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Thread: Fault Current Ratings (AIC versus RMS Symmetrical Amps

  1. #1
    Join Date
    Oct 2004
    Location
    Forth Worth, TX
    Posts
    7

    Fault Current Ratings (AIC versus RMS Symmetrical Amps

    Hello,

    I am trying to understand the relationship of fault current rating between Ampere Interrupting Capacity (AIC) and RMS Symmetrical. For example if a Motor Control Center (MCC) is braced for 42,000 RMS Symmetrical Amps and a product such as a Transient Voltage Surge Suppressor (TVSS) is rated at 200KAIC then how do the two different values relate so I know how the product fault current rating compares with the existing MCC bracing. What if the product had a fault current rating of 13KAIC instead of the 200KAIC. I realize it is imperative to know the fault current rating at that equipment for any installation but I am just trying to relationship of the two different ratings.

    Thank You

    Darrell

  2. #2
    Join Date
    Feb 2003
    Location
    Wisconsin
    Posts
    8,791

    Re: Fault Current Ratings (AIC versus RMS Symmetrical Amps

    There are three components to consider when discussing fault currents.

    The amount of fault available is express in kA RMS symmetrical for 600V systems (i.e. what the utility supplies).

    The amount of current a device can handle, statically, without failing violently. For bus bars this is called bracing, for cables it is called withstand, and for your TVSS it is listed as capacity.

    The amount of current a device (i.e. a circuit breaker or disconnect switch) can interrupt safely is called Amps Interrupting Capacity (AIC) or Amps Interrupting Rating (AIR).

    To have a safe system both the withstand and interrupting ratings of all devices must be less the the amount available at the point they are used in the system.
    Just because you can, doesn't mean you should.

  3. #3
    Join Date
    Feb 2003
    Location
    Seattle, WA
    Posts
    17,198

    Re: Fault Current Ratings (AIC versus RMS Symmetrical Amps

    In other words, if the fault current that is available at the MCC is, for example, 30,000 amps symmetrical RMS, then:
    </font>
    • <font size="2" face="Verdana, Helvetica, sans-serif">The bus bracing of the MCC must be at least as high as the same 30,000 amps symmetrical RMS, and</font>
    <font size="2" face="Verdana, Helvetica, sans-serif"></font>
    • <font size="2" face="Verdana, Helvetica, sans-serif">All of the breakers on the MCC, including the main (if there is one), must be rated for at least 30KAIC, and</font>
    <font size="2" face="Verdana, Helvetica, sans-serif"></font>
    • <font size="2" face="Verdana, Helvetica, sans-serif">The TVSS must be rated for at least 30KAIC</font>
    <font size="2" face="Verdana, Helvetica, sans-serif">
    Originally posted by dwinchel: What if the product had a fault current rating of 13KAIC instead of the 200KAIC.
    If the available fault current was 12,000 amps symmetrical RMS, then you would be fine. If the available fault current was 14,000 amps symmetrical RMS, then the 13KAIC products would not be acceptable.
    Charles E. Beck, P.E., Seattle
    Comments based on 2014 NEC unless otherwise noted.

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