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 Post subject: 208V 125KVA Transformer Increased Clearing Time
PostPosted: Thu Jul 31, 2008 9:22 am 

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I am currently using the Easy Power software suite. Just for speculation, I inserted a 208V, 100KVA transformer downstream from a cat. 0 bus. The bus on the LS of the transformer became rated as "Extreme". This was puzzling to me especially after learning of the 125KVA, 208V "standard". I called an engineer at Easy Power and he stated that the clearing time was increased greatly due to the small transformer size, therefore, the available energy was indeed dangerous. He acknowledged the IEEE standard, but also stated that one could be held liable if the bus was only labeled as a cat 0, since the energy available was much more due to clearing time. What are everyone's thoughts? Should the effect of voltage be considered since it is a catalyst for a self-sustaining arc flash?


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PostPosted: Thu Jul 31, 2008 9:57 am 
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jeppen wrote:
I am currently using the Easy Power software suite. Just for speculation, I inserted a 208V, 100KVA transformer downstream from a cat. 0 bus. The bus on the LS of the transformer became rated as "Extreme". This was puzzling to me especially after learning of the 125KVA, 208V "standard". I called an engineer at Easy Power and he stated that the clearing time was increased greatly due to the small transformer size, therefore, the available energy was indeed dangerous. He acknowledged the IEEE standard, but also stated that one could be held liable if the bus was only labeled as a cat 0, since the energy available was much more due to clearing time. What are everyone's thoughts? Should the effect of voltage be considered since it is a catalyst for a self-sustaining arc flash?


Great question, I am sure you will get many different answers. The concept is that an arc less than the IEEE standard will not be self sustaining so clearing time is irrevelant. Think of unplugging a vacuum cleaner or hair dryer while it is running, do you get an arc flash at the outlet?, sure you do, is is self sustaining? Nope, just a quick flash and then it is gone, no fuse blows, no breaker trips. Why is this? It is all about the arc gap, you shorten the arc gap and it will self sustain, thats why AFCI's came about, thats how house fires start.

Now I know some guys that have done lab testing on 208V equipment fed by 112.5 kVA transformers that say it will be self sustaining but you have to reduce the arc gap, you can sustain an arc with a wrench off your car batery if you want.

You need to draw the line somewhere, we will never be able to prevent all arc flash accidents, there is a difference between hazard and risk.

I am looking forward to this discussion and the views of others in this forum. I am sure this will get interesting.


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PostPosted: Fri Aug 01, 2008 5:53 pm 
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I agree, I have done some IE calcs for small 208/120 lighting transformers and with the low current and high clearing times come up with values greater than 40 cal/cm2. Reality dictates the opposite, there is no way you would see this type of energy released in the arc. Some of the clearing times were huge, 80 seconds or more, its just not possible to expect an 80 sec arc - something is going to melt like a fuse and extinguish the arc before then. The 2 sec rule is good for these applications.


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PostPosted: Mon Aug 11, 2008 12:37 pm 
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In the draft / preprint of NFPA 70E ROC an new exception was added to 130.3 about the Arc Flash Hazard Analysis.

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 and below
2) The circuit is supplied by one transformer
3) The transformer supplying the circuit is rated less than 125 kVA.

It looks like this is from IEEE 1584 and in similar fashion, it does not say what to do for PPE if you work on this circuit i.e. does this automatically become category 0?

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PostPosted: Wed Aug 13, 2008 6:43 am 
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I am of the opinion that you just will not see a large enough sustainable arc below a small LV (<480V) transformer to warrant the calculations. You may get enough right at the transformer to burn someone, but it would probably be minor, given ordinary circumstances. After that the impedence of the system becomes much larger relative to the available power.

Of course, as soon as I say this, some fool will go out and prove me wrong, win himself or his widow million$, and we'll all be calculating the energy at the coffee pot in the break room...


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PostPosted: Mon Sep 01, 2008 7:24 am 
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Phillip, 208 or 240V exception?

Phillip,
I reread IEEE 1584. The no need for analysis phrase was directed to circuits of 208V or less and 125 KVA. IT mentioned that sustained arcs were possible at 240V with some frequency but they could only get one arc at 208V. Did I miss someplace where they changed this voltage class to 240V? Where are you seeing 240V was it NFPA and not IEEE.


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PostPosted: Mon Sep 01, 2008 8:32 am 
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brainfiller wrote:
In the draft / preprint of NFPA 70E ROC an new exception was added to 130.3 about the Arc Flash Hazard Analysis.

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 and below
2) The circuit is supplied by one transformer
3) The transformer supplying the circuit is rated less than 125 kVA.



The exceptions listed are from the 2009 NFPA 70E.

On page 6 paragraph 4 last sentence of IEEE 1584 is the following:

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

There seems to be a minor conflict between IEEE and NFPA.

IEEE says below 240 Volts
NFPA says 240 Volts and below

What a difference the word "and" makes.

Also IEEE mentions low impedance transformers. Utilities often have transformers with impedances down around 2 percent which can still produce a reasonable amount of fault current with lower kVA ratings.

The logic is lower fault currents at lower voltages are not easily sustained. The problem is defining exactly where that point is. There is some test data to support the low cut off, but like so many other areas with arc flash, more test and study is needed. It would be great to have some lower cut off so some of this just goes away :) .

I am hopeful in the future with more testing, more case histories, more data etc., that this will all become a lot simpler. Right now this whole subject (and its confusion) continue to evolve.

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PostPosted: Mon Sep 01, 2008 7:50 pm 
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brainfiller wrote:
IEEE says below 240 Volts
NFPA says 240 Volts and below

What a difference the word "and" makes.



Good catch Jim, I hope 70E catches this before the final release.


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PostPosted: Wed Sep 03, 2008 5:06 am 
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I passed this along to one of my 70E contacts. I'm sure it is too late to change it for the 2009 edition but perhaps this can be resolved in the next edition.

Thanks for bringing up the confusion Haze. Just another one of those things that did not quite line up between the standards. Hopefully if we get more people "poking" at the standards and practices we can help resolve some of this in the future.

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PostPosted: Wed Sep 03, 2008 2:12 pm 
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More on the 240V limit

I don't mean to be a pain, but its part of my understanding process.

Jim,
I see your reference in 1584, 4.2 Step 1
"Equipment below 240V need not be considered unless it involves at least one 125KVA or larger low-impedance transformer in its immediate power supply"

However, in 9.3.2 Voltage it says, and I'll paraphrase:
"It was difficult to sustain an arc at the lower voltages. An arc was sustained only once at 208V in a 508 cubic box. In all other tests with that box and the 305x368x191mm box, the arc blew itself out as soon as the fuse wire vaporized. An arc was sustained several times at 215V in a device box 100x100x50mm....Arc faults can be sustained at 208V and have caused severe injuiries with very high short circuit current applications in meter enclosures....While the accuracy of the model at 208V is not in the same class with the accuracy at 250V and higher, it will work and will yield conservative results. The arc-flash hazard need only be considered for large 208V systems: systems fed by transformers smaller than 125KVA should not be a concern."

Now in the definition it states: Voltage (nominal) A nominal value assigned to a circuit or system for the pupose of conveniently designating its voltage class (as 120/240, 480Y/277, 600V). The actual voltage at which a circuit operates can vary from the nominal within a range that permits satisfactory operation of equipment.

We can argue the point, but to me when I see a nominal voltage of 240V, the first thing I think of is that its a single phase voltage, and the next thing is that 240V nominal would also include 230V and 220V. These are all line to line voltages found in houses throughout the US with line to neutral values of 120/115/110V respectively.

So if the IEEE is saying "below 240V" then they MAY mean the next 'nominal' value below which is 208V. Which is why the reference 208V specifically in 9.3.2 and also add the cautionary talk about 215V and above.

Jim, you wrote:
In the draft / preprint of NFPA 70E ROC an new exception was added to 130.3 about the Arc Flash Hazard Analysis.

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 and below
2) The circuit is supplied by one transformer
3) The transformer supplying the circuit is rated less than 125 kVA.

I can't find this in the ROP2009 either in the 130Article or Annex C, can you give me a the exact article number outline location.


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PostPosted: Thu Sep 04, 2008 9:01 am 
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brainfiller wrote:
I had a copy and obtained approval from it's source at PG&E. It was presented at one of our IEEE 1584 meetings about a year ago. It confirms what Catcher stated. There are a few graphs of data points as well. It shows that we are still learning the physics of arc flash and have a lot of work ahead of us. You can download it at:

[url="http://www.brainfiller.com/documents/PGETestingbrainfillerposting.pdf"]PG&E Presentation[/url]

I added an intro page on it just so people know what it is based on and to use it at your own risk.


Haze,
This seems to be a pain for many - you're not alone!
Yep, there's lots of confusion about this one - thanks for pointing it out - It was needed! That' part of what this forum was about - help bring all this stuff to the surface so it can be discussed and hopefully straightened out in the standards.

I posted the info above a while ago from some PG&E tests that were presented at an IEEE 1584 meeting. They could not get a 208V arc to sustain itself much at all. With a small (1/2 inch) gap they could get one with higher fault current to sustain around 10 cycles but lower fault currents and 1 inch gaps tended to be less than 2 cycles. With low fault currents and 2 cycles, the incident energy would be category zero. The 125 kVA transformer limit is defined because the small transformer size would limit the fault current. There is a problem with terms and wording among the standards. As I pointed out, there is a discrepancy between 1584 and 70E with voltage.

Below 240V
240V and Below
208V etc.

You are quite correct about the language of 9.3.2 where they describe the tests. Part of sustaining the 215V arc was box size.

The general consensus from testing seems to be 208 V three phase is the lower limit although that is not how it was presented in 1584 or 70E
standards. Stopping at 208V 125 kVA is up to the person performing the study but there does seem to be some evidence to support this cutoff. Additional research in the future may confirm this even more(or dispute it - who knows). As WDeanN pointed out, someone will end up having a problem with a coffee pot.

Hope it helps!

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PostPosted: Wed Oct 15, 2008 1:02 pm 

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brainfiller wrote:
In the draft / preprint of NFPA 70E ROC an new exception was added to 130.3 about the Arc Flash Hazard Analysis.

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 and below
2) The circuit is supplied by one transformer
3) The transformer supplying the circuit is rated less than 125 kVA.

It looks like this is from IEEE 1584 and in similar fashion, it does not say what to do for PPE if you work on this circuit i.e. does this automatically become category 0?


Also, say you decide to apply this exclusion and don't perform an analysis on a particular building. If you then decide to apply a 2 second clearing time with any available bolted fault current over 833 A, you'll end up with the product of the clearing time and available fault current over 100kA (I think it was 300kA in 2004 edition). And per 130.3(A)(1) if this product exceeds 100kA cycles, the arc flash boundary must not be calculated instead of using the default 4 feet.

Does this imply that anytime one chooses to apply a 2 second maximum clearing time that an analysis must always be done whether or not the equipment in questions meets the exclusion criterion?


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PostPosted: Thu Oct 16, 2008 10:03 am 
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There are a couple of things going on here. The 2 second rule is used only if you are performing calculations. If you have a case with a long clearing time from a time current curve (usually due to lower arcing current and high instantaneous setting) you can cap the time at 2 seconds subject to some judgement about whether a person can jump out of the way or whether they are in a confined space and can not jump out of the way. 2 seconds is a reaction time not arc extinction time.

The 4 foot rule was developed long before the 1584 methods and is still in 70E. They did have Ralph Lee's formulas for the Arc Flash Protection Boundary as well but moved these to the annex for 2009. I think as more people learn about 1584, the "other" methods are slowly falling by the wayside. Instead of deleting the 4 foot rule, they lowered the limits of when you can apply it. It was 300 and now it is 100 kA - cycles. whcih is not very much short circuit current and clearing time so it is begining to force the use of other methods like 1584.

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PostPosted: Fri Oct 17, 2008 8:03 pm 
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Jim,
Regarding the change in the fault energy for the 4 foot default. Was there any recorded injuries from person in the boundary level that warranted a change? Also, the original 300KAcycles at least provided a condition that was realistic in a modern facotry. The new 166KAcycles is not much fault energy and a large percentage of sites will not qualify. Since the purpose of the default ratings is to make it easy for the smaller companies that can't afford the full analysis, wouldn't it have made more sense to represent the real world and keep the 300KAcycles but extend the default distance to 6 feet.

Making changes of this nature to existing code should only be done on a 'must do' basis. What about the poor Joe that tried to comply, spend $20K on engineering and labels, and elected to use the default of 4 feet because his fault energy was only 200KAcycles. He now gets to do it again. Does NFPA realize the severe influence of these changes or our they just cavalier and 'we know best' approach that says we do as we please.


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PostPosted: Mon Oct 20, 2008 5:20 am 

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Where are you two getting the 166kA cycles? The online 2009 edition is showing me 100kA cycles. Even more strict!


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PostPosted: Tue Oct 21, 2008 5:35 am 
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The 4 foot rule was lowered from 300 kA-cycles to 100 kA-cycles (cut to 1/3) so the 5,000 Amp-seconds goes to 1667 Amp-seconds (not Amp-cycles). Like everything else in 70E, people introduce proposals and some make it through the review and comment and some don't. As I mentioned elsewhere in this thread, I really believe this one is an attempt to phase it out.

What I tell people on a pretty regular basis in class is the Arc Flash Protection Boundary affects people that are not wearing PPE and therefore must not be performing the work. Just keep them far back. 4ft. 4.2ft. 3.6ft. no point in splitting hairs, get them out of the way - maybe 8 feet or 10 feet.

Many companies have begun to adopt larger standardized FPBs to make it simple for their workers. I does not affect affect the worker,they have should have PPE on. The exception would be if you need someone close to take notes or some other task. If they need to be close, have them wear correct PPE.

Yes, unlike the NEC which is generally not retro-active, changes in 70E do have an impact and right now appear to be a moving target.

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PostPosted: Wed Oct 22, 2008 7:54 pm 
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Sorry, I was working from memory and just remembered 166, but Jim has set us straight.

What I find ironic is that the IEEE has established the arc flash boundary as the point at which IE drops to 1.2 cal/cm2. We know from the guide that 1.2 cal is the threshold of a 2nd degree burn. So while the note taker is standing just out of the AFB dressed in his 80% polyester pants and shirts, what would be the result to him if exposed to 1.2 cals. I have a guess that his clothing is going to be melted. Rather than 1.2 cal for the naked man to be able to heal from his 2nd degree burns, I think the IEEE should have done studies on the cal threshold to ignite typical Walmart shirts and dockers, and then placed the AFB at a point of 80% of that.


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PostPosted: Thu Oct 23, 2008 6:35 am 

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brainfiller wrote:
The 4 foot rule was lowered from 300 kA-cycles to 100 kA-cycles (cut to 1/3) so the 5,000 Amp-seconds goes to 1667 Amp-seconds (not Amp-cycles). Like everything else in 70E, people introduce proposals and some make it through the review and comment and some don't. As I mentioned elsewhere in this thread, I really believe this one is an attempt to phase it out.

What I tell people on a pretty regular basis in class is the Arc Flash Protection Boundary affects people that are not wearing PPE and therefore must not be performing the work. Just keep them far back. 4ft. 4.2ft. 3.6ft. no point in splitting hairs, get them out of the way - maybe 8 feet or 10 feet.

Many companies have begun to adopt larger standardized FPBs to make it simple for their workers. I does not affect affect the worker,they have should have PPE on. The exception would be if you need someone close to take notes or some other task. If they need to be close, have them wear correct PPE.

Yes, unlike the NEC which is generally not retro-active, changes in 70E do have an impact and right now appear to be a moving target.


Our company has adopted a ten foot AFB standard. Allows for a larger work area for our mechanics, plus keeps the "side walk engineers" at a much safer distance.


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PostPosted: Thu Oct 23, 2008 7:41 am 
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10 feet actually makes it much easier. It's standardized an it only affects "sidewalk engineers".

I have been pushing this for many years to the surprise of many and have upset a few along the way that are bent on how many decimal places they can calculate but it all comes down to "if you're not doing the work and you're not properly dressed - get out of the way!"

More and more companies are adopting this practice. It's simple. You still need to calculated to make sure you don't have a boundary that pops up as greater than the large boundary you want to adopt, (within reason - 1/2 mile boundaries don't count) but once you do that - it's done.

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PostPosted: Thu Nov 20, 2008 1:38 pm 

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Regarding the exception from above:

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 and below
2) The circuit is supplied by one transformer
3) The transformer supplying the circuit is rated less than 125 kVA.


I am performing an arc flash analysis (using ETAP) on a 208 V bus on the secondary side of a 75 kVA transformer. The initial analysis yields a rating exceeding Category 4, the fault clearing time is 7 seconds. After I apply a fixed FCT of 2 seconds, the hazard category drops to 3. This is a more reasonable result, however, Exception no. 1 above claims that an arc flash hazard analysis is not required.

My experience with arc flash is limited, is there a caveat that may explain why an analysis is not necessary on, what turns out to be, a Category 3 bus?


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