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 Post subject: 25 Degree vs. 75 Degree Conductor Impedance
PostPosted: Mon Jul 30, 2007 5:52 pm 
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Short circuit studies are ususally done with conductor impedance data based on 25 Degrees C for lower resistance and higher worst case short circuit current. Since arc flash calculations could actually be worse with lower short circuit currents leading to longer clearing times, does anyone use 75 degree conductor data as a special case to see if it causes problems?


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PostPosted: Tue Jul 31, 2007 5:56 am 
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I haven't heard of this being a problem before, but I think that the 85% of arcing current rule should cover the cases where conditions are not exactly as calculated.
I believe this was originally proposed due to the problems of predicting arc behavior at low voltage, but it is also useful for altering conditions within the system.


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PostPosted: Tue Aug 28, 2007 5:44 am 
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I have heard of a few consultants / equpment manufacturers that were considering a separate 75 degree data base for arc flash studies. So far I don't believe they have begun using it. I guess this situation is similar to performing a voltage drop study i.e. conductor data at the higher 75 degree impedance yields worse results. I concur with WDN that is probably covered by using 85% of the current in addition to 100%.


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PostPosted: Fri Dec 28, 2007 4:11 am 
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Conductor Temperature

I've had a similar problem with the calculations and have tried fully loaded conductors against off load conditions and have had mixed results. When I use the full bolted short circuit conditions, when the conductors are at low temperature, and then apply the 85% rule this does tend to give a more uniform result. Is there general support for this approach? Its makes things more simple but I can't find any reference to using the 85% rule to cover variations in conductor temperature. (I'm a UK Engineer by the way so I'm remote from the mainstream debate) Kind regards, Mike


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PostPosted: Wed Jan 02, 2008 8:31 am 
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85% of Arcing Current

Welcome Mike!
The 85% rule was part of a minor revision to IEEE 1584 in 2004. It was meant to be applied to the arcing current as estimated from the IEEE equations. I understand the concern was there were too many unknowns so 85% became more of a sensitivity analysis to see if the slightly lower arcing current would cause the overcurrent device to not respond instantaneously, increasing the incident energy. I have not heard that 85% was specifically to cover the temperature differences but I am sure that was one of the "unknowns" that they were thinking of.


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PostPosted: Wed Jan 23, 2008 8:44 am 
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Catcher13 wrote:
Short circuit studies are ususally done with conductor impedance data based on 25 Degrees C for lower resistance and higher worst case short circuit current. Since arc flash calculations could actually be worse with lower short circuit currents leading to longer clearing times, does anyone use 75 degree conductor data as a special case to see if it causes problems?


In reality the temperature would be somewhere in between - I would guess 50C - if the conductors are sized per NEC. Most conductors are sized 125% or higher, the equipment is seldom running at nameplate and so on. So if there is a practical table generated it should be based on something else than 75C.


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