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 Post subject: Tables Vs. Incident Energy Method
PostPosted: Mon Jul 27, 2015 7:15 am 
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It seems a lot of clients are wanting to use the "Table Method" instead of having an incident energy analysis performed. I'm just wondering what you guys think? It seems to me that a short circuit study will still have to be performed at each bus of the system in order to determine if the protective device will clear in time, depending on the symmetrical three phase bolted short circuit. If trying to save dollars by using tables, aka not utilizing software such as SKM or ETAP, and thus performing short circuit studies by hand, it would seem it would take a lot of time, effort, and end up costing quite a bit of money anyway for something that is "assumed" and not engineered. I just wanted to find out what everyone's feeling were on using tables. It seems everyone just wants to assume their breakers will clear (in .03 seconds for example) and slap a label on the equipment, not really knowing for sure what the short circuit current is at the bus and not really knowing if the fault will be cleared in time.


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 Post subject: Re: Tables Vs. Incident Energy Method
PostPosted: Mon Jul 27, 2015 7:31 am 
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The tables should only be used as a short term method until a proper study can be performed.


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 Post subject: Re: Tables Vs. Incident Energy Method
PostPosted: Mon Jul 27, 2015 7:35 am 
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Voltrael, I don't disagree, but does it state that anywhere in the NFPA70E? The best I can tell it doesn't, it appears to give the option to do one or the other and, given the option, people are obviously going with the table method since it appears easier and cheaper. If there's a statement or something within the standard that even hints that an incident energy analysis should be used over the tables I'd like to know if anyone has come across it.


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 Post subject: Re: Tables Vs. Incident Energy Method
PostPosted: Mon Jul 27, 2015 7:48 am 
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Best help I can find is a paragraph from the handbook. So not strictly part of the code, but interpretation of the code by NFPA.

"The employer can choose to use these tables to determine the amount of protection that is necessary. When these tables are used, however, the currents and clearing times included in the equipment category/rating headings must be checked to confirm that use of the tables is permitted. If a task is not in Table 130.7(C)(15)(a), or the working distance is closer than those that used in the tables or the clearing time of the arcing current, then the arc flash PPE categories method should not be used, and the incident energy analysis method should be used instead."

So, tables can only be used after some analysis is done, and then only for the exact tasks that are listed in the tables.

My personal opinion is that they only included the table method to begin with because they knew not everyone was going to have the resources to do a full arc flash analysis, but that the intention was to steer people that way.


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 Post subject: Re: Tables Vs. Incident Energy Method
PostPosted: Mon Jul 27, 2015 3:54 pm 
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It is true that 70E is "method agnostic" and this is intentional. 70E by itself does not support any particular estimate method. It provides a method but doesn't necessarily "support" or in any way give preference to it over any other estimation method. The list has changed over time but there are somewhere around a half dozen methods to estimate incident energy given in Annex D in addition to the table method.

The key to using the tables is that to meet the equipment requirements, there are two pieces of information needed:
1. The fault current.
2. The opening time of the overcurrent protection device.

To estimate opening time of the overcurrent protective device, the fault current must be known fairly accurately and the characteristics of the overcurrent device (time-current-curve in most cases) must also be known fairly accurately. And if you already have this information, the same data can be plugged into the IEEE 1584 "difficult" calculation to arrive at a much more exact result. So paradoxically the input information to use the tables is precisely the same data needed for the "expensive" IEEE 1584 model. If the equipment is outside the required parameters (opening time or fault current) then the tables can't be used but since we already have all the data necessary to use a more sophisticated model, there is virtually no cost to doing so.

A second argument can be made based on research done by Neal et al where they looked at 38 actual arc flash injury incidents. In cases where no study was done, victims were injured 90% of the time. In case where the table method was used, 50% of the time there was an injury. In the case where IEEE 1584 was used, it sucessfully protected against an injury 100% of the time.

So the table method is better than nothing at all, but just that...better than nothing. And that's assuming that it was done properly. Rerunning the "misses" with IEEE 1584 showed that the tabular method underprotected. What I couldn't tell from the study was whether they simply used the tables WITHOUT doing the required analysis or not.


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 Post subject: Re: Tables Vs. Incident Energy Method
PostPosted: Mon Jul 27, 2015 7:18 pm 
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bennrichey wrote:
It seems a lot of clients are wanting to use the "Table Method" instead of having an incident energy analysis performed. I'm just wondering what you guys think? It seems to me that a short circuit study will still have to be performed at each bus of the system in order to determine if the protective device will clear in time, depending on the symmetrical three phase bolted short circuit. If trying to save dollars by using tables, aka not utilizing software such as SKM or ETAP, and thus performing short circuit studies by hand, it would seem it would take a lot of time, effort, and end up costing quite a bit of money anyway for something that is "assumed" and not engineered. I just wanted to find out what everyone's feeling were on using tables. It seems everyone just wants to assume their breakers will clear (in .03 seconds for example) and slap a label on the equipment, not really knowing for sure what the short circuit current is at the bus and not really knowing if the fault will be cleared in time.


The modified NFPA 70E 2015 table method is not fit to be used in the first place. Why? The fault clearing time is required in order for the method to be used. The time cannot be determined unless the arcing fault current is known. Although the NFPA 70E incident method provides formula from the IEEE 1584 Guide for calculating arcing current from the IEEE 1584 Guide, the formula is an integral part of incident energy method and the new standard now specifically prohibits mixing both incident energy and table methods. Hence, NFPA has rendered unusable its own table method in the new NFPA 70E 2015 edition.


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 Post subject: Re: Tables Vs. Incident Energy Method
PostPosted: Tue Jul 28, 2015 2:54 pm 
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arcad wrote:
The modified NFPA 70E 2015 table method is not fit to be used in the first place. Why? The fault clearing time is required in order for the method to be used. The time cannot be determined unless the arcing fault current is known. Although the NFPA 70E incident method provides formula from the IEEE 1584 Guide for calculating arcing current from the IEEE 1584 Guide, the formula is an integral part of incident energy method and the new standard now specifically prohibits mixing both incident energy and table methods. Hence, NFPA has rendered unusable its own table method in the new NFPA 70E 2015 edition.

Great point that I didn't think about. The Table's footnotes require knowing the clearing time and you are correct, to be accurate, the arcing current is needed which no one will know unless they perform the calculations. If someone is going to the trouble to determine any of this, why don't they just perform the IEEE calculations. Humm... I wonder what will happen with the tables in the next edition of NFPA 70E?


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 Post subject: Re: Tables Vs. Incident Energy Method
PostPosted: Wed Jul 29, 2015 3:43 am 
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I would guess people aren't using the tables properly. They are assuming their electrical system meets the requirements of the tables and just looking up tasks and wearing the appropriate PPE.

Which, if true in any case, is probably enough of a reason to get rid of the tables.


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 Post subject: Re: Tables Vs. Incident Energy Method
PostPosted: Wed Jul 29, 2015 3:54 am 
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K. Engholm wrote:
arcad wrote:
The modified NFPA 70E 2015 table method is not fit to be used in the first place. Why? The fault clearing time is required in order for the method to be used. The time cannot be determined unless the arcing fault current is known. Although the NFPA 70E incident method provides formula from the IEEE 1584 Guide for calculating arcing current from the IEEE 1584 Guide, the formula is an integral part of incident energy method and the new standard now specifically prohibits mixing both incident energy and table methods. Hence, NFPA has rendered unusable its own table method in the new NFPA 70E 2015 edition.

Great point that I didn't think about. The Table's footnotes require knowing the clearing time and you are correct, to be accurate, the arcing current is needed which no one will know unless they perform the calculations. If someone is going to the trouble to determine any of this, why don't they just perform the IEEE calculations. Humm... I wonder what will happen with the tables in the next edition of NFPA 70E?


I think the tables refer to bolted fault current not arcing current.

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 Post subject: Re: Tables Vs. Incident Energy Method
PostPosted: Wed Jul 29, 2015 5:32 am 
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wbd wrote:
K. Engholm wrote:
arcad wrote:
The modified NFPA 70E 2015 table method is not fit to be used in the first place. Why? The fault clearing time is required in order for the method to be used. The time cannot be determined unless the arcing fault current is known. Although the NFPA 70E incident method provides formula from the IEEE 1584 Guide for calculating arcing current from the IEEE 1584 Guide, the formula is an integral part of incident energy method and the new standard now specifically prohibits mixing both incident energy and table methods. Hence, NFPA has rendered unusable its own table method in the new NFPA 70E 2015 edition.

Great point that I didn't think about. The Table's footnotes require knowing the clearing time and you are correct, to be accurate, the arcing current is needed which no one will know unless they perform the calculations. If someone is going to the trouble to determine any of this, why don't they just perform the IEEE calculations. Humm... I wonder what will happen with the tables in the next edition of NFPA 70E?


I think the tables refer to bolted fault current not arcing current.



Yes, the tables reference the "Maximum short-circuit current available". Generally the maximum is the bolted 3-phase bolted fault, not the arcing current which is less because the air has impedance.


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 Post subject: Re: Tables Vs. Incident Energy Method
PostPosted: Wed Jul 29, 2015 6:11 am 
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Yes the footnotes on the tables are bolted fault currents. But Arcads point is important. The tables need the clearing time and the correct method is to use the arcing current when reviewing a time current curve. Otherwise using the bolted current may indicate a device trips instantaneously when the arcing current may show a time delay. So to get the clearing time right, you need the arcing current which means you need to perform the calculations and then what's the point of the tables. Interesting discussion!


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 Post subject: Re: Tables Vs. Incident Energy Method
PostPosted: Wed Jul 29, 2015 11:55 am 
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K. Engholm wrote:
Yes the footnotes on the tables are bolted fault currents. But Arcads point is important. The tables need the clearing time and the correct method is to use the arcing current when reviewing a time current curve. Otherwise using the bolted current may indicate a device trips instantaneously when the arcing current may show a time delay. So to get the clearing time right, you need the arcing current which means you need to perform the calculations and then what's the point of the tables. Interesting discussion!


I didn't even think of that fact that the utilizing the "max short circuit current" (not the arcing fault current) could result in faster tripping times and lowering the arc flash hazard! I think it's a very important point. Thanks for all the input guys!


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 Post subject: Re: Tables Vs. Incident Energy Method
PostPosted: Wed Jul 29, 2015 9:32 pm 
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I still beg to differ on 'get rid of the tables'. You can't use other methods without field surveys. This is just a single case where we have a 'chicken and egg' problem. The second is for contractors working in places which have not done a study. As I said earlier...the tables are better than nothing, but definitely not better than 'something'. The closest thing is the tables in NESC which are equipment specific.


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 Post subject: Re: Tables Vs. Incident Energy Method
PostPosted: Thu Jul 30, 2015 5:35 am 
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PaulEngr wrote:
I still beg to differ on 'get rid of the tables'. You can't use other methods without field surveys. This is just a single case where we have a 'chicken and egg' problem. The second is for contractors working in places which have not done a study. As I said earlier...the tables are better than nothing, but definitely not better than 'something'. The closest thing is the tables in NESC which are equipment specific.


I agree, somewhat. The issue is that 70E doesn't make an argument one way or the other, tables vs study. Therefore, from my experience, it appears people assume that they can simply go to a table, look to see if PPE is required, look at another table, assume tripping times or short circuit currents and voila, I don't have to pay to have an arc flash study performed. People are either not understanding it or taking advantage of it. Perhaps the code should state something to the fact of "if a study hasn't been performed yet." Agree or not?


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 Post subject: Re: Tables Vs. Incident Energy Method
PostPosted: Fri Jul 31, 2015 9:36 pm 
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I may sound like a committee statement, but I agree in principle but not with the wording. Doing a "study" can mean many things. And in some cases (E.g. under 250 V,over 15 kv), a table method could be preferred. And the argument is against the equipment table, not necessarily the risk assessment table or the clothing table which is at this point substantially similar to annex H. So we're really talking about hazard analysis only in a limited range of voltages and currents.. The equipment table is a hazard analysis method, but there are better methods.

The argument against tables altogether attacks the "better than nohing" issue though. Another example is emergency release. 70E requires training on this topic but contains nothing on how to do it. The only "approved" procedure is de-energizing. And if shock protection rules including proper equipment design, installation, and maintenance are followed, emergency release procedures should never be needed. Everything else at that point is makeshift anyway. So by the same logic we should not require emergency release training. OSHA and NESC rules do not require it...it is unique to 70E. Yet sometimes bad things happen and we are left with making the best out of a bad situation. So emergency release remains.

At best, it may be an improvement to suggest incident energy analysis is the preferred method. This points in one direction without prejudicing the tables when they are the best available method. A properly done risk/hazard analysis should of necessity consider all available methods and make a determination as to which is the best method of methods. As it stands 70E is moot on this as is NESC. Only OSHA actually gives specific guidance and only for generation, transmission, and distribution equipment.


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 Post subject: Re: Tables Vs. Incident Energy Method
PostPosted: Mon Aug 03, 2015 8:12 am 
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IT seems there is a little statement in NFPA 70E that studies "shall be completed by qualified Professional Engineers." I assume the tables are completed by such a group, but if the average Joe doesn't read all the foot notes, the tables will not be applied correctly. I am therefore in favor of either improving the tables and associated instructions to a point where they are all that is necessary or do away with them and require Incident Energy Studies.


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 Post subject: Re: Tables Vs. Incident Energy Method
PostPosted: Mon Aug 03, 2015 8:39 am 
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Hi All

Greetings from North of the border.
As a President of a local chapter of electrical contractors, on any given day the electrician finds themselves interacting with electrical equipment in all sectors- residential, commercial and industrial. In many cases there will not be a detailed warning label to provide guidance on how to prepare oneself with PPE. We have to agree that the tables are better than having no guidance whatsoever. It is as good as it gets based on the information provided. And the same goes for the study. If I am in a distribution panel at a shopping mall or community centre. Forget the study, not going to happen, these owners are not going to spend the money when it it not mandatory- at least here in Canada. So I resort to the table method, at least I have some guidance - is it perfect No, however neither is the study


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 Post subject: Re: Tables Vs. Incident Energy Method
PostPosted: Mon Aug 03, 2015 9:06 am 
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I absolutely agree with Leonard. I am an engineer for a electrical construction company that has a thriving service division. While arc flash analysis is growing rapidly in our service area, which is a major metropolitan area in the US, (we've completed arc flash analysis on about 40 buildings since the beginning of the year, about 200 since early 2011, and there are at least 3 competitors also in the marketplace) there are still a good number of buildings that do not have analysis complete. We should definitely NOT remove the tables. Our service division is called out to buildings constantly for emergencies, and there is no way an analysis can be complete in a timely fashion to get a building up and running quickly (utility information can easily take 2-4 weeks alone). Whenever a building owner or management customer comes to us, we always recommend first that they at least get training on the tables if they must do energized work, and highly recommend a full 1584 analysis. We always explain the downsides of the tables, but never tell them not to use the tables. I feel very strongly that a 50% decrease in possibility of injury is much better than no decrease. Recommend to use the tables if nothing else, but always explain the draw backs.


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 Post subject: Re: Tables Vs. Incident Energy Method
PostPosted: Mon Aug 03, 2015 10:07 am 
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It seems I have upset some people with my reference to the requirement for a Registered Professional Engineer. If so, I apologize. However, I have read somewhere - I'll have to find it again - and have been informed at a seminar by SKM that a PE is required to certify the analysis. With the number of industries and states that do not require single industry engineers to be registered it does not make sense. If I find additional information I will post the location.


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 Post subject: Re: Tables Vs. Incident Energy Method
PostPosted: Mon Aug 03, 2015 10:28 am 
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airjockey42 wrote:
It seems I have upset some people with my reference to the requirement for a Registered Professional Engineer. If so, I apologize. However, I have read somewhere - I'll have to find it again - and have been informed at a seminar by SKM that a PE is required to certify the analysis. With the number of industries and states that do not require single industry engineers to be registered it does not make sense. If I find additional information I will post the location.


It can be a hot button since it implies who can and can not perform studies. I included such a requirement in the original draft of IEEE Std. 1584.1 - 2013 - IEEE Guide for the Specification of Scope and Deliverable Requirements for an Arc-Flash Hazard Calculation Study in Accordance with IEEE Std 1584™ but it was heavily rejected by the working group. Most states do have licensing laws for "engineering services" so we could not ignore it. The language that eventually made it into IEEE 1584.1 was a note in Section 3.4 Results and Recommendations that states:

NOTE—Engineering licensing requirements of individual jurisdictions (states and/or provinces/nationalities) may
require the analysis to be performed by, or under the direction of, a registered professional engineer.


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