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 Post subject: Don't like results of arc flash analysis use tables instead?
PostPosted: Fri Jan 17, 2014 3:26 pm 
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If someone does not like the "DANGEROUS" results of the calculated arc flash hazard analysis because it prevents their staff from working on the equipment while it is energized can they develop their own task tables that mimic NFPA70E as long as the minimum parameters are met? If this is acceptable should the DANGEROUS labels be replaced with a different type label?


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PostPosted: Sat Jan 18, 2014 7:05 am 
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I would offer that the actual study is more accurate than the tables in NFPA 70E and just because they don't like the results of the study, does not mean that the potential hazard does not exist. Replacing the label will jeopardize the employee's safety! I just read an article from EC&M on study versus tables, the link is in the news feed section of this forum. Show that to whoever is trying to change the labels.

Another way to put this action in perspective is think about a Confined Space Entry. If the air monitor shows a dangerous level of either a gas or lack of oxygen but your employer doesn't like the readings because it is holding up work and says the meter must be wrong, so go in anyhow. Would you?

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PostPosted: Mon Jan 20, 2014 6:37 am 
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I think it maybe a little more fundamental than a question of like or dislike. The NFPA 70E offers Task Base Tables in the event that gear is not labeled. To properly follow the tables a full understanding of the risk must be met first. How can that understanding be reached without knowing the bolted fault current? If you know the bolted fault current, why would you not go ahead and do the full assessment? I have yet to find where the full assessment was a higher risk than the tables unless it was an out right higher risk, in that case I want to know what I am facing. Long story short, the tables should be more conservative (higher level PPE) than an actual assessment with labels, unless you really have a problem area, in which case reducing the PPE would be a huge mistake.


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PostPosted: Mon Jan 20, 2014 7:09 am 
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I know it`s not always possible but wouldn`t changing the up stream protective devices in order to lower the risk factor be a viable option especially on things such as conveyor control cabinets that you know are going to have to be worked on while energized.


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PostPosted: Mon Jan 20, 2014 8:56 am 
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If you change a label from Dangerous to something else, and then someone gets hurt because of it, I would assume you would end up being criminally liable.

Once you have done the study, you can't revert back to a table just because you don't like the result.


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PostPosted: Mon Jan 20, 2014 9:40 am 
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Bernard S. Pettiford wrote:
If someone does not like the "DANGEROUS" results of the calculated arc flash hazard analysis because it prevents their staff from working on the equipment while it is energized can they develop their own task tables that mimic NFPA70E as long as the minimum parameters are met? If this is acceptable should the DANGEROUS labels be replaced with a different type label?

I would say it depends on why they "don't like it".
If it is because they think the study is wrong, then perhaps they could,
but they should be sure they can doucument it and be even more sure that they (and the new labels) are correct.

OTOH, the more likely case being that it is just "inconvenient" to have those pesky dangerous labels, then perhaps the labels should be replaced with ones saying "dangerous, AND your Management doesn't care"


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PostPosted: Mon Jan 20, 2014 10:55 am 
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Followup to scott hill's post: I concur that, if possible changing the up stream protective devices in order to lower the risk factor is a good idea. At my facility last year as part of a power system modernization program, s[SIZE=3]ix power distribution panels where replaced that feed 57 downstream load power panels. 27 of these load panels had an arc flash hazard risk category of 3 (8 to 25 cal/cm2). Now all the downstream load panels have an arc flash hazard risk category of 0 (<1.2 cal/cm2). New electrical system installations and area renovations offer the opportunity to select protective devices that will provide the minimum arc flash hazard while still meeting operational needs. And as Scott suggests, if there is equipment that is worked on regularly, looking to see if the upstram deivce can be changed or one added to reduce the risk would be a prudent measure that is not too costly.[/size]


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PostPosted: Wed Jan 22, 2014 10:01 am 
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There are many cases in which it is necessary to understand the results of the software and its limitations and not just blindly follow what it tells you.

For instance, SKM software switches to the Lee equations above 15 kV and puts a little note on the results table pointing to this fact. However, Lee is well known to be wildly incorrect and way higher than real world results above 15 kV. If you are in that situation, you have the following 3 options:
1. Use the SKM result anyways, knowing that at least it won't err to the low side (by a long margin).
2. Use the tables in NFPA 70E (at least until the 2015 edition comes out) or the tables in NESC (probably more appropriate at medium voltages).
3. Use another software package. Note that there is no test data above 15 kV so everyone is truly guessing at that point.

Similarly, many software packages produce what are known to be nonsense results at low (120/240 V) voltages. At this point one has to resort to the test data that is available.

Software frequently falls down completely when dealing with somewhat esoteric protection technologies such as 87 relaying, arc flash relays, zone protection, and various other protective relaying techniques that essentially confound the software which is relying purely on time-current curve data.

Still yet another case is when it is known that the protective device is in extremely poor condition or that there is some other clear reason to recognize that the study may produce obviously incorrect (low) results.

Finally, the arc flash boundary distances are frequently nonsense. When the distance is so large that it exceeds the size of the building, it becomes necessary to apply best judgement to estimate how far the arc flash hazard will actually travel, which can be prohibitively difficult.

So yes, there are lots of times where you need to ignore the results of the power system analysis software. This should be part of the STUDY. The key is knowing when to do this. That is after all why it is an engineering study and not just blind plug-and-chug.

Be careful too with the term "dangerous", and that's a good reason not to use it. The little note about the "40 cal/cm^2 barrier" in 70E is being removed for good reason. This was conjecture. There is no clear relationship between incident energy and arc blast. Other than a theoretical equation known to be not very accurate, there is currently no way to predict arc blast.

Also, just because equipment is considered "dangerous" by some practitioners most likely means that an arc flash is not survivable. It does not mean that depending on the activity being performed that an arc flash is likely. Be careful of the difference here because not all tasks are disallowed. Even 10 years ago the 70E Committee recognized and repeatedly stated that "just walking by" is not going to trigger an arc flash event. It should be part of an arc flash study to understand what activities may cause an arc flash and which ones won't.


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PostPosted: Thu Jan 23, 2014 3:46 pm 
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70E not IEEE 1584 indicate nor imply that this can not be done. Training is usually sufficient with helping a client understand the results and what can and can not be done. it just does not seem to be working with this client.


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PostPosted: Sat Jan 25, 2014 8:32 am 
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The tables in 70E, tables in NESC, Arc Pro software, NFPA 70E 2004 edition, IEEE 1584, Ralph Lee's equations, and many others are all ways of estimating the arc flash hazard. 70E does not pass judgement on which is the best or most correct method, except setting out some limitations on the tables in 70E and indicating when some other method must be used. Similarly, IEEE 1584 has upper/lower voltage and current limits. Part of an arc flash study includes determining the BEST method of analysis. A company is free to either enlist the opinion of an outside consultant or their own judgement for this. Within the limits of the empirical data IEEE 1584 seems to be the most accurate publicly available method.


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PostPosted: Sat Jan 25, 2014 8:42 am 
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That is not to say that it the best method for all cases. For instance it is invalid above 15 kV and does not accurately represent current limiting fuses. It does not address RISK at all. It doesn't work well with open bus scenarios where arc propagation is significant, nor where there are substantial barriers to plasma movement. Also the term "dangerous" comes from a totally unsubstantiated note in 70E that is being removed in 2015 edition. Since both 1584 and 70E are consensus safety standards, and 70E is recognized by OSHA, it would be an affirmative defense to use either. Where this becomes problematic is mixing methods. Anyone doing so needs to have a reasonable and justifiable approach to doing so. For instance applying the tables in "doors closed" conditions and 1584 in "doors open" conditions recognizes that 1584 test data comes from an open box and does not consider likelyhood as opposed to the tables.


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PostPosted: Mon Jan 27, 2014 1:14 pm 
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Also bear in mind that the Lee Equations are used for three phase faults and not for single phase faults. This does make a significant difference when calculating for >15KV, since typically the faults in this range are SLG say w/n a manhole, which doesn’t help matters. We’ve calculated the actual Power (real since V&I curves were very close to being in phase), dissipated w/n a SLG fault using the V & I curves from our SEL relays. Then deriving the Incident Energy released for such fault, the values came up higher than what was calculated using the Lee equations. I don’t agree with the options listed above, but I do agree we don’t know much for truly calculating the Incident Energy for a three phase 15KV or higher fault. One reason is the Lee equation doesn’t factor I the voltage content, which as mentioned above makes a big difference when deriving energy values from power values. There will be a study by NASA-KSC planned for March time frame at KEMA to try and capture the above fault scenario. In the mean time I do agree we don’t have anything in hand to calculate the full content of the hazard, making hazard and risk analysis difficult. I do agree the energy released is not all arc blast, it could be in forms of light, sound, and heat / pressure will be all be assumed… Also, I hear the argument about qualifying the term “dangerous.” In my mind knowing the considerable amount of unknowns (especially in the MV &HV areas of electrical), while trying to protect workers from electrical hazards is dangerous, which is all the more reason for folks to work de-energized wherever possible. Otherwise, set your protective relaying to instantaneous (phase & ground), w/ re-closures set to single shot…
Also, one thing to mention, the HRC values listed in Table Table 130.7(C)(16) correspond to the table Table 130.7(C)(15)(a), and are not to be used for calculated values. That is per discussions with NFPA.


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