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 Post subject: Another Tables vs Calculations Question
PostPosted: Wed Jan 14, 2009 1:06 pm 
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I understand the comments regarding using the calculations in lieu of the table (130.7(C)(9)) if an arc flash study has been performed. But I am very confused and need some help with interpretation - I am fairly new at all of this. See my scenarios below with my 2 questions.

Scenario 1 - I have a 480V mcc and I need to open the bucket door for troubleshooting - will be using a multimeter. The arc flash study shows that the incident energy for this particular exposed energized equipment is 7.1 cal/cm2. Therefore, using Table 130.7(C)(11) I would use Category 2 PPE. If I had used the table I would need to use Category 2* PPE.
My observation is that Category 2* is not even a possibility by using the calculations. Am I correct?

Scenario 2 - I have a 480v mcc and I want to open the switch while the bucket door is closed. According to the table I would use Category 1 PPE. How do I use my calculation (7.1 cal/cm2 incident energy) information to determine the Hazard/Risk Category when there are no exposed energized parts?

There are numerous tasks listed in the table that are similar to my Scenario 2. I like the concept of using the table because it makes it easier to determine PPE in an industrial facility. Ultimately I would like to have a PPE table for various tasks but I do not know how to do it until someone can shed some light on Scenario 2 above.


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PostPosted: Wed Jan 14, 2009 1:41 pm 
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psThomas wrote:
I understand the comments regarding using the calculations in lieu of the table (130.7(C)(9)) if an arc flash study has been performed. But I am very confused and need some help with interpretation - I am fairly new at all of this. See my scenarios below with my 2 questions.

Scenario 1 - I have a 480V mcc and I need to open the bucket door for troubleshooting - will be using a multimeter. The arc flash study shows that the incident energy for this particular exposed energized equipment is 7.1 cal/cm2. Therefore, using Table 130.7(C)(11) I would use Category 2 PPE. If I had used the table I would need to use Category 2* PPE.
My observation is that Category 2* is not even a possibility by using the calculations. Am I correct?

Scenario 2 - I have a 480v mcc and I want to open the switch while the bucket door is closed. According to the table I would use Category 1 PPE. How do I use my calculation (7.1 cal/cm2 incident energy) information to determine the Hazard/Risk Category when there are no exposed energized parts?

There are numerous tasks listed in the table that are similar to my Scenario 2. I like the concept of using the table because it makes it easier to determine PPE in an industrial facility. Ultimately I would like to have a PPE table for various tasks but I do not know how to do it until someone can shed some light on Scenario 2 above.


Here is the thing, you cant use the tables and study results, you pick one or the other. The tables have limitations and there are many, many assumptions made to determine the HRC (And a little "best guess"). Calulations are based on the actual available fault current and clearing times of your system and should superseed the tables if you have a study in place.

There does not need to be exposed live parts to have an arc flash hazard, look at the definition of arc flash hazard in the 70E.


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PostPosted: Fri Jan 30, 2009 11:53 am 
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psThomas's point in Scenario 2 is also giving me heartache. I see where the calculation is the best method and I have done the calculations and applied the labels. But my staff asks me about the table. In the table they lower the HRC for various tasks. But if I use the label all tasks on that piece of equipment remains the same. If I understand correctly, there is no "derating" of the HRC by task if you do the calculations. Yet, a large facility is really forced to do the calculations due to the environment. Is this a correct assumption?


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PostPosted: Fri Jan 30, 2009 12:46 pm 
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Zog wrote:
Here is the thing, you cant use the tables and study results, you pick one or the other.


Zog,

I have seen other posts where it has been said that you can use a study and use the tables. What I'm thinking about is a large industrial facility where a study goes to a certain level where it is determined that items below this level meet the criteria for the use of the tables (short circuit level, clearing time).

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Barry Donovan, P.E.
www.workplacesafetysolutions.com


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PostPosted: Fri Jan 30, 2009 1:01 pm 
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wbd wrote:
Zog,

I have seen other posts where it has been said that you can use a study and use the tables. What I'm thinking about is a large industrial facility where a study goes to a certain level where it is determined that items below this level meet the criteria for the use of the tables (short circuit level, clearing time).


Yes, that is fine. Most plants draw the line at the <240V <125kVA point and do the analysis above that and use the tables below that. i meant you dont mix the two on the same equipment.


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