A question regarding Series Rating and Short Circuit Analysis

[quangmsc]

Can someone help me with this?
Situation:
A 42kA rated wye-delta starter of a chiller is fed from a 1600A feeder breaker of a 100kAIC rated substation via 60ft long 4 sets 500kcmil cables. The available fault current at the starter is 63kA > 42kA.

Questions:
1) Can cable limiter be used at both ends (feeder breaker and starter) of each cable per phase to reduce the short circuit? Bussman do make cable limiters for industrial applications.
2) For the above described single line diagram, do I need to worry about series ratings NEC 240.86?
Thank you very much.

 

[augie47]

My understanding is that the sole purpose of cable limiters is to protect cables. I have never know them to be figured into any fault current application.
If might be worth your time to check with the starter manufacturer to see what breakers or fuses will series rate with your starter. I inspected an job yesterday where a 10k rated contactor series rated with the upstream breaker for a 100k rating.

 

[kingpb]

IMO don't ever series rate for anything.

I believe current limiting fuses is what your thinking of.

If they make fuses large enough for the application, then yes you can use them, provided the peak let through current is below the equipment rating. Need to look at the let through curves.

Cable limiters isolate the cable in parallel cable runs to limit damage they are not intended to take the place of fuses.

 

[petersonra]

If all you are trying to do is protect the starter, there is likely a combination rating of some CB or fuse that will work.

 

[don_resqcapt19]

IMO don't ever series rate for anything.

I believe current limiting fuses is what your thinking of.

If they make fuses large enough for the application, then yes you can use them, provided the peak let through current is below the equipment rating. Need to look at the let through curves.

Cable limiters isolate the cable in parallel cable runs to limit damage they are not intended to take the place of fuses.

But the use of upstream current limiting fuses is providing "series protection" for the OCPD that is part of the downstream starter. This must be a listed and tested combination of devices or it must be selected under engineering supervision for an existing installation per 240.86(A).

 

[kingpb]

But the use of upstream current limiting fuses is providing "series protection" for the OCPD that is part of the downstream starter. This must be a listed and tested combination of devices or it must be selected under engineering supervision for an existing installation per 240.86(A).

I agree, and that's why I would not do what the OP is proposing. But, as you state, per 240.86(A) it is allowed.

NEMA has a good paper that discusses this topic and without extensive testing the analytical method only provides some added degree of protection; hence as stated; IMO as an engineer I am not willing to accept the responsibility that it may not properly operate and someone ends up being injured or killed. I will never sacrifice safety for money.

 

[quangmsc]

Thank you for all your very helpful replies. So cable limiters are out. I don't prefer current limiting fuses either because changing fuses typically exposes the electrician to live parts and there is always a danger of installing a current limiting fuse. Many times, wrong fuse sizes were installed.

That leaves me with the final option: series rated circuit breakers combo approved by UL.

 

[petersonra]

Thank you for all your very helpful replies. So cable limiters are out. I don't prefer current limiting fuses either because changing fuses typically exposes the electrician to live parts and there is always a danger of installing a current limiting fuse. Many times, wrong fuse sizes were installed.

That leaves me with the final option: series rated circuit breakers combo approved by UL.

You may find your options are very limited. Unfortunately on such a large motor your choices are going to be difficult and expensive anyway.

I suggest starting with having someone else look very closely at the SCC calculations and see if there is someplace that a mistake was made or where an assumption was used that made the downstream SCC a lot worse than it really is. Make a careful survey of what is actually there so you are not making calculations based on bad data.

Just because you have a "100kAIC rated substation " does not mean there is 100 kA of SCC available there.

 

[quangmsc]

Bob,
I double checked the input number and couldn't find any thing wrong with it.
One thing I wonder then:
The starter with the breaker inside is only designed for overload protection. In the event of the short circuit at the starter, the upstream 1600A feeder breaker should clear the fault because the short circuit protective device in this case is that 1600 breaker, not the starter/breaker. So we don't have to worry about the fact that the rating of the starter is only 42kA < 63kA.
Any comment is welcome. Thanks.

 

[kingpb]

Where is the 63KAIC available? Did you take 100KAIC and reduce it by the cable run, so the 63KAIC is at the terminals of the starter?

As stated by others, 100KAIC rated substation is going to have a lower than 100KAIC amount of fault current. In that case you need the actual SC number.

Does the 63KAIC include contribution from the chiller too? If so, then the starter would not see the full 63KAIC.

Its pretty easy to model if you have the right numbers.

 

[iceworm]

Bob,
I double checked the input number and couldn't find any thing wrong with it.
One thing I wonder then:
The starter with the breaker inside is only designed for overload protection. In the event of the short circuit at the starter, the upstream 1600A feeder breaker should clear the fault because the short circuit protective device in this case is that 1600 breaker, not the starter/breaker. So we don't have to worry about the fact that the rating of the starter is only 42kA < 63kA.
Any comment is welcome. Thanks.

We have no knowledge of what the starter is. However, generally speaking, there will be overloads in the starter that provide overload protection. The breaker in the starter is there to provide short circuit protection for faults on the load side of the starter (also provide gf protection).

The issue is that once the current gets up into the instantaneous range for both the starter CB and the upstream CB both are opening at the same time. There is no way to tell which one is taking the hit. The energy is porportional to I^2 (t) and is dissapated in the cb contact opening arc. As I recall, cbs are rated to open on rated fault current, be able to be reclosed, open once more on rated fault current. After that, you have no guarentee of anything other than a pile of glowing slag where the cb used to be

If there is 63kA available SCC at the line side of the starter, then the starter/CB combination must be able to open under the fault current from a fault on the load side of the starter.

This one is not rated for 63kA. As others have said, your options are recalculate, series rating, new gear rated for 63kA.

ice

 

[topgone]

We have no knowledge of what the starter is. However, generally speaking, there will be overloads in the starter that provide overload protection. The breaker in the starter is there to provide short circuit protection for faults on the load side of the starter (also provide gf protection).

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By not knowing the kind of starter used, we can only guess. But if the OP meant to have a problem with the motor circuit protection breaker only, the OP can refer to the breaker manufacturer's catalog and verify if the breaker he is using is listed as series-rated with the 1600A feeder breaker. Otherwise, he should secure a breaker fit for the required breaker fault current rating, i.e. higher than 63kA.