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 Post subject: 240.87 Arc Energy Reduction - Proposed Change to 2017 NEC
PostPosted: Mon Jun 22, 2015 12:20 pm 
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Finally - After the 2014 Edition of the NEC changed 240.87 and did not include an instantaneous trip as a method for arc energy reduction, new language is proposed for the 2017 Edition which would include the instantaneous:

240.87(B) Method to Reduce Clearing Time
An instantaneous trip setting that is less than the available arcing current.

Progress......


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 Post subject: Re: 240.87 Arc Energy Reduction - Proposed Change to 2017 N
PostPosted: Tue Jun 23, 2015 8:48 am 
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Jim Phillips (brainfiller) wrote:
An instantaneous trip setting that is less than the available arcing current.
...



Now if they would just include the requirement to actually determine the arcing fault current. Then do we go for 85% of the arcing fault current as recommended in IEEE1584?

And while they are at it, it would also be a good idea to see if a fuse is in its instantaneous (e.g. current limiting) region below the arcing fault current..


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 Post subject: Re: 240.87 Arc Energy Reduction - Proposed Change to 2017 N
PostPosted: Thu Jul 02, 2015 10:28 pm 
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The actual measured arcing current from the IEEE 1584 test data at low voltages (under 1 kv) has a long tail distribution. The 85% value captures enough of the "tail" to where when using PPE chosen with the predicted incident energy value according to an ATPV calculated using ASTM 1959, the victim is not injured beyond a 1st degree burn according to numerical results 95% of the time. Without the 85% calculation, the prediction accuracy goes down significantly, because even small reductions result in greatly increased opening times with most devices that are not in their instantaneous region.


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