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 Post subject: 2012 nfpa 70e 130.5(a)
PostPosted: Thu Sep 01, 2011 9:57 am 
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The section in reference is 2012 NFPA 70E 130.3A

"The arc flash boundary for systems 50 volts and greater shall be the distance at which the incident energy equals 1.2cal/cm2."

Does this expand the necessary scope of calculated values? It looks like this also ceases "generic labels".

Thoughts?


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PostPosted: Thu Sep 01, 2011 10:22 am 
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It just seems that 2012 NFPA 70E 130.5(A) is contradicting 130.3(A) which limits the scope to equipment:

"Exception No. 1: An arc flash hazard analysis shall not be
required where all of the following conditions exist:
(1) The circuit is rated 240 volts or less.
(2) The circuit is supplied by one transformer.
(3) The transformer supplying the circuit is rated less than
125 kVA."

previously if pieces of the power system meet "exception No. 1" we would generate a generic label and use the 4ft boundary.

Now this looks like per NFPA 2012 130.5 the boundary requires a calculation which would expand the scope of the system model, down to all 3 phase circuits.

It also mentions 50V or greater, how can we calculate a boundary if there is no industry accepted method for arc-flash on single-phase systems?


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PostPosted: Fri Sep 02, 2011 7:32 am 
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Exception No. 1 in 130.3(A) is supposed to be replaced by a FPN (see the ROP, or various other reports like this one by Jim: http://www.ecmag.com/?fa=article&articleID=12696).


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PostPosted: Fri Sep 02, 2011 8:41 am 
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JCV wrote:
It just seems that 2012 NFPA 70E 130.5(A) is contradicting 130.3(A) which limits the scope to equipment:

"Exception No. 1: An arc flash hazard analysis shall not be
required where all of the following conditions exist:
(1) The circuit is rated 240 volts or less.
(2) The circuit is supplied by one transformer.
(3) The transformer supplying the circuit is rated less than
125 kVA."

previously if pieces of the power system meet "exception No. 1" we would generate a generic label and use the 4ft boundary.

Now this looks like per NFPA 2012 130.5 the boundary requires a calculation which would expand the scope of the system model, down to all 3 phase circuits.

It also mentions 50V or greater, how can we calculate a boundary if there is no industry accepted method for arc-flash on single-phase systems?


The 240V / 125 kVA exception is being removed from the 2012 Edition of NFPA 70E. It will instead refer to IEEE 1584 for guidance. I just submitted proposed language for the next edition of 1584. My language provides the exception where the short circuit current is 5 kA or less and the voltage is 250 Volts and less. If the exception is taken, then a default incident energy of 4 cal/cm^2 must be used.

***This is ONLY a proposal - do not use this for your studies (yet)*** It is based on some recent test data (confidential at this point - sorry). The IEEE 1584 committee meets in a few weeks so I'll see how this proposal is recieved.

As far as the AFB above 50V, yes this could cause confusion. There was a suggestion a while back to require Ei calculations down to 50V. The committee had a look of "where did that come from" and everyone quickly chimed in "how would you do it? the equations are only valid down to 208V 3 Phase"

Welcome to the world of standards development. There are many well intentioned people out there with ideas but sometimes they don't always fit into the big picture of what we know.

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PostPosted: Tue Sep 06, 2011 2:25 pm 
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Thanks for the quick response! Your input is much appreciated.


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PostPosted: Tue Oct 23, 2012 3:23 pm 
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Jim, hate to ask maybe a really obvious question; but does the most current version of IEEE 1584 recommend arc flash analysis on three-phase systems of 208 volts or greater? So including equipment of 208 volts?


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PostPosted: Tue Oct 23, 2012 4:35 pm 
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Jim, when you write
My language provides the exception where the short circuit current is 5 kA or less and the voltage is 250 Volts and less. If the exception is taken, then a default incident energy of 4 cal/cm^2 must be used.
You are missing the protection device time to clear the AF fault. I have obtained on some projects, panelboards with Isc=1.5kA, 220V and the result is an incident energy of 8cal/cm2 due to the tripping time of a FD MCCB.
I better stick to the calculations rather than exceptions.


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PostPosted: Wed Oct 24, 2012 4:30 am 
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zelwood - We are still using the 2002 edition of IEEE 1584. The range is from 208 to 15 kV. The only exception is based on the 125 kVA <240 V exception. The actual requirement for the arc flash hazard analysis is from NFPA 70E and IEEE 1584 provides the methods of performing the calculations.

FGuillenS - The 125 kVA exception is assuming there is no upstream device i.e. from a transformer secondary. With a smaller transformer and lower voltage, the idea was it would be difficult to have a significant amount of incident energy - the arc would likely self extinguish. A hazard still exists, it was just thought not be a big one. We are presently reviewing this exception as part of the next edition of IEEE 1584 and it will likely be revised downward. Perhaps 30 kVA or 45 kVA We will know more as testing and discussions continue.

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PostPosted: Wed Oct 24, 2012 7:06 am 
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brainfiller wrote:
zelwood - We are still using the 2002 edition of IEEE 1584. The range is from 208 to 15 kV. The only exception is based on the 125 kVA <240 V exception. The actual requirement for the arc flash hazard analysis is from NFPA 70E and IEEE 1584 provides the methods of performing the calculations.


Pardon my beginner-level questions, Jim; if I understand correctly, the only exception to having to have an arc flash analysis done is if the voltage is <240 (but NOT including 240) AND the equipment is served by a transformer smaller than 125 kVA.


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PostPosted: Wed Oct 24, 2012 8:34 am 
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zelwood wrote:
Pardon my beginner-level questions, Jim; if I understand correctly, the only exception to having to have an arc flash analysis done is if the voltage is <240 (but NOT including 240) AND the equipment is served by a transformer smaller than 125 kVA.

Everyone begins somewhere. It's a pretty amazing field.

NFPA 70E has several methods for the analysis including the hazard risk tables. IEEE is just the calculation method. Either way, according to NFPA 70E, a hazard analysis has to be done for systems not placed in an electrically safe work condition that are greater than 50 volts. For an arc flash hazard analysis, this means determining the arc flash boundary and protective clothing/equipment that is worn when working within the boundary when an arc flash hazard is present. This can be done either with the NFPA tables or with IEEE 1584 calculations.

The 125 kVA exception is for systems less than 240V i.e. 208. The 2009 edition of NFPA 70E had similar language but they mis-copied it and had 240 volts or less instead of less than 240 volts. There really is not an exception to the hazard analysis, the exception is for the calculations - small transformers, + low voltage = low incident energy. The problem with the calculation exception is that it does not tell you what to do as far as protection if you do not perform the calculations. It is a good idea to have a minimum PPE/protection level even if you use the calculation exception.

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PostPosted: Wed Oct 24, 2012 9:01 am 
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Thank you, Jim. Educational, as always. I was wondering: say a piece of equipment is determined to be 208 volts and to meet the 125 kVA exception; would this piece of equipment require a label? (I assume it would.) What information would go on the label in such a case?


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PostPosted: Wed Oct 24, 2012 3:55 pm 
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zelwood wrote:
Thank you, Jim. Educational, as always. I was wondering: say a piece of equipment is determined to be 208 volts and to meet the 125 kVA exception; would this piece of equipment require a label? (I assume it would.) What information would go on the label in such a case?

Great question! I asked this question as part of the "Question of the Week" some time back to see how people are handling this. There does not seem to be one answer and many very talented people on the forum have different opinions. The survey question is at the following link: [url='http://www.arcflashforum.com/threads/1650/'][Label Question][/url]

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PostPosted: Sat Oct 27, 2012 11:04 pm 
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Small nitpicking. 70E says that you have to do a risk analysis, not just a hazard analysis. There is a major difference. If you do a hazard analysis only, then you better wear a 40 cal suit when going to the beach because the energy (cal/cm^2) gets well above 40 cal/cm^2 in a half hour or so, and the tables don't provide for using sunscreen.


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PostPosted: Fri Nov 02, 2012 2:45 pm 
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The <240V, single 125kVA transformer exception has left me baffled.

I really need to clarify. The NFPA70E-2012 edition removed the exception line, (correct?) and refers people to the IEEE 1584-2002 standard. The IEEE 1584-2002 standard allows us to neglect anything less than 240V and fed by a single 125kVA transformer or less. So, in a sense, the same rules apply as before, however instead of explicitly calling out the exception in the NFPA70E-2012, they refer us back to the IEEE 1584-2002 exception. Is this right?

Reason I'm asking: The 208V panel and 4 accompanying 208V panels show in my attached PDF have an arc flash category of 3. This seems too high. I believe that because arcing fault at the breaker is so low, that it releases in the thermal region at about 40 seconds. I have attached a PDF with the oneline, TCC, and arc flash table.

Any advice would be greatly appreciated. Thank you.


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PostPosted: Fri Nov 02, 2012 3:37 pm 
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belangerje wrote:
they refer us back to the IEEE 1584-2002 exception. Is this right?


Not quite there yet. 70E itself does not make any reference to any particular method at all in the body. The annex gives no less than 8 different methods. Of those, just one is IEEE 1584. And IEEE 1584 is the only method that contains the exception you are referring to. So if you are using one of the other methods, then the exception would not apply. It is left up to you as the end user to determine which method you feel is the most valid. In many cases, it isn't. For instance we have 23 kV buses where I work at. IEEE 1584 is not valid above 15 kV. The only calculation given in IEEE 1584 for this voltage range is the Lee method and that method is exponentially less accurate as voltage increases.

So strictly speaking it was correct to delete the low voltage exception out of 70E because that is a peculiarity with a particular calculation method. If you use IEEE 1584 as the method of choice then at least with the current edition, the exception exists.

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Reason I'm asking: The 208V panel and 4 accompanying 208V panels show in my attached PDF have an arc flash category of 3. This seems too high. I believe that because arcing fault at the breaker is so low, that it releases in the thermal region at about 40 seconds. I have attached a PDF with the oneline, TCC, and arc flash table.


There is another "exception" in IEEE 1584. They acknowledge that beyond 2 seconds, anyone within the vicinity of an arc flash would either attempt to escape on their own or be physically pushed out of the area by the arc blast. The language within IEEE 1584 in this regard isn't that good but almost all engineers cut off the arcing time at 2 seconds. I don't have the parameters used in your calculation but if you used 40 seconds for an opening time instead of 2 seconds, then the incident energy would be 20 times higher than typically calculated. Applying the 2 second cutoff would reduce the incident energy to <1.2 cal/cm^2.


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PostPosted: Fri Nov 02, 2012 4:08 pm 
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Thanks for your reply Paul. I appreciate the clarifications (I didn't know there were 8 different calculation methods...).

I used 2 seconds in my calculations. I am using the "NFPA70E-2012 Annex D.7 IEEE 1584 - Preferred Method (CSA-Z462 Annex D.7)" calculation method in SKM.

I guess the moral of the story is that without employing the "exception" a person at this panel could be exposed to a high amount of incident energy based upon my calculations. And, without employing the exception I would advise wearing a cat. 3 suit unless the client wanted to spend the money on a more faster-acting breaker or maybe install a fast-acting fuse. At the end of the day I'm most concerned with worker safety and I want to feel confident in the end results of the analysis.


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PostPosted: Sun Nov 04, 2012 4:37 pm 
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belangerje wrote:
I guess the moral of the story is that without employing the "exception" a person at this panel could be exposed to a high amount of incident energy based upon my calculations.


If you don't apply the exception then instead of SPF 15, you'd recommend a 40 cal/cm^2 for spending an hour at the beach!


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PostPosted: Mon Nov 05, 2012 6:25 am 
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PaulEngr wrote:
If you don't apply the exception then instead of SPF 15, you'd recommend a 40 cal/cm^2 for spending an hour at the beach!

I don't think arc-rated clothing is specifically tested for protecting the skin from sun exposure. I wouldn't recommend wearing arc-rated clothing in that case.
SPF 15 has been tested in those conditions, so it can be easily recommended.
:p
Although I think most clothing do offer some kind of sun protection...


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PostPosted: Thu Nov 08, 2012 4:15 pm 
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brainfiller wrote:
The 125 kVA exception is assuming there is no upstream device i.e. from a transformer secondary. With a smaller transformer and lower voltage, the idea was it would be difficult to have a significant amount of incident energy - the arc would likely self extinguish. A hazard still exists, it was just thought not be a big one. We are presently reviewing this exception as part of the next edition of IEEE 1584 and it will likely be revised downward. Perhaps 30 kVA or 45 kVA We will know more as testing and discussions continue.


Jim,

Can you clarify what you mean by "no upstream device" ? I had posted a question about the <240V, 125kVA transformer exception last week. The situation I'm describing is: 112.5kva trans secondary side ---> 240V Main Breaker --> (5) 208V MLO feed-through panels. As shown in my uploaded PDF (attached). I'm meeting with my client soon and want to get their blessing to employ the exception, but I'm unclear about this comment.

On another note, I know you had previously discussed going with a minimum level of PPE for all panels in which you apply the exception. For example, I've heard of people saying that if you have under 5k of fault current on a 208V system, perhaps you could label everything as a category 2. This was common at a utility that I worked for previously, long before I was neck-deap in this arc flash hazard stuff. Any thoughts on that? Thanks!


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PostPosted: Sat Nov 10, 2012 5:26 am 
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belangerje wrote:
Jim,

Can you clarify what you mean by "no upstream device" ? ['/quote]

In other words assuming that say fuses on the primary side of the transformer do not trip. In other words the only limit on the incident energy is essentially the transformer impedance and the available fault current and voltage.

Quote:
On another note, I know you had previously discussed going with a minimum level of PPE for all panels in which you apply the exception. For example, I've heard of people saying that if you have under 5k of fault current on a 208V system, perhaps you could label everything as a category 2. This was common at a utility that I worked for previously, long before I was neck-deap in this arc flash hazard stuff. Any thoughts on that? Thanks!


The utilities that I've seen have never pushed for true H/RC 2 that I've seen. This rule comes from an older recommendation in NESC. What they were requiring was 8 cal/cm^2 FR clothing but they did NOT require any protection for the face. So it's not really H/RC 2. It's more like H/RC 1 before balaclavas and goggles or face shields were added to the requirements. When I was engineer for a foundry where fire retardant clothing was required, I switched the requirement to 8 cal/cm^2 shirts and pants so that every electrician effectively started at H/RC 1 (again, before balaclava and face shield was added) or 4 cal/cm^2 protection.

At that time, we could essentially automatically treat anything less than 4 cal/cm^2 as "no special PPE required" and only required greater PPE when we exceeded 4 cal/cm^2. After adjusting a few fuses and breakers, this rule worked for over 90% of the buses in the plant. Since we also allowed operation of disconnects with the same "standard PPE", during a 3 year period we only had to crack out the higher level PPE (we just went straight to 40 cal/cm^2) twice.

Speaking from experience quite often especially in older plants actually getting under 1.2 cal/cm^2 is hard to do. But if you can raise the limit to 4 cal/cm^2 (almost 4 times higher) this is comparatively easy.

These days are gone. Now based on the tables in 70E, you'd mandate a minimum set of PPE (hard hat, leather boots, safety glasses, ear plugs, leather or rubber gloves, nonmeltable long sleeve clothes) but again this PPE generally resembles "standard work wear" for most industrial plants. The big burning issue right now where I work at is that 90% of the reflective vests (required when working around locomotives or a lot of mining equipment) are meltable and we probably need to find better alternatives.


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