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 Post subject: Implementing Arc Flash Study Results
PostPosted: Mon Feb 28, 2011 1:16 pm 
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Greetings,

New poster here, but I've been browsing for about a year - lot of good information and knowledgeable folks around here.

I'm working as an electrical engineer at an industrial manufacturing plant, and over the past year or so we've been working through a short circuit coordination and arc flash study. The short circuit coordination work I'm fairly experienced with, it's what I enjoy. The Ei analysis, and how it relates to short circuit coordination is still new to me.

The past few years we've been following Table 130.7(C)(9) because we had not yet performed the hazard analysis. Now that we have results, migrating away from the table and to the Ei boundaries is challenging. That combined with the fact that we have a very extensive work permit system, with all PPE and energized work documented leads to a lot of questions about a variety of work.

My first question is regarding 480V MCC and enclosures, which are rated Cat 0. In the past we've worn face shields when opening energized starter buckets, along with insulated gloves and tools. We already wear FR 8 Cal uniforms. Now that we have completed the analysis and we know it's a Cat 0, is that Cat 0 anytime we are interacting with the equipment...regardless of the level of interaction??

In other words, Cat 0 PPE applies if we were removing a starter bucket from a live bus? I guess I'm having a hard time believing one PPE rating for a variety of work. Also, as I understand it, we must wear Cat 0 PPE within the flash protection boundary, in this case 17", of exposed energized conductors? Is this the torso of the body, or limbs? Thanks for any guidance or sharing of experiences.

Ron


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PostPosted: Thu Mar 03, 2011 9:14 am 
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Welcome to a great learning experience - performing an arc flash calculation study. You are following the foot steps of many others. First by using the NFPA tables and then migrating towards incident energy calculations.

The tables are more of a collective opinion of a group with NFPA 70E. They are used if calculations are not performed. The tables factors in risk as well as the hazard. i.e. the hazard is the lion in the cage. The risk is how strong is the cage, am I inside or outside the cage etc.

The incident energy calculations just look at the hazard. Whether you are operating a switch, inserting a bucket, voltage testing, the Ei calculations indicate if something goes wrong (regardless of the cause) how much energy could be there. Also, the Ei calculations are based on specific parameters of your system, equipment location, equipment type etc. Becuase of this, sometimes you find the required PPE is less using the calculations than the generalized NFPA tables.

However, it is possible that the PPE based on calculations could be greater than the PPE from the tables too. It seems like your case is that the PPE becomes lower. You probably have a lower short circuit current and clearing time.

Wearing a minimum 8 cal PPE regardless of how low the actual calculated PPE requirement becomes is my favorite option. When working around an arc flash hazard, even if the Ei is lower, my opinion is there is always the possibilty of injury. I have seen some 1 cal/cm2 bangs (technically Cat 0) that I would not want to be standing near (let alone in Cat 0 natural / non melting fabric)

Enjoy the learning experience!

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Jim Phillips, P.E.
Brainfiller.com


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PostPosted: Thu Mar 03, 2011 9:48 am 
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brainfiller wrote:
The tables are more of a collective opinion of a group with NFPA 70E. They are used if calculations are not performed.


It is important to remember that the HRC and PPE requirements are also based on verifying the NOTES that apply to each task or equipment class. In most cases, calculations and verification must still be performed to confirm that available fault currents and clearing times of overcurrent protective devices are within the maximum limits specified in the NOTES.


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PostPosted: Thu Mar 03, 2011 10:31 am 
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kencybart wrote:
It is important to remember that the HRC and PPE requirements are also based on verifying the NOTES that apply to each task or equipment class. In most cases, calculations and verification must still be performed to confirm that available fault currents and clearing times of overcurrent protective devices are within the maximum limits specified in the NOTES.


You are correct - the footnotes are important but let’s look at 2 of the footnotes in detail for discussion purposes. Specifically Page 31 of the 2009 Edition of NFPA 70E. 600V Motor Control Centers.

Footnote 2 - upper limit 65 kA for 2 cycles.
Using this data for incident energy calculations based on IEEE 1584, I get around 3.5 cal/cm^2. Using the HRC table, some locations are category 0, 1 or 2*

If the PPE selection for this data was made based on calculations, it would lead to a minimum 3.5 cal/cm^2 PPE which most say is Category 1 (4 cal/cm^2 minimum)

Footnote 3 - Upper limit 42 kA at 20 cycles.
Using this data for incident energy calculations I get around 24 cal/cm^2. (clearing time is very important) Using the tables where footnote 3 applies and Category 4 and Category 1 are both referenced.

If the PPE selection for this data was made based on calculations, it would lead to a minimum 24 cal/cm^2 PPE which most say is Category 3 (25 cal/cm^2 minimum)

The NFPA 70E Tables are a great way to come up with PPE in the absence of calculations and should be used where calculations have not been performed. However the Ei calculations provide a more precise idea of the actual hazard based on specific conditions.

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