean? We just dont worry about it and hope for the best?

 

[jim dungar]

They did however say, "This transfer switch does not include short time current ratings", which I think is what I was getting at in post #13. What exactly does that mean? We just dont worry about it and hope for the best?

I would take this to mean your protective device must have an instantaneous trip function that cannot be disabled.

Most of this is governed by the UL 1008 standard for transfer switches.
Here is a link to some ASCO tech data. https://www.ascopower.com/us/en/resources/technical-briefs/applying-transfer-switch.jsp

Edit.
I found the short time rating has to due with being able to withstand faults for 6 cycles or 0.1sec.