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Thread: Fused Disconnect Required to meet AIC rating

  1. #1
    Join Date
    Aug 2017
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    Florida
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    Fused Disconnect Required to meet AIC rating

    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?

  2. #2
    Join Date
    Jul 2006
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    San Francisco, CA, USA
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    6,950
    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.
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  3. #3
    Join Date
    Feb 2003
    Location
    New York, 40.7514,-73.9925
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    4,350
    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.
    Ron

  4. #4
    Join Date
    Aug 2017
    Location
    Florida
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    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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  5. #5
    Join Date
    Feb 2003
    Location
    New York, 40.7514,-73.9925
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    Quote Originally Posted by Sean.Day72 View Post
    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?IdcSe...1C96944H01.PDF
    Ron

  6. #6
    Join Date
    Oct 2013
    Location
    MD
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    46
    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-el...rentR-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.

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