It is currently Wed Jan 26, 2022 9:55 pm

Author Message

 Post subject: Arc Flash Methodology for 25 KVA single phase; 240/120 voltPosted: Mon Mar 09, 2015 2:15 pm

Joined: Mon Aug 04, 2014 5:19 pm
Posts: 15
IEEE 1584 does not apply for single phase and using the equivalent three phase model seems to be overly conservative.
What are study people doing to model ?
thanks JIM

Top

 Post subject: Re: Arc Flash Methodology for 25 KVA single phase; 240/120 vPosted: Tue Mar 10, 2015 6:34 am
 Plasma Level

Joined: Tue Oct 26, 2010 9:08 am
Posts: 2174
Location: North Carolina
Since it is known that the 3 phase case is equal to or greater than the single phase case, there's an obvious answer (hint, just ignore the single/three phase issue and use 3 phase results), for 300 V or greater.

IEEE 1584 really isn't valid under 300 V because it does not model arc self-extinguishment, or "weak" arcs that occur. Under 300 V, the formula is based on a SINGLE data point, not the huge range that is given where it is "valid". I have seen quite a bit of experimental/testing data below 300 V though, and frankly, it is all over the board, showing anything from the idea that IEEE 1584 is vastly overly conservative to being somewhat under-rated. Neither one gives me a warm fuzzy, and other calculations (Lee, etc.) aren't any better in this regard.

So what I currently do for the under 300 V case is to simply rely on existing IEEE consensus safety standards as follows:
1. IEEE C2 (NESC) gives a table that states that anything under 300 V is less than 4 cal/cm^2. The 4 cal/cm^2 is the minimum rating given in any IEEE C2 table. Since the intended audience is generation, transmission, and distribution equipment and OSHA 1910.269 currently specifies that the minimum required PPE for all work whether there is an arc flash hazard or not is to wear arc rated PPE, this all-inclusive approach covers everything. Diving deeper into the source for the NESC data (EPRI) shows that at 20 kA they never measured anything over 4 cal/cm^2.
2. IEEE 1584 has a "rule" that any equipment fed by a single source with a 125 kVA or smaller transformer and with a voltage under 240 V with three phase rating does not need to be analyzed...presumes that it is less than 1.2 cal/cm^2. It is my understanding that the 125 kVA cutoff in future editions will likely be a short circuit (kA) rating so that it is more encompassing and that the lower cutoff will be lowered but without a clear definition of what that will be, I'm sticking with IEEE 1584.

Top

 Post subject: Re: Arc Flash Methodology for 25 KVA single phase; 240/120 vPosted: Tue Mar 10, 2015 10:26 am

Joined: Mon Aug 04, 2014 5:19 pm
Posts: 15
Thanks Paul Good insight.

I will second that since I got a rude awakening when I was doing an Arc Flash presentation for my utility metering group and I was using IEEE 1584.
For example, on the secondary of a three phase 208 volt, 750 KVA, padmount transformer, IEEE 1584 yielded enormous arc flash energy while ANSI/IEEE C-2 (NESC) lists 4 cal/cm^2.
Prior to the listing in the 2012 edition of the NESC, I saw some of the reports from agencies cited in the NESC that were showing this lower level as opposed to what IEEE 1584 yields and I had a hard time accepting this big difference!
Now it is in black and white. But a word of caution. Electric Utility facilities are governed by the NESC whereas Industrial are not per se but I do think the NESC has some bearing. (if I am playing both sides?)
thanks JIM

Top

 Post subject: Re: Arc Flash Methodology for 25 KVA single phase; 240/120 vPosted: Tue Mar 10, 2015 6:48 pm
 Plasma Level

Joined: Tue Oct 26, 2010 9:08 am
Posts: 2174
Location: North Carolina
OSHA has ruled when you are a cogen,.269 applies. In fact I have to deal with railroads, cogen, mine, air strip, port, ships, and various chemical plants all on one site. If anyone mentions jurisdictional issues,I roll my eyes. Functionally, 70E is based almost entirely off IEEE 1584. NESC mostly uses experimental data (not an empirical formula) from EPRI sponsored or conducted tests for <1000 V and ArcPro for the overhead line tables. Therein lies the rub...NESC (EPRI) equipment-specific tests are a better approach, when it is a valid one. Many times depending which jurisdiction applies, the various codes may or may not apply. For instance NEC is the only one for most electrical cases that is regulatory except in rail, cogen, mine, port, ship, and air strip areas. In those areas depending on applicability, NESC or NEC for that matter may apply as a voluntary reference standard, but it is not “law“. So specifically NESC is always just a consensus safety standard but that doesn't make energized work standards in it any less valid than 70E, so long as it is applicable to the task and equipment.

Top

 Post subject: Re: Arc Flash Methodology for 25 KVA single phase; 240/120 vPosted: Mon Mar 16, 2015 5:51 am
 Arc Level

Joined: Wed Jun 04, 2008 9:17 am
Posts: 428
Location: Spartanburg, South Carolina
NFPA 70E does not require any particular analysis method. I believe that if the NESC tables ,or the Arc Pro software that is used for some tables, is more appropriate for a particular situation than IEEE 1584, then these should be used.

Top

 Post subject: Re: Arc Flash Methodology for 25 KVA single phase; 240/120 vPosted: Mon Mar 16, 2015 6:17 pm
 Plasma Level

Joined: Tue Oct 26, 2010 9:08 am
Posts: 2174
Location: North Carolina
70E and NESC do not specifically 'endorse' any calculation method. But the tables in 70E are based on 1584. The tables over 1,000 V in NESC are based on ArcPro. So no endorsememt but the table approach in each uses a ca;culation method. Neither directly in the standard documents the calculation metjod for tables, except the NESC table fo4 <1 kV is heavily footnpted.

Top

 Post subject: Re: Arc Flash Methodology for 25 KVA single phase; 240/120 vPosted: Tue Mar 17, 2015 11:57 am

Joined: Tue Apr 08, 2014 12:03 pm
Posts: 8
Hi Paul,
Thanks for your extensive feedback and insight. I have some general questions for the group and look forward to additional feedback.

You mentioned that the NESC "gives a table that states that anything under 300V is less than 4 cal/cm^2". Are you refering to Table 410-1? I believe that there are some specific areas in that table and voltage range that that call for Class 3 (self contained meters) and and Class 4 (Metal Clad switch gear) but please let me know if you are using a different table. I would be interested to know if I am missing something.

With regards to the "rule" in IEEE about the equipment that is fed by a 125kVA or smaller transformer not needing to be analyzed. I first interpreted this to mean that sources of energy below this level do not need to be considered as sources but have felt that that three phase loads at 208V and above should still be analyzed for the PPE category level, regardless of the source feeding it. What are the group thoughts on this?

I have also heard from others that they automatically want to label areas under 240V with a less than 125kVA feed as class 0 and since my general experience has been with software calculations in lieu of using the tablesin NFPA and NESC I don't feel that I can agree with this unless there is some definitive industry standard that states that this thinking is acceptable by OSHA.

Thanks for the feedback

Craig

Top

 Post subject: Re: Arc Flash Methodology for 25 KVA single phase; 240/120 vPosted: Tue Mar 17, 2015 1:58 pm
 Plasma Level

Joined: Tue Oct 26, 2010 9:08 am
Posts: 2174
Location: North Carolina
CLM wrote:
Hi Paul,
Thanks for your extensive feedback and insight. I have some general questions for the group and look forward to additional feedback.

You mentioned that the NESC "gives a table that states that anything under 300V is less than 4 cal/cm^2". Are you refering to Table 410-1? I believe that there are some specific areas in that table and voltage range that that call for Class 3 (self contained meters) and and Class 4 (Metal Clad switch gear) but please let me know if you are using a different table. I would be interested to know if I am missing something.

Yes, I meant that table and have it open now. Specifically although "300 V" is something of a "magic number" in NFPA 70E for shock protection, it changes to "250 V" in NESC Table 410-1 and I always have to somewhat keep track of this. I have no idea what "Class 3" and "Class 4" mean because those are not the terms in that table. They use "4 cal/cm^2" for every piece of equipment EXCEPT metal clad switchgear and MCC's for 250 V or less. For that one case, the table uses 8 cal/cm^2. My general focus on a personal basis has been with panelboards which are the focus of the second "half" of the table. I'm not entirely sure why the "split" in the table exists. I have never seen 208 V switchgear so I have no idea what that even looks like, and although I'm sure it exists I haven't seen 208 V "MCC's" either but it sort of sounds like metal enclosed gear I guess.

Quote:
With regards to the "rule" in IEEE about the equipment that is fed by a 125kVA or smaller transformer not needing to be analyzed. I first interpreted this to mean that sources of energy below this level do not need to be considered as sources but have felt that that three phase loads at 208V and above should still be analyzed for the PPE category level, regardless of the source feeding it. What are the group thoughts on this?

IEEE 1584 has a single data point at 208 V. Everything else self-extinguished. As I understand it from later tests for instance from Wilkins it will probably be lowered to around say 4.5 kA or less, but until a revised IEEE 1584 comes out this is the cloest thing we have to a lower cutoff other than the previously mentioned NESC. It is definitely not a "source size" but is a combination of both voltage and current. Above somewhere around 250-480 V, arcs are stable enough to not be self-extinguishing or some other form of "weak" arcing so IEEE 1584 methodology applies. The 1584 "rule" was more of a side comment referencing this but it has been accepted as a "rule" regardless pretty widely in the industry. Even taking data from say Wilkins and the basis behind NESC (EPRI data), there is still a lower cutoff. The only argument is how low should it be.

Quote:

I have also heard from others that they automatically want to label areas under 240V with a less than 125kVA feed as class 0 and since my general experience has been with software calculations in lieu of using the tablesin NFPA and NESC I don't feel that I can agree with this unless there is some definitive industry standard that states that this thinking is acceptable by OSHA.

Thanks for the feedback

Craig

Don't take my word for it. Take a look at Mersen's web site and EPRI's reports on the subject. You can see the data for yourself. Simply stated, IEEE 1584 relies on just one data point at 208 VAC and is simply not accurate because it does not estimate the likelihood that the arc will self-extinguish among other things. Actual test data is about the best you can go on right now. Also EFCOG has produced a couple reports on the subject that analyze the available test work and set some lower limits than IEEE 1584. And the "208 V" cutoff is problematic because it captures a bunch of single phase 240/120 stuff that should not be captured. The primary reason of course for not doing the analysis is that relative to power structures, most of us don't have the time and/or money to quadruple the workload in doing arc flash analysis, and I've pretty succinctly seen where IEEE 1584 results are crap. As a case in point I just looked at a transformer yesterday that had a 300 kVA unit with a 22.9 kV:208/120 Y secondary. IEEE 1584 (SKM) analysis determined that it had a 18 kA short circuit, 6.8 kA arcing current, and 53 cal/cm^2 incident energy at 18" away. Test data from EPRI for a 20 kA source (mind you, that's actually 20 kA, not the bolted fault potential) only came up to a little over 3 cal/cm^2. This makes IEEE 1584 off by an order of magnitude. And bear in mind that in this particular case due to the presence of only primary fusing that was somewhat oversized, it would NEVER trip at 6.8 kA so the incident energy was reported for 2 seconds of arcing time.

Hence the reason you can't trust software programs and IEEE 1584 at this voltage level.

Top

 Post subject: Re: Arc Flash Methodology for 25 KVA single phase; 240/120 vPosted: Tue Apr 14, 2015 1:07 pm

Joined: Tue Apr 08, 2014 12:03 pm
Posts: 8
Hi Paul,
Thanks for the response. I have been meaning to get back to your post but haven't had the time. you wrote:
PaulEngr wrote:
I have no idea what "Class 3" and "Class 4" mean
.

What I was referring to was the arc flash PPE category levels, where Class 3 is rated for a minimum of 25 cal/cm^2 and Class 4 is the accepted max level of PPE , rated for a minimum of 40 cal/cm^2. Thus there are Class 3, Class 4 PPE, and above requirements called out in Table 410-1 since the self contained meters / cabinets and metal clad switch gear / mcc's indicate PPE requirements (although conservative values) above these established limits.

Can you point me toi the Mersen and Wilkins references that you mention? I looked for the Mersen reference but I could not find it.

There is an additional question that I also wanted to ask the forum:
The NESC tables have a 4, 8 and 12 cal systems defined. Using the values in Tables 410-3 I have not yet run into a situation that calls for higher levels of PPE than a 12 cal "system", but I am wondering if anyone has? Since the PPE levels defined in IEEE 1584 are for 4, 8, 25 and 40 cal/cm^2 limits, what is the approach when using the NESC tables and the upper limit of 12 is reached? Shouldn't there be a commonality between approaches with regards to the PPE levels? Does anyone have any insight on this difference between standards?

Your point about the magnitude difference between IEEE (SKM) and the EPRI test results is very interesting and I would again be keen on obtaining a link to the reports that indicate these values.

Thanks for your time and feedback. The insight that you have provided to date has been much appreciated.

Craig

Top

 Post subject: Re: Arc Flash Methodology for 25 KVA single phase; 240/120 vPosted: Wed Apr 15, 2015 7:49 pm
 Plasma Level

Joined: Tue Oct 26, 2010 9:08 am
Posts: 2174
Location: North Carolina
CLM wrote:
Hi Paul,
Thanks for the response. I have been meaning to get back to your post but haven't had the time. you wrote:
PaulEngr wrote:
I have no idea what "Class 3" and "Class 4" mean
.

What I was referring to was the arc flash PPE category levels, where Class 3 is rated for a minimum of 25 cal/cm^2 and Class 4 is the accepted max level of PPE , rated for a minimum of 40 cal/cm^2. Thus there are Class 3, Class 4 PPE, and above requirements called out in Table 410-1 since the self contained meters / cabinets and metal clad switch gear / mcc's indicate PPE requirements (although conservative values) above these established limits.

Can you point me toi the Mersen and Wilkins references that you mention? I looked for the Mersen reference but I could not find it.

There is an additional question that I also wanted to ask the forum:
The NESC tables have a 4, 8 and 12 cal systems defined. Using the values in Tables 410-3 I have not yet run into a situation that calls for higher levels of PPE than a 12 cal "system", but I am wondering if anyone has? Since the PPE levels defined in IEEE 1584 are for 4, 8, 25 and 40 cal/cm^2 limits, what is the approach when using the NESC tables and the upper limit of 12 is reached? Shouldn't there be a commonality between approaches with regards to the PPE levels? Does anyone have any insight on this difference between standards?

Your point about the magnitude difference between IEEE (SKM) and the EPRI test results is very interesting and I would again be keen on obtaining a link to the reports that indicate these values.

Thanks for your time and feedback. The insight that you have provided to date has been much appreciated.

Craig

Mersen is a fuse manufacturer. Should be no problem finding it. Bob Wilkins is a retired Mersen researcher. EPRI has a website. Any paper over 10 years old is free. Just enter a search for arc flash on their website..

No compatibility problem between 70E and NESC. Incident energy is physics, not Code. As voltage increases for the same power, ANSI transformer designs, etc., incident energy decreases dramatically. There is no maximum with incident energy. I have seen large 480 V systems over 100 cal/cm2. NESC just chose some “arbitrary“ points based on typical available PPE stopping short of multilayer flash suits. 70E similarly picked “arbitrary“ values. I have heard but cannot confirm that early green colored multilayer flash suits (pickle suits) were limited to 40 cal. Oberon makes stuff over 100 cal. Not long ago single layer PPE was only available in 8-10 cal. Now 12 cal is popular. I have heard thats the limit with a single layer. Regardless my personal goal is to drive towards no PPE or single layer otherwise.

Top

 Display posts from previous: All posts1 day7 days2 weeks1 month3 months6 months1 year Sort by AuthorPost timeSubject AscendingDescending
 Page 1 of 1 [ 10 posts ]

 All times are UTC - 7 hours

 You cannot post new topics in this forumYou cannot reply to topics in this forumYou cannot edit your posts in this forumYou cannot delete your posts in this forumYou cannot post attachments in this forum

 Jump to:  Select a forum ------------------ Forum Library / Articles The Lounge    Question of the Week - What Do You Think?    Arcflashforum.com Feedback and Announcements    Off Topic Discussions    News in Electrical Safety Arc Flash and Electrical Safety    General Discussion    Electrical Safety Practices    Equipment to Reduce Arc Flash Dangers    Personal Protective Equipment (PPE) Arc Flash Studies    General Discussion    Arc Flash Labels    Software for Arc Flash Studies    System Modeling and Calculations    NEW! Electrode Configuration Library – 2018 IEEE 1584 Codes and Standards    CSA Z462 Workplace Electrical Safety    EAWR Electricity at Work Regulations, HSE - Europe    OSHA CFR Title 29    IEEE 1584 - Arc-Flash Hazard Calculations    NFPA 70 - National Electrical Code - NEC (R)    NESC - ANSI C2 - National Electrical Safety Code    NFPA 70E - Electrical Safety in the Workplace    2015 NFPA 70E Share It Here    Arc Flash Photos    Your Stories    What's Wrong Here? by Joe Tedesco
© 2022 Arcflash Forum / Brainfiller, Inc. | P.O. Box 12024 | Scottsdale, AZ 85267 USA | 800-874-8883