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 Post subject: IAC rated boards
PostPosted: Wed Aug 09, 2017 6:34 pm 
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Hi Guys,

I am new to the forum and the topic of arc flash. My query is if there is a board IAC rated to 25kA,1sec, would the board be treated to be able to withstand the following arc fault condition:

a. Lower current, higher duration - for example: 15kA, 2sec
b. Higher current, lower duration - for example: 30kA, 0.5sec

If the answer to any of the above is a yes, I would be keen to understand as to how the evaluation is done and to know if there is any reference paper discussing the topic. From my limited understanding, I do not think it is a matter of simply comparing the incident energy (i.e. if the calculated incident energy for the expected fault is lower than the calculated energy derived from the board rating, then the board will be okay). I look forward to your feedback/guidance. Thanks in advance guys!.


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 Post subject: Re: IAC rated boards
PostPosted: Thu Aug 10, 2017 7:02 am 
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You are way off for two reasons. IAC has nothing to do with arcing faults. It applies only to bolted faults. In arcing conditions, AIC does not predict anything. The only rating we currently have equivalent is the "arc resistant gear" standards which are not really design standards...they are simply a result of subjecting the equipment to a specific fault current as per the specific IEC or ANSI standard that it is rated under. If it passes the test, cotton material suspended it front of it does not burn through but it stops short of for instance an incident energy rating. So it does not equate directly to say cal/cm2. Depending on practitioners I've seen recommendations suggesting that it is "<1/2 cal/cm2" and that some sort of FR PPE is necessary (although not much) with varying opinions...the arc resistant test itself just doesn't give us much of a specific practical situation for application. It's more of a contrived test.

Getting back to AIC during a bolted fault condition, the primary concern is really magnetic force which is proportional to the square of the current pushing against the bus bar mounting. Time really doesn't matter except that for instance asymmetrical fault current matters hence the length of time. If it is rated for 25 kA the force at 26 kA is going to be 8% higher. There is obviously going to be some element of conservative rating in the 25 kA AIC but 25 kA is what it was tested for so it is simply not rated to go higher no matter what the time limit is. You apply AIC based on short circuit currents calculated using a standard such as IEC 60909 or ANSI. Either the current is under AIC or not.

Going in the other direction in terms of magnetic force then obviously the rating is a maximum. As you extend the time span out towards long term conditions then we don't care about magnetic force anymore...the concern is thermal issues. So at that point you go by the continuous current rating which is really a 2 hour rating. At shorter time intervals thermal energy will be proportional to the current squared again and there are plenty of charts and formulas if you google thermal damage curves.

Obviously at the point where AIC and thermal damage curves meet is in the no-man's land of short term current. There really aren't any standards for this current region. Generally the only time I look at it is for instance motor startups and the extreme tail end of inductive inrush particularly with transformers and making sure that it doesn't trip something out there.


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 Post subject: Re: IAC rated boards
PostPosted: Mon Aug 21, 2017 4:27 pm 
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The Internal arc clssification type tests of IEC 62271-200 are intended to demonstrate that the pressure effects and thermal effects from an internal arc in conjunction with the venting system do not result in an exposure to an operator from arc products being released due to the overpressure acting on doors, vent openings and windows. A simple extrapolation cannot be applied for higher currents and the resultant higher magnetic forces may result in other effects, and in any case the rate of pressure rise may result in higher internal pressures being developed.
For the lower current case, it might seem reasonable that some extrapolation could be applied, however burn-through effects of the arc for example might be difficult to predict. Further type testing would be required to confirm the result.
Cigre TB602 provides a methodology to calculate the pressures developed in the various chambers of the switchgear that may be of interest. Again, this method requires some calibration of thermal transfer coefficients by type testing.


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 Post subject: Re: IAC rated boards
PostPosted: Tue Aug 22, 2017 6:36 am 
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True about the IEC methodology when it comes to arc resistant gear. However the original poster was talking about IAC which is not an arc resistant gear-type of test. Both the IEC and ANSI standards for arc resistant gear are really performance standards...either the equipment when designed, installed, and maintained according to manufacturer recommendations, fully contains or redirects the energy from an arc flash or it doesn't. It says nothing about the performance of the gear at any condition outside the manufacturer's design requirements.

It is also a solution looking for a problem. Under normal operating conditions there is only a very remote chance of an arc flash in the first place. The likelihood is so small that compared to injuries from other industrial accidents, the likelihood is far less than would raise a concern. Thus 70E-2015 doesn't even require PPE during normal operation. Arc resistant gear improves on this by substantially reducing the likelihood of an injury during an already extremely remote event where PPE wasn't even necessary in the first place. Once the doors on arc resistant gear are opened all the arc resistant properties disappear and it becomes no safer than any other gear, energized work is precisely the time when arc flash becomes a concern. Thus arc resistant gear is a solution looking for a problem.


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 Post subject: Re: IAC rated boards
PostPosted: Tue Aug 22, 2017 8:12 pm 
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I stand corrected. IAC is an IEC term. I was thinking of AIC which is an ANSI term referring strictly to bolted faults, not arc resistant gear.


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