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 Post subject: Cf constant
PostPosted: Mon Mar 10, 2008 7:46 am 
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Hi,

For voltages above 1 kV, IEEE-1584-2002 proposes Cf = 1.5. I just read in the Electrical Safety and Arc Flash Handbook that engineers are changing this constant in the equation to Cf = 2.2 to better match laboratories results.

Have any one changed this value with confidence?

Thanks!


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PostPosted: Mon Mar 10, 2008 8:26 am 
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Joined: Thu Jul 19, 2007 7:54 am
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Location: St. Louis, MO
John,
I read Cf=1.5 for voltages below 1kV.
I haven't heard of changing this value. Changing it up, especially to 2.2, would increase the energy levels significantly (by an additional 33%!). I would be interested in any justification for this as well.

I try to avoid being overly conservative in my calculations, without some documented justification, because it reduces buy-in from the electricians, as well as management, to effectively implement an arc flash program.


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PostPosted: Mon Mar 10, 2008 11:07 am 
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Hi WDeanN,

you are correct, my mistake. Cf = 1.0 k for above 1 kV and Cf = 1.5 for below 1kV. Yes, increasing the Cf = 2.2 would increase the incident energy. It seems that IEEE-1584 std isn't enough conservative for voltages over 4.16 kV. Lee's method seems too conservative and IEEE-1584 not enough.

I'm currently testing the Heat Flux calculations which seem to give better result for open air equipment at distribution voltage level.

Thanks for you opinion.

Electric John


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PostPosted: Thu Oct 16, 2008 2:15 pm 
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Based on the IEEE test data, I think going beyond 1.5 would be quite an overkill. The Cf is based on establishing a "confidence factor" when comparing calculated PPE categories vs. PPE categories from test results.

A comparison was made of the required PPE category based on tests, and then the PPE category was also calculated based on the (then) proposed IEEE incident energy equations. At lower voltages, the equations often gave PPE values that were lower than the test results gave. Multiplying by 1.5 adjusted the answers so that the equations gave a PPE class that was equal, and in a few cases, greater than the test data 95% of the time. They then called this a 95% confidence factor. If you increase the Cf higher than 1.5 you would likely end up with an even greater PPE level, beyond what a test might indicate.

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