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 Post subject: Use of Tables after Study Completed
PostPosted: Mon Nov 01, 2010 12:31 pm 
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We have completed our Arc Flash Hazard Analysis and have labeled everything in the plant appropriately . Before we completed this, we were attempting to use the information in Table 130.7(C)(9) - realizing that the arc flash study was the "right thing to do" and provided a much more accurate analysis of the hazards involved.

The concept I'm trying to understand now pertains to task-specific PPE. With the tables, there are specific tasks listed w/ varying levels of required protection. For example - on 600V class switchgear to operate a fused switch with the doors closed is HRC0. Working on energized parts on that same switchgear is HRC 2*.

Now we have the labels everywhere, and most of our 600V class switchgear is HRC2. It seems unreasonable to me that we would now require the same HRC2 turnout gear to operate a closed door switch as we would to perform energized electrical work, but perhaps that is the case. Does the act of completing a study enact the burden of the full listed non-task-specific PPE for any and all tasks, even those that were previously listed as HRC0 on the tables? Personally, I think that this could use some clarification. I have spoken with multiple people on this and most of they have a different understanding. Most of them have reached their understanding in a logical fashion, but they are all a bit different.

Could someone who has perhaps spoken with the NFPA directly on this help shed some light? I have heard a bunch of different "opinions", but they were all from folks who have read through the regulations and sat through a class or three - like myself.


- Jason


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PostPosted: Tue Nov 02, 2010 4:30 am 
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The three words that will sum up the answer are "hazard" "risk" and "opinion" .

The 70E tables were developed some time ago as an attempt to provide guidance for PPE selection before calculation studies were performed. They looked at the hazard and associated risk of the hazard. i.e. more risk by racking a breaker than simply operating a switch although the hazard is the same.

The tables were created based on the "opinion" of a group within NFPA 70E. Not on research. I will never forget an IEEE 1584 meeting several years ago where an NFPA person was praising the merits of the tables over the calculations and it came out that it was just opinion. Someone asked if they felt opinion was more scientific than research?

The IEEE 1584 calculations determine only the hazard and does not consider the risk / probability of the event. i.e. how much incident energy could result during an arc flash - not what is the liklihood of an arc flash occuring.

We could equate this to a short circuit study. You calculated the fault current and compare it to the interrupting rating of the protective device. There is no consideration of the probablity that the fault will be the maximum bolted condtion - it will likely be less but you just consider the worst since that could happen.

The "normal" (?) interpretation right now for arc flash studies is when calculations are performed, it becomes a black and white issue i.e. Wear PPE that is appropriate for the incident energy - period.

Hope that helps. Others may have a few additional comments - lots of great opinions on the forum.

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PostPosted: Tue Nov 02, 2010 5:43 am 
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Thanks for the information Jim. I have heard three different interpretations of how to apply this:

1) Use the PPE on the label regardless of task - even if you are doing lower risk tasks with the doors fastened closed.

2) Use the PPE on the table for explicitly listed lower risk tasks, like closed door swtiching. Use the PPE on the label for the higher risk tasks.

3) No PPE required for closed door switching - use PPE on the label when interacting with exposed energized conductors only.

Understand that option 1 will change the way a lot of companies do LO/TO activities, where unqualified personnel are using disconnects or MCC cubicles to lock out a pump, fan, compressor, etc . . . These individuals would have to receive task-specific training and be provided with PPE or an electrician would have to be involved for these tasks. There's also the thought process of simply walking through an MCC room with contactors and relays activating within the gear automatically. Do we make these rooms completely off-limits without the PPE? This can take an extreme path if we are not careful.


- Jason


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PostPosted: Tue Nov 02, 2010 6:06 am 
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Jason,

The tables use assumed fault currents and device interrupting times. The tables are better than nothing and cover typical areas but not all. To get an idea of the assumptions that were made when the tables were developed, see the footnotes at the end of Table 130.7(C) (9). Note that in most cases the device interrupting times are 2 cycles (0.03 sec), 20 cycles (0.33 secs), and 30 cycles (0.5 secs) with varying limits on the available short circuit study.

A true Arc Flash Study uses the arcing fault current and the upstream device operating time to determine the AF energy level. This is the most accurate method to calculate the AF energy. In short, the tables are an approximation and are the best method when formal detailed AF equations and calculations cannot be used. Use the labels attached to the equipment.

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PostPosted: Tue Nov 02, 2010 9:40 am 
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grapejape wrote:
Thanks for the information Jim. I have heard three different interpretations of how to apply this:

1) Use the PPE on the label regardless of task - even if you are doing lower risk tasks with the doors fastened closed.

2) Use the PPE on the table for explicitly listed lower risk tasks, like closed door swtiching. Use the PPE on the label for the higher risk tasks.

3) No PPE required for closed door switching - use PPE on the label when interacting with exposed energized conductors only.

Understand that option 1 will change the way a lot of companies do LO/TO activities, where unqualified personnel are using disconnects or MCC cubicles to lock out a pump, fan, compressor, etc . . . These individuals would have to receive task-specific training and be provided with PPE or an electrician would have to be involved for these tasks. There's also the thought process of simply walking through an MCC room with contactors and relays activating within the gear automatically. Do we make these rooms completely off-limits without the PPE? This can take an extreme path if we are not careful.


- Jason


Item 1) Unless you are using tables only, yes this is the correct method.

Item 2) This is a common mis-interpretation. The standard interpretation is you CAN NOT mix tables and calculations. I think you will find this unanimous. It's either or.

Item 3) This was the case prior to the 2009 Edition of 70E. Now it defines the arc flash hazard as existing when there is "interaction" which is switching even with the door closed unless it is an arc resistant design. I launched a bus bar once in the lab at an alarming speed. I know of others that have blown open / off doors. They might hold the arc flash for small incident energy levels but there is no way of knowing.


Robertefuhr wrote:
Jason,

The tables use assumed fault currents and device interrupting times. The tables are better than nothing and cover typical areas but not all. To get an idea of the assumptions that were made when the tables were developed, see the footnotes at the end of Table 130.7(C) (9). Note that in most cases the device interrupting times are 2 cycles (0.03 sec), 20 cycles (0.33 secs), and 30 cycles (0.5 secs) with varying limits on the available short circuit study.

A true Arc Flash Study uses the arcing fault current and the upstream device operating time to determine the AF energy level. This is the most accurate method to calculate the AF energy. In short, the tables are an approximation and are the best method when formal detailed AF equations and calculations cannot be used. Use the labels attached to the equipment.


Great point. In the end even if you use the tables, calculations are STILL required to verify the short circuit current and clearing time. ...and it is impossible to know the correct clearing time if you do not know the ARCING short circuit current as Robert stated.

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PostPosted: Tue Nov 02, 2010 9:59 am 
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Thanks for the good discussion everyone. I think that there are a lot of common misconceptions about this standard. I have spoken with multiple people, all who were "sure" that they were doing it correctly. I honestly don't think that anyone was trying to skirt the rules either.

I'm headed towards allowing non-qualified personnel to operate class 0 disconnects only (our local disconnects) and required qualified personnel with PPE to operate anything higher (our MCCs and fused 480v dist panels). This is going to cause some weeping and gnashing of teeth, so I want to make sure I have my facts together before I propose this.


- Jason


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PostPosted: Sun Nov 07, 2010 5:26 am 
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grapejape wrote:
Thanks for the good discussion everyone. I think that there are a lot of common misconceptions about this standard. I have spoken with multiple people, all who were "sure" that they were doing it correctly. I honestly don't think that anyone was trying to skirt the rules either.

I'm headed towards allowing non-qualified personnel to operate class 0 disconnects only (our local disconnects) and required qualified personnel with PPE to operate anything higher (our MCCs and fused 480v dist panels). This is going to cause some weeping and gnashing of teeth, so I want to make sure I have my facts together before I propose this.


- Jason


A note of caution here as class 0 still requires appropriate PPE and training


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PostPosted: Fri Apr 08, 2011 5:06 pm 
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My work place (a large paper mill company) has spent a few years in developing a hazard/risk task matrix that we will apply to the calculated arc flash ie as printed on the label. We have decided to use a caloric reduction instead of a category reduction as table 130.7.c.9. We documented the risk assessment in our safety eswp document. Tasks such as working on a PLC that is installed in an MCC bucket does not warrant the same PPE as replacing a bucket. A hazard risk assessment only makes good sense even when the labels have been afixed to equipment.


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PostPosted: Thu Apr 05, 2012 7:48 am 
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TheArk wrote:
My work place (a large paper mill company) has spent a few years in developing a hazard/risk task matrix that we will apply to the calculated arc flash ie as printed on the label. We have decided to use a caloric reduction instead of a category reduction as table 130.7.c.9. We documented the risk assessment in our safety eswp document. Tasks such as working on a PLC that is installed in an MCC bucket does not warrant the same PPE as replacing a bucket. A hazard risk assessment only makes good sense even when the labels have been afixed to equipment.


TheArk,

In spite of what the committee did with the tables in lowering PPE requirements based on the probability of an arc flash occurring, it seems that the consensus opinion of this forum is that once the incident energy is calculated, the only way to lower the PPE requirement regardless of the task is to lower the incident energy or put in other controls to reduce exposure to the incident energy. I would be interested in seeing how you performed and documented your risk assessment for electrical tasks if you are willing to share. My company is trying to do the same thing and I hope to not reinvent the wheel, but I don't see how evaluting probability of an event allows one to make a PPE reduction from that calculated. We are ready to begin applying detailed labels on our equipment and if we don't have a good understanding of what the label means and how to apply it to electrical tasksl, I'm afraid we will cause more harm than good.


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PostPosted: Thu Apr 05, 2012 1:20 pm 
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I don't agree that a hazard/risk assessment cannot logically reduce the required arc rating of the PPE for unlikely hazards. When you calculate the IE, you are calculating the IE for the worst scenario, with the arc at a set distance from the worker. It there is an arc, there is some probability that the worker will be exposed do a lower IE than the calculated IE. If the probability of an arc is low, you could logically allow a lower arc rating because the total risk of injury might be the same with lower rated PPE as that of a higher probability arc with higher rated PPE.

What else would you do for a situation with low probability of an arc, have no PPE at all? An all or nothing approach is not necessarily any more logical than having some protection in the rare event that there is an arc in a low probability situation. It might save your life if you are further away from the arc that the assumed working distance or if the fault current is different from the worst case scenario.


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PostPosted: Sun Apr 08, 2012 10:42 am 
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The consequence (hazard) is not reduced because the likelihood is reduced. You can reduce the consequence by doing things like moving the worker further away, reducing the fault clearing time, or increasing the impedance (assuming the fault clearing time doesn't substantially increase). This is a problem with the tables in 70E...in many cases, the consequence has not been reduced.

In some cases (e.g. below 240 VAC but definitely below 50 V), an arc may not be self-sustaining so the incident energy calculation is meaningless. At that point, the arc flash hazard is minimal and need not be calculated.

In the cases described though, what you are doing is reducing the likelihood. At some point the likelihood becomes so low that PPE is no longer required. For instance, what is the chance that I will be struck by a meteor while I'm at work? And what pray tell are you going to do to protect against that occurrence? This is an example of an event that has a very severe consequence but where the likelihood is very low. If the chance of an arc flash is greater than the chance of being hit by a meteor, I think we can agree that wearing PPE is pointless. We need to start looking at arc flash hazards the same way...at some point although it is indeed possible for an arc flash to occur, and the consequence is definitely beyond acceptable (1.2 cal/cm^2), BUT if the likelihood of occurrence is below our tolerance for risk, then PPE should not be required.

Equipment reliability statistics are already readily available. One need only look at those with an eye towards the type of failure that may occur (arcing fault) and you have your answers.


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PostPosted: Mon Apr 09, 2012 7:42 am 
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When this shortcoming was identified in the 2000 edition of NFPA 70E I discussed it at length with our State department of labor and industries medium voltage inspector. His unofficial opinion at that time was that the need to suit up could be weighted by likelihood of disturbing an exposed, energized conductor. That still seems to make sense though there is probably legal peril in such individual interpretation.


There are many aspects to this discussion. One example of what doesn't fit the general answers: Some 480V motor controllers are constructed with solid dielectric and extended tracking distances such that they cannot sustain an arc at rated voltage.


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PostPosted: Mon Apr 09, 2012 5:26 pm 
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When this shortcoming was identified in the 2000 edition of NFPA 70E I discussed it at length with our State department of labor and industries medium voltage inspector. His unofficial opinion at that time was that the need to suit up could be weighted by likelihood of disturbing an exposed, energized conductor. That still seems to make sense though there is probably legal peril in such individual interpretation.

There are now a variety of risk assessment standards (IEC 61508, 61511, PMMA, RIA, ANSI B11.TR3) that address this problem through a methodology called quantitative risk assessment. Since all of these are already recognized standards there is no legal peril in using them as long as you adopt the procedures contained in them. All of them are general procedures...one only need insert arc flash hazard data into the procedures to generate results.

The risk assessment annex in 70E is clearly an attempt to adopt ANSI B11.TR3 or something very similar but it is woefully incomplete in a lot of areas. I've been looking specifically at IEC 61511 myself because although it does have a "default" method using SIL terminology and some automatic risk assignments, you can also use the generalized version of it. IEC 61511 itself is really nothing more than a specialized version of IEC 61508 adopted to process industries instead of manufacturing plants.


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PostPosted: Wed Apr 11, 2012 5:45 am 
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PaulEngr wrote:
There are now a variety of risk assessment standards (IEC 61508, 61511, PMMA, RIA, ANSI B11.TR3) that address this problem through a methodology called quantitative risk assessment. Since all of these are already recognized standards there is no legal peril in using them as long as you adopt the procedures contained in them. All of them are general procedures...one only need insert arc flash hazard data into the procedures to generate results.

The risk assessment annex in 70E is clearly an attempt to adopt ANSI B11.TR3 or something very similar but it is woefully incomplete in a lot of areas. I've been looking specifically at IEC 61511 myself because although it does have a "default" method using SIL terminology and some automatic risk assignments, you can also use the generalized version of it. IEC 61511 itself is really nothing more than a specialized version of IEC 61508 adopted to process industries instead of manufacturing plants.


Has anyone actually done a risk assessment of common tasks on energized electrical equipment to determine when the calculated level of PPE needs to be deployed? Is anyone using risk assessment of tasks to assign reduction of PPE values to tasks similar to the 70E tables? I would like to see some good examples of the use of one of these risk assessment methodogies to help me apply it correctly or at least apply it consistant with the thinking of the arc flash professional community.


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PostPosted: Wed Apr 11, 2012 7:05 pm 
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jtinge wrote:
Has anyone actually done a risk assessment of common tasks on energized electrical equipment to determine when the calculated level of PPE needs to be deployed? Is anyone using risk assessment of tasks to assign reduction of PPE values to tasks similar to the 70E tables? I would like to see some good examples of the use of one of these risk assessment methodogies to help me apply it correctly or at least apply it consistant with the thinking of the arc flash professional community.


Yes. See the library on this forum.


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