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 Post subject: Citation requested
PostPosted: Wed Oct 10, 2012 5:13 am 

Joined: Tue Oct 09, 2012 4:55 am
Posts: 4
Location: Michigan
What is the actual NESC citation requiring arc flash studies to be performed?


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PostPosted: Wed Oct 10, 2012 7:03 am 
Sparks Level

Joined: Tue Apr 03, 2012 1:28 pm
Posts: 53
Location: Louisville KY
Don't have my book with me (out of town) but look at article 410 of the NESC. NESC methodology only applies to open switchyards and open conductors on overhead and underground lines and only consider line to ground arc flash as I understand it. There is a chart in 410 (I believe) that gives the guidelines. If no one else replies, I get home on Saturday and can look it up.

Ken


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PostPosted: Wed Oct 10, 2012 8:21 am 

Joined: Tue Oct 09, 2012 4:55 am
Posts: 4
Location: Michigan
Thanks. It is 410.3. The NESC index is about useless. We are having interal discussions on how best to comply with NESC, NFPA 70E, and IEEE. Since the science and the field of study is not mature, we are trying to use the best judgement on appropriate reference materails and methods.


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PostPosted: Wed Oct 10, 2012 8:25 am 
Sparks Level

Joined: Tue Apr 03, 2012 1:28 pm
Posts: 53
Location: Louisville KY
I agree. The NESC needs an after-market index like Tom Henry does for the NEC (Tom Henry's Key Word Index for the NEC). You would think that someone could come up with a useful index for these code books and actually put them IN the book! :)


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PostPosted: Wed Oct 10, 2012 4:45 pm 
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Joined: Tue Oct 26, 2010 9:08 am
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Location: North Carolina
EHSS wrote:
Thanks. It is 410.3. The NESC index is about useless. We are having interal discussions on how best to comply with NESC, NFPA 70E, and IEEE. Since the science and the field of study is not mature, we are trying to use the best judgement on appropriate reference materails and methods.


NESC works best in "distribution" type equipment. 70E works best for "utilization" type equipment. NESC is an IEEE standard (C2). Other IEEE standards are the backing standard behind both of them in many cases. For instance IEEE 1584 is the best for arc flash calculations within the limit of it's scope (<=15 kV for instance). NESC is using ArcPro by the way for the calculations that derive the table values. ArcPro is specifically meant for outdoor/open type equipment. There is no test data above 15 kV and ArcPro is a theoretical model so it's anyone's guess if it's wrong. 70E uses IEEE 1584 for the values (H/RC) used in it's tables. IEEE 1584 has some issues but is currently the best published standard for arc flash calculations within its limited range of voltages and currents.


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PostPosted: Wed Oct 10, 2012 4:57 pm 

Joined: Tue Oct 09, 2012 4:55 am
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Location: Michigan
Our panels and equipment is mainly 34,500 or 690 volts with a sizable number of amps behind it. The protection breakers and switches are robust and sophisticated, so we can limit duration fairly easily. We have been modeling arc flash with various software models and trying to ensure that we are providing the best information possible to our personnel for PPE and limiting the amout of incident energy through engineering and disbursing the amount of current to multiple switches. It is the best we can do right now without more hard data.


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PostPosted: Thu Oct 11, 2012 7:00 pm 
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Location: North Carolina
EHSS wrote:
Our panels and equipment is mainly 34,500 or 690 volts with a sizable number of amps behind it. The protection breakers and switches are robust and sophisticated, so we can limit duration fairly easily. We have been modeling arc flash with various software models and trying to ensure that we are providing the best information possible to our personnel for PPE and limiting the amout of incident energy through engineering and disbursing the amount of current to multiple switches. It is the best we can do right now without more hard data.


Watch out for SKM, ETAP, EasyPower, etc. Typically these software packages will revert to an arc flash model which is published and is fairly close to correct at 600 V. However it becomes exponentially inaccurate as the voltage increases and is off by a factor of 300% at 15 kV. One of the major fallacies is that it treats voltage and current as interchangeable which arc physics says they are not. ArcPro seems to be more accurate within the existing published data sets (up to 15 kV) but uses an unpublished, proprietary model. The third choice would be to simply refer to the tables in a consensus safety standard (IEEE C2 or NFPA 70E) and don't try to model above 15 kV. The latter is the approach we took because we recognized that we are on legally very weak ground if we utilized either of the available theoretical calculation models.


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