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Thread: Reducing AIC at a machine.

  1. #1
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    Reducing AIC at a machine.

    Hello,
    Our company was ask to install a 3 phase 480/277v GE panel about 4 months ago. We installed several 30 & 60 amp type TEY 14K AIC 3 phase breakers. Ran the conduit and wire to where each new machine. Each machine has 3 fuses that our feeds connect to. Just received a call from the company that the machine manufacture wants the AIC on some of the machines to be 10K AIC. I couldn't find any 10K AIC 480 breakers at any supply houses. I called GE. They said they don't make them. My question is does anyone know how to reduce the AIC from 14K to 10K. Could I possible change the fuses on the machines that need to be 10K AIC. If so what style of fuse? Thanks, in advance!

  2. #2
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    it would not take much wire to change a 14 kA SCC to 10 kA. Has anyone calculated what the actual SCC is at the panels?

    In any case, it won't do you any good to change out the 14 kAIC breakers. the SCC will stay the same.
    Bob

  3. #3
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    Quote Originally Posted by derelectric View Post
    . . . the machine manufacture wants the AIC on some of the machines to be 10K AIC.
    Let me take Bob’s answer a step further. Let’s start by getting our terms right. AIC is the amount of current that an overcurrent protection device (i.e., fuse/holder or breaker) can interrupt, without being destroyed in the process. The term can only apply to an overcurrent device. AIC has nothing to do with the amount of fault current that can be imposed at the location of a component (i.e., your machines or the panel that feeds them).

    If the manufacturer wants assurance that the amount of short circuit current available (SCCA – that is the correct term to use here) is no higher than 10,000 amps, then someone needs to perform a fault calculation to determine the SCCA.


    Charles E. Beck, P.E., Seattle
    Comments based on 2017 NEC unless otherwise noted.

  4. #4
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    first, doesn't matter what the CB's are rated as long as > than the available fault current
    so the 14k are fine

    you need to reduce the fault current at the machine
    as noted, you must first calculate what it currently is
    then explore options
    current limiting fuses
    possible a current limiting reactor (CLR) http://www.lcmagnetics.com/inductors...iting-reactor/, introduces a little vdrop but will add Z to drop current
    The difference between genius and stupidity is that genius has its limits.

  5. #5
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    Ok, thanks for the quick replies. I will contact someone at the plant to see if they can calculate the Fault current. Just a note, I talk to Cooper Bussman Technical group. They informed me that Fuses are not rated AIC.

  6. #6
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    Quote Originally Posted by derelectric View Post
    Ok, thanks for the quick replies. I will contact someone at the plant to see if they can calculate the Fault current. Just a note, I talk to Cooper Bussman Technical group. They informed me that Fuses are not rated AIC.
    I suspect you did not hear what they actually said. You cannot reduce the available short circuit current by the use of current limiting fuses. Many times though you can put them in series with another device and get a series rating of the devices as a pair that exceeds what the other component's rating is. But this has to be an NRTL tested pairing.

    Just out of curiosity, how is that you selected the 14kAIC breakers in the first place?
    Bob

  7. #7
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    Quote Originally Posted by derelectric View Post
    OJust a note, I talk to Cooper Bussman Technical group. They informed me that Fuses are not rated AIC.
    I didn't think they were. I was not certain whether the combination of fuse and fuse holder has an AIC rating, as my designs almost never include fuses. Anyone know?

    Charles E. Beck, P.E., Seattle
    Comments based on 2017 NEC unless otherwise noted.

  8. #8
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    Quote Originally Posted by derelectric View Post
    Ok, thanks for the quick replies. I will contact someone at the plant to see if they can calculate the Fault current. Just a note, I talk to Cooper Bussman Technical group. They informed me that Fuses are not rated AIC.
    you need a calculation that has been signed and sealed by an engineer regularly engaged in the practice of calculating fault currents.

    you want to ask if they have an existing short circuit study or set of plans listing the amount of fault current available at the equipment.

  9. #9
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    If you assume that the 14kAIC breakers are the correct size for the available fault current at the panel would it be very hard to calculate the fault current at the fuses? Isn't there an app for that.
    Rob

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    All responses based on the 2014 NEC unless otherwise noted

  10. #10
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    Actually, the entire concept, as you have posted it, is preposterous. So either you heard and recorded it incorrectly, or the machine manufacturer is way off base.

    We can ASSUME that what they MIGHT have MEANT was that the AVAILABLE FAULT CURRENT at their machine must be 10kA or less. That seems far more likely, but is not what you posted. So first off you must get clarity on exactly what they want using appropriate terminology.
    • AIC = Amps Interrupting Capacity; a term for PROTECTIVE DEVICES that will interrupt the flow of current in the event of a fault. Breakers have an AIC rating based on the maximum amount of current they can handle going through them during a fault, because UNTIL the breaker interrupts the flow of current, ALL of the AFC will be attempting to flow through it and the mechanical stresses that represents (magnetic repulsion / attraction) can literally make it explode..
    • IR = Interrupting Rating. Fuses have an Interrupting Rating which is technically the same, so it's a little odd that Cooper was being so literal. But yes, technically it's not an interrupting CAPACITY I guess.
    • AFC = Available Fault Current, also known as Short Circuit Amps (SCA); the term for how much fault current (typically in kA or thousands of amps) that are available at the line side terminals of something. It's derived from the transformer size, the capacity of the primary circuit of that transformer, the impedance of it and any other impedance values between that and the line terminals, typically the resistance of the length of cables (and anything like reactors added).
    • SCCR = Short Circuit Current Rating; the TESTED value of a piece of equipment to withstand the mechanical forces that might take place during a fault of the rated current. This is typically a combination OF the AIC / IR of the protective device, the Let-Through current of that device and mechanical strength of the power devices down stream.
    • Let-Through = the amount of current that will get through a protective device (CB or fuses) in the time it takes to stop current from flowing during a fault.


    If it IS the case that what they meant was 10kA Available Fault Current (AFC), then their asking YOU to do something about it via component selection in the panels you installed is bogus. That's not how that would work. Attempting to limit the AFC at a machine is a tricky business and often not realistic, but is something that would more likely involve the services of a registered PE. Machine OEMs tend to not like to hear that though, they want to build the cheapest thing they can get away with and try to shift the burden off to someone else. The CORRECT way to do this would have been for them to ASK their customer what the AFC already IS, then build their control panel accordingly, along with a UL listing that showed an official SCCR (Short Circuit Current Rating) on the label as is now required by code. After the fact is not the way to approach that issue.
    Last edited by Jraef; 01-09-18 at 04:45 PM.
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