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 Post subject: NFPA70E 2012 & NESC 2012 - Outside current and voltage range
PostPosted: Mon Sep 15, 2014 1:46 am 
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Joined: Sun Jul 22, 2007 5:00 pm
Posts: 27
Location: Stratham, SW Australia
Question 1.
NFPA 70E 2012 Annex D.5 methodology is valid for the range of max bolted 3P short-circuit currents from 16kA to 50kA.
How is incident energy calculated for max bolted 3P short-circuit currents outside the 16kA to 50kA range ?

Is NFPA 70E-2012 documentation available advising the methodology to use when the max bolted 3P short-circuit current is outside the 16kA to 50kA range specified in NFPA 70E 2012 Annex D.5 ?


Question 2.
NESC 2012 methodology is valid within :
- The max bolted 3P short-circuit current range of 20kA to 50kA shown in look-up Table 410-3 of NESC 2012,
- The system phase to phase voltages shown in look-up Table 410-3 of NESC 2012.

What are the NESC-2012 requirements for calculating incident energy in a 132kV system with max bolted 3P short-circuit currents below 20kA ?

Is NESC 2012 documentation available advising the methodology to use when:
- The max bolted 3P short-circuit current is below the 20kA to 50kA range shown in look-up Table 410-3 of NESC 2012,
- The system phase to phase voltage of 132kV is not in the voltage ranges shown in look-up Table 410-3 of NESC 2012 ?


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 Post subject: Re: NFPA70E 2012 & NESC 2012 - Outside current and voltage r
PostPosted: Mon Sep 15, 2014 6:06 am 
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Joined: Wed Jun 04, 2008 9:17 am
Posts: 409
Location: Spartanburg, South Carolina
Question 1
IEEE Std 1584 method (Annex D.7) does not have fault current limits.
NFPA 70E does not advise what methodology to use. It presents various methodologies in Annex D, but Annex D is not a part of the NFPA 70E requirements.

OSHA Appendix E to ┬ž 1910.269 does have a table to help select a calculation method, but OSHA does not endorse any particular method.

Question 2
NESC Table 410-3 fault currents are maximum values.
I hadn't noticed the voltage gaps in Table 410-3. I guess you could use the lower voltages since these give more conservative values.
You could get Arc Pro from HD Electric Co and calculate the incident energy for any value of fault current or voltage. This program was used to develop Table 410-3.


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 Post subject: Re: NFPA70E 2012 & NESC 2012 - Outside current and voltage r
PostPosted: Mon Sep 15, 2014 1:42 pm 
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Joined: Tue Oct 26, 2010 9:08 am
Posts: 1796
Location: North Carolina
mnewman wrote:
Question 1.
NFPA 70E 2012 Annex D.5 methodology is valid for the range of max bolted 3P short-circuit currents from 16kA to 50kA.
How is incident energy calculated for max bolted 3P short-circuit currents outside the 16kA to 50kA range ?


That is an old method due to I believe Doan. It is also limited to a maximum of around 600 VAC. It was an improvement over the Lee method which gives no limits but is known to produce "conservative" (ridiculously high) values as the current and/or voltage increase, but it was a predecessor to the current most popular IEEE 1584 method for estimation.

In no way are any of these restricting you from doing actual laboratory testing. It is just far less expensive to do an engineering estimate rather than do testing if at all possible.

Quote:
Is NFPA 70E-2012 documentation available advising the methodology to use when the max bolted 3P short-circuit current is outside the 16kA to 50kA range specified in NFPA 70E 2012 Annex D.5 ?


No. 70E and NESC do not recommend any particular methodology at all but merely catalog several. Both give a table method which is definitely in the Code, or allow you to use your own engineering study instead.


Quote:
NESC 2012 methodology is valid within :
- The max bolted 3P short-circuit current range of 20kA to 50kA shown in look-up Table 410-3 of NESC 2012,
- The system phase to phase voltages shown in look-up Table 410-3 of NESC 2012.

What are the NESC-2012 requirements for calculating incident energy in a 132kV system with max bolted 3P short-circuit currents below 20kA ?


Wow, you really hit an odd place on the table. First with regards to voltage you'd have to read the 138-145 kV line since that's the next higher voltage. Second the table is actually "0-20 kA", "20.1-30 kA", "30.1-40 kA", and "40.1-50 kA" (currents are listed as maximums). Reading across that line you'd then have 4 cal/cm^2 for 12.1 or faster clearing times in cycles, 8 cal/cm^2 for 24.1 cycles or faster, and 12 cal/cm^2 for 36.2 or faster clearing times.

Quote:
Is NESC 2012 documentation available advising the methodology to use when:
- The max bolted 3P short-circuit current is below the 20kA to 50kA range shown in look-up Table 410-3 of NESC 2012,
- The system phase to phase voltage of 132kV is not in the voltage ranges shown in look-up Table 410-3 of NESC 2012 ?


With both NESC and 70E, you will get lower values if you do the calculations yourself. IEEE 1584 results will be less than those in the task tables of 70E. Since your current (and voltage) fal below the line in NESC 2012, you will get lower values if you use ArcPro, which is the software used to generate the NESC table. Note that IEEE 1584 empirical method (the so-called theoretical method is Lee) is only valid from around 300 V (IEEE 1584 claims 208 V but only has a single data point at that voltage) up to 15 kV. So it would not be valid for 16 kA, 137 kV. At that point you will be limited to Lee (ridiculous results), NESC table results, or ArcPro. Duke Heat Flux is another software program out there but comparison data shows that it produces lower values than predicted by either IEEE 1584 (within the valid range) or ArcPro so the general consensus is that it is suspect. OSHA 1910.269 has a table discussing these differences with respect to voltages above 10 kV.


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