Re-visiting short circuit current rating in 5k control cabinet...

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arcsnsparks98

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Jackson, TN USA
Not long ago there was a thread about a chiller with a control cabinet labeled 5kA SCCR. I believe it was a 208 chiller. I recently saw an install of two 480v chillers, each having their own control cabinet with a 5kA SCCR. They were fed with a 600A MCCB, 65kAIC (each) and located approximately 100' from the switchgear. Parallel 350 Cu feeders so negligible current limitation there. The transformer supplying the gear is a 2000 kVA delta/wye to a 2500A OCP protecting the entire switchgear. Most transformers of this configuration that I come across (please contain the 'nit picks') are usually marked with a Z of about 5.5%, give or take a fraction of a % here and there. Clearly there is more than 5kA available at the chillers. The way I calculate it though, any transformer large enough to supply the chillers with sufficient operating current will be capable of delivering more than 5kA of short circuit current. Is this correct? Even with an isolation transformer, line reactor, etc. in place, it seems that there will be more SCC than what these cabinets are listed to handle. Thoughts?
 
Wow!
Difficult to imagine that size equipment with a 5k withstand.
I agree with you that with that size circuit it will be difficult to get your SCA down to 5k.
My first thought would be to investigate what components are limiting you to that low a number and see if you could protect them with a current limiting device.\
A good engineer seems in order.
To add to the dilema, TN requires NRTL listings and altering the equipment poses it's own problems.]
Have you discussed it with the manufacturer ?
 
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arcsnsparks98 said:
Not long ago there was a thread about a chiller with a control cabinet labeled 5kA SCCR. I believe it was a 208 chiller. I recently saw an install of two 480v chillers, each having their own control cabinet with a 5kA SCCR. They were fed with a 600A MCCB, 65kAIC (each) and located approximately 100' from the switchgear. Parallel 350 Cu feeders so negligible current limitation there. The transformer supplying the gear is a 2000 kVA delta/wye to a 2500A OCP protecting the entire switchgear. Most transformers of this configuration that I come across (please contain the 'nit picks') are usually marked with a Z of about 5.5%, give or take a fraction of a % here and there. Clearly there is more than 5kA available at the chillers. The way I calculate it though, any transformer large enough to supply the chillers with sufficient operating current will be capable of delivering more than 5kA of short circuit current. Is this correct? Even with an isolation transformer, line reactor, etc. in place, it seems that there will be more SCC than what these cabinets are listed to handle. Thoughts?
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Just ball parking, the available fault current at the terminals of the transformer, assuming 3% impedance, infinite primary and no motor contribution is about 90,00 amps. At 5% impedance it would be about 53,000 amps. Of course at the terminals of the chiller it would be considerably less, but certainly likely to be more than 5K.
Sounds like a study is in order.
 
arcsnsparks98 said:
What baffles me is this equipment was spec'd and provided by carrier. Not some chuck in a truck that wouldnt know any better.
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agree. certainly surprising as it is rare to have that high a operating amperage and only 5k SCA.
maybe the infamous low bidder :)
 
Unsure how strict your inspectors are or if it will be inspected, but the State (along with OSHA) does require most equipment to have NRTL certification.
 
Funny that this should come up, but funny in a tragicomedic way.

If you're looking for a reason why they only have a 5kA rating, it's simple; they don't care... Really, they don't, they consider it "someone else' problem". I was deeply involved in a contract negotiation with Carrier years ago to get the 5 year contract for their starter business (worth about $3 million /year at the time). The term SCCR was just in its infancy and had just been put into the NEC with the addition of Article 409, but the concept behind it was there all along anyway. We used to call it "withstand rating", it's just that there was no formal process for attaining it for assemblies, only individual components. That's all that really changed, the formalized /standardized process so that one could trust the numbers.

Anyway, I brought up this subject during the negotiations with Carrier, because our chief competitor was supplying cheap customized panels built from Korean components that I KNEW had never been tested as combination starters, so there was no way they could offer a reasonable withstand rating as an assembly and would just use the "courtesy" 5kA value that they could attain without expensive testing. Carrier literally told me they didn't care, wasn't their problem. I explained what it would mean to installing contractors in the field, that was met with total indifference. We lost the contract.

Chickens coming home to roost I suppose, but unfortunately you are the new coop master.
 
arcsnsparks98 said:
When I was on site I looked for the SCCR but didnt look for a UL label. I suppose if it was a non UL cabinet they stuck a default rating on it and called it good.
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An NRTL 'tested and listed' equipment could still only be 5kA.
NRTL's test to a level requested by the manufacturer, they do not determine the actual 'highest amount'. For example, a piece of equipment may be designed for 18kA, but the manufacturer may choose to have it tested and labeled as 10kA if that is the nearest, but not higher, standard level.

The only way to get equipment that is rated for the available fault current is through enforced specification and procurement procedures.
 
Jraef and Jim: Both posts are very interesting.
I had an inspection today on some new foundry equipment. Transformer SCA is 43k, gear is 65k. New UL listed industrial control panel, 100 amp load within 25 ft of MDP.
Chock full of Class J fuses but the UL nameplate SSCR states 5k.
Factory rep on site was asked about the low number and advised "That's the customer's problem".

Anybody think of a quick fix ?
 
augie47 said:
Jraef and Jim: Both posts are very interesting.
I had an inspection today on some new foundry equipment. Transformer SCA is 43k, gear is 65k. New UL listed industrial control panel, 100 amp load within 25 ft of MDP.
Chock full of Class J fuses but the UL nameplate SSCR states 5k.
Factory rep on site was asked about the low number and advised "That's the customer's problem".

Anybody think of a quick fix ?
Click to expand...

There are two possibilities and they seem equally likely.

The first is that no one asked for anything else so they put the default label on.

The other is that despite the class J fuses, the thing really is 5 kA SCCR. Class J fuses are often the least expensive OCPD option, and are often used for that reason. The fact that they can help with your SCCR is secondary to the cost factor.

You might be able to get the manufacturer to supply a new nameplate. They can't send you a new UL label for you to install but they can give you a corrected nameplate.

We ask every customer what SCCR rating they want. The majority opt for the cheapest option. We do have one customer who has standardized on 65 kA. Not all that hard to deal with these days, except when they go to 575 V. Another has standardized on 42kA. I am not sure why. For the most part if it meets 42kA it will meet 65kA as well. There may be some stuff they are using that is tough to get past 42kA. But it is not with us.
 
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jim dungar said:
... The only way to get equipment that is rated for the available fault current is through enforced specification and procurement procedures.
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I think that's really the crux of the matter right there. A lot of engineers just copy and paste specs from older projects, never looking at them to see if they need updating. So the specs go out without being clear on who is responsible for having the equipment SCCR rating match the AFC. I see it day in and day out.

Worse yet, as the OP's situation seems to indicate, is the VENDOR being allowed to write the spec for the equipment! They are not going to include ANYTHING that might cost them more money if they don't have to.
 
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