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 Post subject: Disconnection times (t)
PostPosted: Fri Nov 16, 2012 2:47 am 
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Joined: Wed Oct 17, 2012 1:59 am
Posts: 7
I would like clarification as to to what value should be taken for circuit protection disconnection times. My understanding is that it ought to be I (bf) but reconsidering the calculations again I am now not sure whether it is considered that it ought to be the total arcing current Total I(a).
Please advise.

Another comment I would make is that the Arc Flash Calculation guide omits to make reference to fuses as circuit protectors. I would like to see in future revisions of this excellent publication references and diagrams for at least the common standard BS88 HRC fuses.

thanks


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PostPosted: Fri Nov 16, 2012 6:35 am 
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Joined: Thu Jul 07, 2011 6:03 am
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Location: Netherlands
The disconnection time for calculating arc flash hazard according to IEEE 1584 has to be determined from the arcing current Ia. Additionally for voltages under 1000V it also has to be calculated at 85% Ia, calculate the hazard for both cases and take the highest incident energy as a result.

The arcing current can be a lot lower than the bolted fault current and could for example result in a fast acting stage of a protection to no longer trip as expected, leading to (very) high results.


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PostPosted: Mon Nov 19, 2012 6:52 pm 
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Joined: Sat Apr 04, 2009 9:50 pm
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Location: San Antonio, TX
I agree with jvrieling


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