Selective Coordination Realistic?

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Is selective coordination possible with none adjustable thermal magnetic breakers? Like a 20 amp QO coordinating with a 40 amp QO?



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Without going deeply into it . . . I have used fused 120/208V panelboards and they coordinate quite nicely. You can also use fuses up the line as well.

RC
 
It is interesting that they say provide "selective coordination" per NFPA 99, when NFPA 99 only requires coordination and not selective coordination.

The difference being that selective coordination is down to time "zero" while coordination is only down to 0.1 seconds. The coordination rule is much easier to meet, and your breakers probably would work for that...probably won't for selective coordination.
 
don_resqcapt19 said:
It is interesting that they say provide "selective coordination" per NFPA 99, when NFPA 99 only requires coordination and not selective coordination.

The difference being that selective coordination is down to time "zero" while coordination is only down to 0.1 seconds. The coordination rule is much easier to meet, and your breakers probably would work for that...probably won't for selective coordination.
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Is 0.1 realistic for all faults or does it just takes reasonably probable faults into account?
 
Maybe.
Manufacturers have test breaker combinations for selective coordination. Typically you need to have the upstream breakers be 1 frame size larger than the downstream one. There is also an issue with the available fault current, often it needs to be just a few kA.
 
It's not likely that a QO20 will coordinate with an upstream QO40. I put together a TCC using a 225 kVA transformer which would be a reasonable size when compared to the 150 kW genset. In the scenario on the attached TCC the pair would only coordinate up to a maximum fault current of 283 amps. Selective coordination would need electronic-trip breakers that would cost a whole lot more than the thermal-magnetic Square D QO type.
 

Attachments

  • MH QO TCC.pdf
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mayanees said:
It's not likely that a QO20 will coordinate with an upstream QO40.
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A QOx20 and QOx40 are the same frame size (basically the same magnetic element), so there is likely no possible coordination even though there is a 2X difference is trip rating.
You typically cannot see selective coordination, for small breakers, by using trip curves unless the fault current is below the Instantaneous pick up point. You need to go to the published tested combinations from each manufacturer.
 
jim dungar said:
You typically cannot see selective coordination, for small breakers, by using trip curves unless the fault current is below the Instantaneous pick up point. You need to go to the published tested combinations from each manufacturer.
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That's a really good point, and it's illustrated in the attached TCC. The instantaneous portions overlap such that selective coordination is not apparent. But the referenced literature shows the two breakers coordinate selectively up to 10kA. (Table 4, p.11)
 

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  • MH LA QO TCC.pdf
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Cannotlogin said:
Is 0.1 realistic for all faults or does it just takes reasonably probable faults into account?
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You would have to look at the instantaneous part of the trip curves and the available fault currents. It is much easier and cheaper to coordinate at 0.1 seconds than at zero seconds.
 
mayanees said:
That's a really good point, and it's illustrated in the attached TCC. The instantaneous portions overlap such that selective coordination is not apparent. But the referenced literature shows the two breakers coordinate selectively up to 10kA. (Table 4, p.11)
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Any idea why? Thats just really confusing.
 
Cannotlogin said:
Any idea why? Thats just really confusing.
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There is a bit of slop in the way TCCs are actually drawn. Breakers often perform far faster, in the Instantaneous region, than the TCC shows, due to improvements in components and design Historically the industry shows the worst case performance on their TCC even if it covers only .05% of the breakers which might be sold (things liked switched neutral and auxiliary switches could impact breaker performance. By putting breakers in series it is possible to take advantage of a specific breaker configuration's speed as well as any dynamic impedance that may be present at lower fault currents.

For the most part selective coordination is about having branch circuits cleared before feeder circuits when something that has been operating fails. Bolted short circuits, with their high fault, currents usually only occur at power up while arcing short circuits, with relatively low fault currents, are much more common.
 
mayanees said:
That's a really good point, and it's illustrated in the attached TCC. The instantaneous portions overlap such that selective coordination is not apparent. But the referenced literature shows the two breakers coordinate selectively up to 10kA. (Table 4, p.11)
Click to expand...

They coordinate because of dynamic resistance which does not show up on a TCC...it does in tested combination data.
 
mbrooke said:
Dynamic resistance?
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During short circuits breakers current limit just as fuses do, even if they aren’t specifically built to do this. The effect slows down the trip time of a larger breaker so that the smaller one trips first even with identical short circuit trip times.

However even with complete data on both breakers there is no known way to predict this behavior. Every breaker combination must be tested. Hence we have series rating charts.
 
Does anyone else object to the main breakers on the load side of the ATS's?

How will anyone ever find a 40A breaker on the load side of a LS-ATS that will coordinate with either 40A breaker on the line side of LS-ATS? I suppose it could be done if 40A electronic trip breakers are available. But think of the cost it would add. And why?

Same with the 400A switch.

Eliminate the mains on the load side of the ATS's.
 
steve66 said:
Does anyone else object to the main breakers on the load side of the ATS's?

How will anyone ever find a 40A breaker on the load side of a LS-ATS that will coordinate with either 40A breaker on the line side of LS-ATS? I suppose it could be done if 40A electronic trip breakers are available. But think of the cost it would add. And why?

Same with the 400A switch.

Eliminate the mains on the load side of the ATS's.
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Even if you got rid of the 40 amp breaker on the load side of the ATS, it is not likely that the branch breakers in the panel would coordinate with the upstream 40 amp devices.

You need to remember that 700.32 and 701.32 were written by someone who works for Bussmann....and their fuses work just fine for selective coordination applications.
 
One thing to remember when performing selective coordination, is to use the actual fault currents for the circuits requiring selectivity.
Many people try to use the maximum bolted fault current on a device's line side which can be problematic, even 20' of #12 conductor on the load side of a 20A breaker can considerable impact the amount of fault current and improve coordination.

One problem with fuses is, you must maintain the size relationship between devices (8X is not uncommon unless the more expensive style fuses are used).
 
don_resqcapt19 said:
Even if you got rid of the 40 amp breaker on the load side of the ATS, it is not likely that the branch breakers in the panel would coordinate with the upstream 40 amp devices.

You need to remember that 700.32 and 701.32 were written by someone who works for Bussmann....and their fuses work just fine for selective coordination applications.
Click to expand...

The fact that the 20's and 40's won't coordinate makes having the 40A breaker on the load side of the ATS even worse.

If there is any short circuit on any 20A LS branch, there is a very good chance the 40A on the load side of the ATS will also trip, which will completely disconnect the LS panel.

The generator will never get a chance to do its job or to provide power to the LS branch. There will be an open circuit breaker right in the way.

Panels on the load side of ATS's should not have main breakers if it can be avoided. If it can't be avoided, they should be coordinated so they never trip before the upstream breakers.
 
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