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Thread: Reading AIC Ratings on a breaker

  1. #1
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    Reading AIC Ratings on a breaker

    Hello,

    Newbie question alert!

    I have a generic question regarding reading the ratings on a circuit breaker.

    I have a 700 Amp Breaker that says it is 600V.

    The Interrupting ratings are as follows: 100kA for 240V, 65kA for 480V and 50kA for 600V. All applicable for 50Hz/60Hz

    The system it is connected to is 400V / 50Hz. (It's in A foreign country.)

    If I am doing the calculations, do i need to make sure my devices have less than 90% of 65kA or 50kA ? The device itself will handle 600V, but is at 400V, so wouldn't that mean it has a higher AIC rating?

    Thank you for your time.

  2. #2
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    My guess is it is around 75k for 400V but I would check with the manufacturer. I didn't know that you had to be 90%-- not sure about that.

  3. #3
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    I think the rating is still 65K. I would calculate the fault and see where it falls. I don't understand the 90% factor. As Dennis suggested, check with the mfg but i don't think they will give you a higher rating.
    Please let us know.
    Last edited by bob; 03-19-12 at 05:19 PM.

  4. #4
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    Do not ever try to compare AIC device ratings from the US with device rating from elsewhere in the world.

    The UL testing for AIC has different characteristicss than the IEC parameters.
    For the most part there is also a slight difference in the way the US calculates short circuit currents for device evaluation.
    Just because you can, doesn't mean you should.

  5. #5
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    As the voltage goes down you can "anticipate" that the KAIC will go up because it is a matter of the contacts being able to interrupt power. Voltage goes up, interrupting current capability goes down, voltage goes down, interrupting current goes up.
    But take a look at an internationally rated breaker which most breaker manufactures have and maybe what you should be using. Check the Eaton series G line.

  6. #6
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    To clarify:

    This is an existing system, I am simply trying to model it in software to determine short circuit ratings for an arc flash study.

    A field services person went to the site and took pictures, so for each of these breakers, there's a different AIC rating dependent on voltage. either 240/480/600 V. If I know the utility system (or generator) is 400V, i can at least safely assume the AIC rating at the breaker is the value printed at 480V rather than the 600.

    Running my scenario, almost everything is failing because the breakers are set to their max rating of 600. If I change their rating in the software to 480, (seeing as it's only getting 400V) the AIC should go up from 18kA to 65kA.

    Unless I'm missing something, that is. Please point out any failures in my logic. No ego to bruise!

  7. #7
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    I appear as though you have to do some interpolation as there are not publish values for the voltages in question. It is a common practice to default to the next lower KAIC should there not be a specific value for the voltage in question. If you make a calculated guess as to what it may be at the voltage you are looking for that should give you a ballpark figure as to whether you are in trouble of not. If it looks as if your in the ball park there is no issue to pursue. If you are on the ragged edge plus then the research starts. But the problem i that there still no published values and as such whatever values that you come up with would they be accepted.

  8. #8
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    Quote Originally Posted by jim dungar View Post
    Do not ever try to compare AIC device ratings from the US with device rating from elsewhere in the world.

    The UL testing for AIC has different characteristicss than the IEC parameters.
    For the most part there is also a slight difference in the way the US calculates short circuit currents for device evaluation.
    Bingo. Apples and oranges as far as breaker ratings go. You cannot apply ratings across rating agency specifications. Meaningless exercise.

    Quote Originally Posted by Voltaco View Post
    To clarify:

    This is an existing system, I am simply trying to model it in software to determine short circuit ratings for an arc flash study.

    A field services person went to the site and took pictures, so for each of these breakers, there's a different AIC rating dependent on voltage. either 240/480/600 V. If I know the utility system (or generator) is 400V, i can at least safely assume the AIC rating at the breaker is the value printed at 480V rather than the 600.

    Running my scenario, almost everything is failing because the breakers are set to their max rating of 600. If I change their rating in the software to 480, (seeing as it's only getting 400V) the AIC should go up from 18kA to 65kA. Unless I'm missing something, that is. Please point out any failures in my logic. No ego to bruise!
    I think I understand what you are attempting to do here; you don't know what is there other than the breakers and you are attempting to extrapolate an Available Fault Current based on the theory that the breakers that are being used are an indirect indicator of a minimum amount of fault current. For example if there was only 35kA available, they would not have used 65kA breaker, so you know the AFC must be over the next lowest value but not higher than the lowest stated value of any one breaker you see installed.

    It's a noble attempt to do something without the proper information, but really, you cannot ASSume anything from what you have so far with regards to any reasonably accurate Arc Flash study. Whatever you extrapolate is useless when it comes to a real Arc Flash Hazard Risk Analysis. You must have REAL data on what is AVAILABLE as far as Fault Current, which has almost no meaningful bearing on what was used in the equipment design. For all you know, the equipment you have pictures of may actually have a "Series Rating" with something else up stream, so the values shown on the individual breakers themselves may be rendered meaningless in terms of what you are basing your assumptions on. For example you may actually have 85kA of AFC, but this panelboard may be rated 85kAIC when used behind some other Current Limiting breaker upstream, so that they can use 65kAIC rated components. But that will have no bearing on the AFC as far as your Hazard Risk Analysis. Sorry.
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    Quote Originally Posted by Jraef View Post
    Bingo. Apples and oranges as far as breaker ratings go. You cannot apply ratings across rating agency specifications. Meaningless exercise.



    I think I understand what you are attempting to do here; you don't know what is there other than the breakers and you are attempting to extrapolate an Available Fault Current based on the theory that the breakers that are being used are an indirect indicator of a minimum amount of fault current. For example if there was only 35kA available, they would not have used 65kA breaker, so you know the AFC must be over the next lowest value but not higher than the lowest stated value of any one breaker you see installed.

    It's a noble attempt to do something without the proper information, but really, you cannot ASSume anything from what you have so far with regards to any reasonably accurate Arc Flash study. Whatever you extrapolate is useless when it comes to a real Arc Flash Hazard Risk Analysis. You must have REAL data on what is AVAILABLE as far as Fault Current, which has almost no meaningful bearing on what was used in the equipment design. For all you know, the equipment you have pictures of may actually have a "Series Rating" with something else up stream, so the values shown on the individual breakers themselves may be rendered meaningless in terms of what you are basing your assumptions on. For example you may actually have 85kA of AFC, but this panelboard may be rated 85kAIC when used behind some other Current Limiting breaker upstream, so that they can use 65kAIC rated components. But that will have no bearing on the AFC as far as your Hazard Risk Analysis. Sorry.
    The point that I was trying to make is that unless there is an interrupting rating on the device you can't extrapolate one for another voltage. You may interpolate what the KAIC may be at the applied voltage to see where that ends up. BUT it is not to be assumed as a figure that is to be used. It should only provide a "ballpark" idea as to what the device may be rated for at if in fact it did have a rating. All if may do is to be an indication as to how much trouble that you are in and to illustrate that to the contractor, owner or however. It strictly is to be considered as educated guess and not fact. As a former application engineer for one of the larger MCCB manufacturers I have dealt with this issue many, many times.

  10. #10
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    Quote Originally Posted by Jraef View Post
    Bingo. Apples and oranges as far as breaker ratings go. You cannot apply ratings across rating agency specifications. Meaningless exercise.



    I think I understand what you are attempting to do here; you don't know what is there other than the breakers and you are attempting to extrapolate an Available Fault Current based on the theory that the breakers that are being used are an indirect indicator of a minimum amount of fault current. For example if there was only 35kA available, they would not have used 65kA breaker, so you know the AFC must be over the next lowest value but not higher than the lowest stated value of any one breaker you see installed.

    It's a noble attempt to do something without the proper information, but really, you cannot ASSume anything from what you have so far with regards to any reasonably accurate Arc Flash study. Whatever you extrapolate is useless when it comes to a real Arc Flash Hazard Risk Analysis. You must have REAL data on what is AVAILABLE as far as Fault Current, which has almost no meaningful bearing on what was used in the equipment design. For all you know, the equipment you have pictures of may actually have a "Series Rating" with something else up stream, so the values shown on the individual breakers themselves may be rendered meaningless in terms of what you are basing your assumptions on. For example you may actually have 85kA of AFC, but this panelboard may be rated 85kAIC when used behind some other Current Limiting breaker upstream, so that they can use 65kAIC rated components. But that will have no bearing on the AFC as far as your Hazard Risk Analysis. Sorry.
    Jraef,

    Thanks for taking the time to respond back to me. What you have surmised is actually not what I'm doing. I have an extremely well documented set of field reports of what is available at the site for modeling. I have model/serial number of every panel, switchboard, switch gear, transformer, etc...

    My issue is much simpler than what people are making it out to be.

    When inputting data into SKM Powertools for a branch breaker or a MCB... it gives you three options per type of breaker, 240v 480v or 600v. The way I understand these breakers is that they are designed for operation under 600v. This is great because our system is 400v. Well within operational parameters for the breaker. But because I can only select one of these three options in the software, I assume it is safe to say that the ACTUAL AIC rating of the device is going to be greater than whatever it is as if I have this device hooked up to a 480v system, because the lesser the voltage, the higher the AIC rating.

    When I started, I assuming the lowest AIC (@600v) for the breakers because there was no information of its rating at 400v. I ended up with a good number of failures as the available fault current exceeded the ratings of a number of the panels/breakers. However, because we're at 400v, I am guessing I can safely apply the 480v printed AIC ratings to these devices as thats going to be closer to what the actual AIC rating is, but also does not go over it.

    I was a bit worried that even though the Breaker's printed data had cases for 240v, 480v, and 600v, that maybe you had to use the 600v data because that's the devices maximum rating, and that there might be an identical device that tops out at 480.

    I am also making an educated guess that when this system was designed, the PE actually used NEC to do so, since NEC rules apply at this particular site, and therefore things should be coordinated and have been appropriately sized in terms of ratings.

    However, as we all know, nobody's perfect.

    Oh, and I did go over everything yesterday and applied series ratings based on manufacturer's UL guide, which reduced a number of my failures as well.
    Last edited by Voltaco; 03-21-12 at 11:45 AM. Reason: clarity.

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