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 Post subject: Obligation to notify other parties?
PostPosted: Tue Apr 02, 2013 1:41 pm 
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Hello,

Here is the situation I am contemplating a correct course of action on. I have recently completed an arc flash hazard analysis and have the following situation.

A 300kVA padmount feeds two separate services. One service is a 2000A panelboard (excessive but this facility has been undergoing alot of change and recently added 480V services). The other service goes to a line up of meters with circuit breakers on the load side of the meter. There are 3 200A breakers with either Class 200 or 320 meters as there is no CTs and 1 600A breaker on the line side of a meter with CTs. Since the only protection for this metering setup is the utility primary side fusing, the IE is 38 cal/cm2. This metering setup will be labeled.

My concern is with the utility meter personnel who may be changing or servicing the meters. The NESC lists in Table 410-1 a 4 cal/cm2 PPE system for 50-250V for meter work and has a footnote that states "Industry testing on this equipment by two separate major utilities and a research institute has demonstrated that voltages 50 V to 250 V will not sustain arcs for more than 2 cycles, thereby limiting exposure to less than 4 cal/cm^2." The reference cited is 208-V Arc Flash Testing: Network Protectors and Meters. EPRI, Palo Alto, CA: 2010.

Based on the NESC note, using the 2 cycles, the arc flash hazard drops to 0.6 cal/cm^2.

1. Why is it ok for the NESC to say 208 V can't sustain itself while IEEE 1584 states a limit where no analysis needed is <125 kVA & <240V? If 208V can't sustain itself, why not use a 2 cycle time cutoff for 208V no matter how large a transformer serves it? Note that the NESC is also a IEEE document.
2. Which way is correct? 208V does not know whether it is inside an industrial facility or on the utility side or the PPE is picked from NESC or a study done using 1584 methodology. Note that the secondary equipment, with the exception of the meter, is owned by the customer.
3. How can one have the label there saying PPE of 39 cal/cm^2 is required and a utility person comes to change out the meter using the meter bypass in the meter socket while wearing PPE of 4 cal/cm^2?
4. What obligation do I have to raise this issue with the utility and how does one argue it when two IEEE documents say 2 different things.

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PostPosted: Wed Apr 03, 2013 5:22 am 
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IEEE 1584 is an empirical calculation based on a specific test setup. It does not adequately simulate many utility scenarios. EPRI has been doing testing of specific conditions and publishing the results, which have been collected in NESC. Lee, and other calcs will also give different results. More recent testing which is going into 1584 on the next version will lower the 125 kva rule for some circumstances. So it is easily possible to have deviations. You need to determine for yourself absent testing which one is representative of your conditions.


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PostPosted: Thu Apr 04, 2013 9:58 am 
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Both methods are correct. One uses the theoretical clearing time of the primary fuse (or the 2 second rule) while the other recognizes the practicality that these faults are self clearing. http://www.epri.com/abstracts/Pages/ProductAbstract.aspx?ProductId=000000000001022218

4. None. The utility follows the rules of their industry while you follow the rules of yours.


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PostPosted: Thu Apr 04, 2013 5:06 pm 
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In a similar situation, I have a lot of 25 kv equipment for distribution. Above 15 kv do I blindly follow 1584 in spite of the fact that it defers to the Lee method which gives grossly overstated values, the table in 70E, the table in NESC, or something else? For overhead lines, clearly NESC is the best authoritative source. For indoor switchgear, 70E may be the best authoritative source. 1584 (Lee) is just plain wrong. So since I have both types of equipment, both tables are correct. By the way, Lee recommends around 60 cal for the overhead line and 80 or so for the indoor subs.


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PostPosted: Mon Apr 08, 2013 6:35 am 
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NFPA 70E does not specify what method must be used to calculate incident energy. Annex D presents some calculation methods but is not part of the NFPA 70E requirements. It is up to the engineer doing the analysis to use the appropriate method for the situation.

When I have made incident energy calculations for overhead distribution lines that are owned by an industry, not a utility, I have used the methods used in the NESC. I believe this is more appropriate than IEEE 1584.


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PostPosted: Fri Apr 19, 2013 2:20 pm 
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4. None. The utility follows the rules of their industry while you follow the rules of yours.

So, 208V arc flashes act differently if you work for a utility versus private industry? Here you have a meter and a breaker, right next to each other. Pulling the meter as a utility person you could be in 4 cal/cm^2 PPE while the private industry person operating the breaker is in 40 cal/cm^2 PPE. Both are subject to the same arc flash however one method (NESC/utility) says the arc flash would be limited to 2 cycles while the prevailing method for private (non-utility) allows only a 2 second cutoff. Am I the only one that thinks this is absurd?

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PostPosted: Sun Apr 21, 2013 4:57 am 
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The arc flash does NOT work differently. What is going on is using two different methods to determine the hazard. Neithet 70E nor NESC specify HOW to determine an arc flash hazard...70E does not require IEEE 1584. An engineer at each operation made a determination based on their opinion as to the applicable method. However, the NESC recommendation is based on lots of actual tests performed by EPRI. IEEE 1584 is based on lots of tests in boxes that don't necessarily represent meter sockets, AND currently all tests that do not result in a self-sustaining arc were ignored. Thus in this case IEEE 1584 calculation will result in an error that is about 6000% higher than test data indicates and means that unless someone on the industrial side recognizes this and adopts the specific instance from NESC, you get an absurd situation. 1584 also gives absurd results for open air conditions especially above 15 kv, and gives absurd results for fuses when they current limit. You just have to recognize the limitations in 1584 and when it might be better to rely on something else. As a general rule it produces results that error on the high side with a few exceptions near the edge of the valid region where odd things such as predicting arcing fault currents larger than bolted fault currents occuf.


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PostPosted: Sun Apr 21, 2013 7:23 am 
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So, it seems that the EPRI testing might support using a less than 2 second cutoff time, which may be in line with Chet Davis's paper. There is an old thread on his paper which discusses a 0.5 sec time for 208V systems.

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PostPosted: Sun Apr 21, 2013 5:25 pm 
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There is also more recent test data (not public yet) that suggests that under extreme circumstances, arcs are sustainable. My advice is...do not guess. Use any available consensus safety standards and be consistent. This places you in a defendable position from a liability standpoint. Peer reviewed papers are good when that is the best available. But if you can say that you relied on say NESC for say 15 kv or greater, overhead gear, and service entrance equipment, 70E tables for panelboards and identifying "no arc flash risk" scenarios, and IEEE 1584 for everything else, this is based on a very logical approach that marries the appropriate guidance with the condition. You could also use Lee for everything.


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PostPosted: Mon Apr 29, 2013 7:41 am 
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Two aspects to this quandary: liability and practicality.

For liability purposes I would label the panels according to industry standard calculations. By doing so you have warned any electrician or non-utility personnel of the potential energy.

For practicality purposes, not much reason to worry about the Utility. Utilities were doing explosive meter tests before Arc Flash energy was a well known entity. Their means are to address the hazard might be different than NFPA70E recommendations, and indeed their track record is fairly good.

This does point out the ambitious nature of NFPA 70E. It seems the committees were focused on specific, calculated or specialized aspects of electrical safety and did not integrate the material well into greater context. An (non-arc flash) example of such specialized scope is the general employer requirement to provide adequate illumination for working on power electronic equipment 340.7 (A) (8), battery rooms 320.3(b) that would seem to already be covered in 130.6(C). That the authors are duplicating other sections of the same code, might also explain how they fail to anticipate how this meshes into Utility and other real world work.


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