Fused Disconnect Required to meet AIC rating

[Sean.Day72]

Hello,

There has been some debate in our office on this subject. The scenario is we have 200 Amp breaker on the first floor feeding a 112kva transformer on the 3rd floor. The transformer then feeds a panel with a MCB. The calculated available fault current at the primary side of the transformer on the third floor is approximately 60kaic. We have a disconnect switch on the primary side of this 3rd floor transformer. Question, is the disconnect required to be fused to meet the AIC rating of the circuit it's on? My understanding is that an unfused disconnect is only rated 10kaic. The answer seem to be obviously Yes you need a fuse, but I'm receiving some push back. We meet the OCPD for the transformer from the breaker, but if a fault occurs it could potentially damage that disconnect. Am I missing something?

 

[Jraef]

First the correct semantics. Non-fused disconnects will not have any IC (Interrupting Capacity) rating, they don't interrupt faults by themselves. They do have a WITHSTAND rating, now called an SCCR (Short Circuit Current Rating). The terminology is important.

So you are correct, non-fused disconnects only carry a 10kA SCCR at best (some as low as 5kA). The way around that is to use a fused disconnect, because the current limitation of the fuse will allow the SAME disconnect to be used at the IC rating of the fuse (in most cases), i.e. 100kAIC or 200kAIC. It's a series rating of the switch and fuse together. No fuse, no rating.

Technically IF the non-fused switch were series listed WITH the specific circuit breaker ahead of it, you might be able to use it. I don't know of any that are, but I don't know everything. I will say however that IF that situation exists, it will ONLY exist if the breaker and switch are from the same mfr, because someone must pay for the destructive series testing and listing of them together, and no mfr bothers to pay to series list their devices with a competitor's device. So if you have a Sq. D breaker and Sq. D switch, maybe, but Sq. D breaker and Siemens switch, no chance. So if both are the same mfr, contact that mfr and ask them if they are "series listed". If not, it's a no-go.

 

[ron]

Jraef is right on.

You need a fused switch, or a properly rated enclosed circuit breaker for the XFMR primary.

Maybe just consider removing the primary disconnect and making the OCPD on the 1st Fl lockable.

 

[Sean.Day72]

Thanks guys I’m in agreement . I sent a email out to Eaton if I can get a disconnect and a breaker series rated


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[ron]

Thanks guys I’m in agreement . I sent a email out to Eaton if I can get a disconnect and a breaker series rated

Eaton's stuff is here

Look at the fuse/breaker tables
http://www.eaton.eu/ecm/idcplg?IdcS...s=0&Rendition=Primary&dDocName=1C96944H01.PDF

 

[smoothops10]

SqD heavy duty safety switchs do have these series ratings as well but generally only with the powerpact line of breakers. Not the E frame breakers that is standard in their lighting nf panelboards. If you had an I-line panelboard with powerpacts best you could get for the switch is a 35kA @ 480V rating. https://www.productinfo.schneider-e.../ULListedMaximumShortCircuitCurrentR-8071BF65

Are you sure your calcs are not overly conservative? You didn't metnion voltage but assuming 277/480 you would need atleast a 2000kVA utility transformer at a low 4% impedance to get near those numbers at the main. Then run a 3/0 feeder 40' and you are going to lose 20kA by the time you get to the 3rd floor.

That said, if your numbers are accurate and all you want is a disconnecting means you could look at using a molded case switch.

 

[Sean.Day72]

Ok I found a series rated system (Upstream OCPD and disconnect )that meets the AIC requirements. I guess this leads me into another code question. I know if you have a series rated panel you're are to mark it as such per 110.22 . Will this marking be required on the non-fused disconnect to the effect of "Caution Series Combination Rated XXX. Identified replacement component required" I assume yes because if it's replaced in the future a tested system would need to be in there.

I run into series rated panels a lot for school board work because they're cheap and can reduce the AIC ratings of the branch OCPDs.

 

[Jraef]

Ok I found a series rated system (Upstream OCPD and disconnect )that meets the AIC requirements. I guess this leads me into another code question. I know if you have a series rated panel you're are to mark it as such per 110.22 . Will this marking be required on the non-fused disconnect to the effect of "Caution Series Combination Rated XXX. Identified replacement component required" I assume yes because if it's replaced in the future a tested system would need to be in there.

I run into series rated panels a lot for school board work because they're cheap and can reduce the AIC ratings of the branch OCPDs.

I think it's a good idea and I would do it, but I'm not sure it's required. When it's time for inspection, you will want to have your documentation with you in case the AHJ asks to see it, but field identification signage is not in the NEC as far as I know (although I don't have a 2017 version yet).

 

[kwired]

If upstream device is a fuse then the non fused switch often will be series rated for that, I think.