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Do you think an exception based on current OR kVA will be easier to use?
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 Post subject: Part 2 - 125 kVA Exception
PostPosted: Sun Jan 15, 2012 6:51 am 
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Every now and then, the results of the “Question of the week” don’t turn out the way I thought they would. This is one of those times. Here are the details of what is being proposed. Link here

In December, 10 of us on the IEEE 1584 committee met to resolve a few issues on a future edition of the IEEE 1584 standard. One of those items involved the 125 kVA exception. As many forum regulars know, this is one of the areas of the standard that I have been very close to. The ideas have been evolving as to how to handle a new, more refined version of the exception.

The committee was in agreement that an exception is still necessary. However several had concerns about making the cut off too high. Cases were cited where an arc could be sustained at very low currents under certain conditions. I commented that given the correct conditions you can sustain almost anything. That is what an arc light was over 100 years ago.

This lead to a compromise. Right now the proposed language that is floating around has a low short circuit cut off of 2500 Amps or less for 208 Volt 3 phase systems or circuits downstream from a 30 kVA transformer with a minimum percent impedance of 3.5%.

There is a caveat that injury can still occur below this level and PPE may be necessary but no recommendations were given (that is not within the scope of IEEE 1584) This is a good caveat since I have seen people state there is no hazard present when the 125 kVA exception is used.

Keep in mind this is only preliminary draft language and not an official position yet. The language could always continue to evolve.

Now on to a follow up question. Do you think having the exception based on either:

  • 2500 A minimum short circuit current OR
  • 30 kVA 3ph. transformer

is easier to use than the present exception?

Your thoughts are certainly welcome and encouraged!

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PostPosted: Sun Jan 15, 2012 7:56 am 

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I am glad a kVA is still part of this. Current only was going to be a bit difficult since we would not know when we hit 2500 amps until we gathered data. However, having 2500 amps is also good so we can ignore calcs on long 208V circuits once we hit this number. 2500 seems a bit low but we'll live with it (literally)


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PostPosted: Sun Jan 15, 2012 12:55 pm 
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If the proposed exception is for a fault current level or a transformer kVA rating with a minimum impedance, it requires you to know the impedance of all of the 120/208V secondary transformers. This is more difficult than excluding all transformers below 125 kVA regardless of impedance.

The new NFPA 70E references IEEE-1584 and not the 125 kVA transformer limit. Will arc hazard analyses that were done with the old limit have to be revised when the new IEEE-1584 is issued?


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PostPosted: Mon Jan 16, 2012 7:50 am 
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The existing IEEE 1584 text states:

“Equipment below 240 V need not be considered unless it involves at least one 125 kVA or larger low impedance transformer in its immediate power supply.”

NFPA decided to defer to IEEE for this since it involves the calculations and now just references IEEE 1584.

There were many questions and concerns with the existing language.
  • Is 125 kVA too high?
  • Low impedance - How low?
  • What do you do if you use the exception?
The proposed language attempts to address each of the questions. 30 kVA and 3.5% was selected because that results in a secondary short circuit current of around 2500 Amps on the 208 volt secondary. 2500 being the value that is proposed as the low cut off.

When the exception is used, I suggested listing a default incident energy level like 4 cal/cm^2 or something else but it was decided to just say PPE may be needed. No one wanted to use an exact number.

As far as re-running studies - I don’t have a good answer for that one. I see the likelihood of re-running studies as a function of new equations with the next version of 1584 more than because of the revised 125 kVA exception. However, I’m sure there will be locations such as 45, 75 and 112.5 kVA transformers (other sizes in other countries) that will end up with different results.

It’s still evolving.

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PostPosted: Tue Jan 17, 2012 8:07 am 
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brainfiller wrote:
I see the likelihood of re-running studies as a function of new equations with the next version of 1584....


Can you elaborate on these “new equations” and when they will be published?


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PostPosted: Tue Jan 17, 2012 11:12 am 
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I'm not really at liberty to get too specific about the equations yet. I am hearing the AC equations will be completed this year (given various dates) but then the new 1584 still needs completed and judging from getting other documents throught "the process" it could take a while.

As always, when I have something concrete to pass along (and not restricted) I'll post it here.

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PostPosted: Tue Jan 17, 2012 12:55 pm 
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Jim,
Without getting too specific can you discuss what these new equations are based on? Are they testing and developing equations based on “actual equipment” and real world situations?

Do you have any feel as to the results? I find it hard to believe any new equations, based on real equipment, would predict higher values than the current methodology does.


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PostPosted: Thu Jan 19, 2012 12:19 pm 
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One of the big things with the new model is accuracy. The existing equations have a calculation factor (adjustment because the results sometimes were a bit low) and often are referred to as "estimate"

It's hard to test actual equipment since there are so many combinations and variations. I'll have more info to share once it is all complete.

Thanks for everyone's interest.

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PostPosted: Thu Jan 19, 2012 12:25 pm 

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. . . probably called for

I voted 'no', it looks to me that the contemplated change would probably not be easier, but that doesn't mean a change isn't called for.

I personally know a man (not myself) whose hand was severely burned (bad 2nd degree burn over approximately 30% of one hand, that took weeks to heal, and yes, the accident was due to lack of standard safety measures) by an arc on a 240V circuit from a 37.5 kVA transformer. While I don't know how 'sustained' the arc was, this does go to show that these lower power circuits can still pose a significant arc flash safety hazard.


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PostPosted: Sun Jan 22, 2012 12:59 pm 
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Joules Vernier wrote:
I voted 'no', it looks to me that the contemplated change would probably not be easier, but that doesn't mean a change isn't called for.

I personally know a man (not myself) whose hand was severely burned (bad 2nd degree burn over approximately 30% of one hand, that took weeks to heal, and yes, the accident was due to lack of standard safety measures) by an arc on a 240V circuit from a 37.5 kVA transformer. While I don't know how 'sustained' the arc was, this does go to show that these lower power circuits can still pose a significant arc flash safety hazard.


Thanks! That is the kind of story that needs to get out there. An exception is needed but the question is how low should it be. You just confirmed that the present exception does need lowered.

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PostPosted: Wed Jan 25, 2012 5:52 am 
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I think an exception based on kVA would be easier to initially apply.


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PostPosted: Wed Jan 25, 2012 9:35 am 
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Either one would be good, but with a caveat the there still exist a danger of an arc flash as shown by a previous thread, especially if you are working closer than 18". This is the case with your hands, as you are much closer to the arc and therefore would receive a more serious injury there then to the rest of the body at the lower fault levels. A minimum PPE should be Cat 00 insulated gloves with leather protectors.


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PostPosted: Mon Feb 06, 2012 12:01 pm 

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brainfiller wrote:
“Equipment below 240 V need not be considered unless it involves at least one 125 kVA or larger low impedance transformer in its immediate power supply.”
.



Could you please define how low "low impedance" is? Does IEEE 1584 refer to an exact level?


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PostPosted: Mon Feb 06, 2012 4:56 pm 
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Good question. It was never really defined in the original version of 1584. It was more of a concept that the short circuit current should not be very large and "very large" was left somewhat open to interpretation.

The 1584 committee just met last week and just when we thought we had the exception language figured out, it was picked appart by the full committee. The majority believe we need an exception that is where the agreement ends.

We were hoping to have the language worked out for the next edition of 1584. A new group was put together to look at it again. I think this might be the 3rd time around for an attempt to reach an agreement about a low level cut off.

It was quite an interesting 1584 meeting.

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PostPosted: Tue Feb 07, 2012 3:01 pm 

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"It was quite an interesting 1584 meeting."

So is there anything new to report from IEEE 1584? New equations or anything like that?


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PostPosted: Tue Feb 07, 2012 3:57 pm 
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Jeff B. - New Equations?

I’ve trying to think about how to approach this one. Since the IEEE 1584 meeting is open to anyone that wants to attend, and since I have been receiving quite few emails about the meeting so this is becoming known about rather quickly, then there is nothing confidential about what happened at the meeting. So here it goes.

We were told at the IEEE 1584 meeting last week that there will be no equations provided. Instead, there will be an executable file provided - basically a black box as everyone at the meeting began calling it.

Needless to say everyone was surprised and the software people were quite concerned. That’s all I can really pass along right now since that is what was provided in an open meeting.

What happens next? We’ll wait and see.

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PostPosted: Tue Feb 07, 2012 4:57 pm 

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You can't be serious?


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PostPosted: Wed Jun 06, 2012 6:18 am 

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In my opinion it would be better to have the KVA(max) with impedance(min) or even better a table of exempted transformer values. This is because the most common use of the exemption is to determine the scope of a modeled Arc Flash Hazard Analysis study. If a calculation is required before using the exemption then time/resources wouldn't be optimized. The fault current exemption would adjust the equations used, which would still be worthwhile, but not really needed for an exemption. If a single KVA/%Z value is used then I think it should also include the minimum fault current so people can develop their own table. For sure the ambiguity between 240 and less than 240 should be removed along with threshold fault current values for 480, 240, 208, and 120 where current can no longer support an arc.

As for the hazards of equipment fed from a smaller size transformer it seems there are a couple things at work. First off there is the limitation of an arc flash hazard analysis and shock analysis for protecting workers. They are not complete in and of themselves and it seems the committees are trying to address situations that these analyses were never intended to be used for (As mentioned by richxtlc). A Category 0 PPE level in the AF analysis DOES NOT mean that no arc flash hazard exists. It only says that at typical working distances for CHEST and FACE, typically 18", what is the expectation of a second degree burn. This DOES NOT deal with burns to the hands which would be well within that distance. The example mentioned by Joules happened on a single phase transformer and presumably without sufficient hand protection for work on live equipment. I don't think it validates the need for lowering of the transformer KVA exemption but rather shows the need to better train people that an Arc Flash Hazard analysis is part of the solution not the total solution. You still need a safety program with basic requirements, and PPE Cat 0 doesn't mean no PPE is required.


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