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 Post subject: 3.9 Calories Cat 1 or 2?
PostPosted: Wed Jan 02, 2008 5:19 pm 
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OK so how close is close? Now and then we have a calculated number that gets pretty close the next hazard risk category. Technically 3.9 Calories is Cat 1 but how many of you make the CYA move and call it Cat 2? :cool:


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PostPosted: Thu Jan 03, 2008 9:17 am 
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We label it "as is" i.e. 3.9 calories in your example. We footnote it in the report that it is marginal and Cat 2 could be used, especially if a 2 category system is in place, i.e. only Cat 2 and Cat 4.


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PostPosted: Thu Jan 03, 2008 2:51 pm 
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Hi "C",

We would use Cat 2 protection for something that close even though technically 3.9 is Cat 1. You could probably argue that the calcs are conservative (3 phase) and all kinds of other things but the opposite is also true. Your hands are closer than the 18 inch working distance. etc. Bottom line..... FEAR THE LAWYERS!


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PostPosted: Fri Jan 04, 2008 5:37 am 
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Hi gang,

Here is the good one, what if you have 7.9 Calories? That is close to the upper limit of Cat 2 which is not that bad of a Category. Do you use the same "safe" approach and push to 3 or 4 or keep it at Cat 2. :eek:


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PostPosted: Fri Jan 04, 2008 9:33 pm 
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I call them exactly as IEEE 1584 calculates them. If it calculates at 3.9, we label it RC-1 and if it calculates at 7.9 we label it as RC-2.

Reasons:
1) The calcs are empirical and almost arbitrary. For example, the methods assume some fault resistance that could vary more than just about any other parameter. What we are doing is applying an industry standard and not absolutes or pure scientific theory. I am comfortable with this and have witnessed safety improved as a result.

2) It is expected that IEEE 1584 would already include safety margins.

3) It is expected the PPE includes safety margins.

4) I do not wish to be responsible for reinterpreting the standard for what could be such a liability fraught situation; where would it stop?


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PostPosted: Sun Jan 06, 2008 11:14 am 
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Good info Gary, thanks.

If you are using 2 category system i.e Cat 2 and 4 for PPE it's pretty easy if your sitting on the fence between cat 1 and 2 or cat 3 and 4. I was curious about a calculation like the 7.9 that I mentioned pushing you into a much higher PPE level. I agree with you it is probably best to use what is calculated and not "round up". I am sure someone could argue "rounding up" introduces a hazard by wearing a more burdonsome level of PPE than required.


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PostPosted: Mon Jan 07, 2008 6:34 am 
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Exact means exact

Our policy is that the value that is calculated is the value we go with. If it's 3.9 then it's Level 1, etc. Our logic in making that decision is that the parties responsible for deciding that 4.0 is the threshold for the next level were privy to data that we are not. Whether that's the bult in safety factor in the calculations or the assumption that one-tenth of a calorie is negligible in terms of thermal transfer or whatever. The 70E and/or 1584 working group decided in their wisdom regarding this subject that 3.9 was Level one and 4.0 was Level 2.

Now, some may say that I am placing too much faith in the expertise of others. Perhaps that's true, but what I know in fact to be true is that my job is to utilize the best methods presently available to perform this work for the people who have entrusted their safety to me. My job is not to evaluate and/or reform the calculation or category assignment process. That's for the aforementioned working groups. When they come up with something better, more accurate or more comprehensive, then I will change my process. In the interim however, I will label circuits that calculate at 3.9 at Level 1.

As far as legalities go, we incorporate a lengthy and verbose disclaimer to our arc flash analysis reports that states that we have calculated these values using the presently available methods, and that energized work should be avoided, etc. The best you can do is the best you can do, and exactly 3.9 means exactly 3.9. Again, just my two cents.


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PostPosted: Mon Jan 07, 2008 6:42 am 
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I post them as calculated. Part of the reason is that the categories are somewhat arbitrary.

The main difference between Cat 1 and 2 is a faceshield and hearing protection. The main difference between Cat 3 and 4 is multilayer suit.

To my mind, you protect yourself from the hazard. What we are calculating is the arc FLASH, not the blast. At least not yet. You must protect from that big ball of fire that you are calculating. Provide protection from the heat as calculated. At the present time, that is the best you can do.


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PostPosted: Tue Feb 12, 2008 7:07 pm 
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K. Jackson wrote:
I am sure someone could argue "rounding up" introduces a hazard by wearing a more burdonsome level of PPE than required.


At the facilities we have performerd arc flash studies, Risk Category one has not been a problem as that was decided as base level to enter the plant (basically one layer of nomex etc.).

For chores above that Risk Category 1 , there is a arc blast suit available rated Risk category 4. We did not want the confusion of multiple different sets of PPE even though there are intermediate locations. Most jobs require that "the suit" are short duration and the craftsmen have been eager to wear the protection.

I guess it is nice to have the extra thickness when dealing with the pucker factor of racking in a breaker. For longer term work we would seek an outage.


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PostPosted: Thu Feb 14, 2008 2:40 pm 
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I agree with you Gary, we can't reinvent standards or guides from the IEEE because we think it's too close. If you really think it's too close, then I would strongly advise to re-evaluate the limiting factors contributing to the calculated arcing fault currents in your study. Are you considering both min and max fault levels from the utility? Are pre-fault voltages being used or nominal? Are you inspecting definite time characteristic curves for a worst case arc flash level that could occur from an intermediary fault level between the minimum and maximum fault levels? If a person isn't looking at the effect of some of these issues, then yes there could be valid conditions in which this 7.9 cal/cm2 could creep up higher to a category 3 classification. If not, then stick with the values calculated as per the standard being used. IEEE 1584 has some built in checks for giving conservative values in the first place as mentioned by others previously. Rather than disclaiming higher out of fear, I would rather recommend that a person fully outline the basis for their results based on the full letter of the standard for why the results come out as such. Besides you might be able to cover this detail by acknowledging that the minimum PPE standard for the location is cat #2 so a borderline cat #1 calculation will still be covered by the standardized safety gear a plant has selected.

Just some insight to help.


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PostPosted: Fri Feb 22, 2008 11:45 am 
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Gutterboy wrote:
I agree with you Gary, we can't reinvent standards or guides from the IEEE because we think it's too close. If you really think it's too close, then I would strongly advise to re-evaluate the limiting factors contributing to the calculated arcing fault currents in your study. Are you considering both min and max fault levels from the utility? Are pre-fault voltages being used or nominal? Are you inspecting definite time characteristic curves for a worst case arc flash level that could occur from an intermediary fault level between the minimum and maximum fault levels? If a person isn't looking at the effect of some of these issues, then yes there could be valid conditions in which this 7.9 cal/cm2 could creep up higher to a category 3 classification. If not, then stick with the values calculated as per the standard being used. IEEE 1584 has some built in checks for giving conservative values in the first place as mentioned by others previously. Rather than disclaiming higher out of fear, I would rather recommend that a person fully outline the basis for their results based on the full letter of the standard for why the results come out as such. Besides you might be able to cover this detail by acknowledging that the minimum PPE standard for the location is cat #2 so a borderline cat #1 calculation will still be covered by the standardized safety gear a plant has selected.

Just some insight to help.


You bring up valid points, most of which can be summarized by saying arc flash exposure is a complicated calculation with more than 14 variables affecting the outcome in ways that may not be intuitive. Even NFPA 70E acknowledges this shortcoming with disclaimers throughout. I attempt to be clear in deliverables that this is an improvement in personnel protection but not an assurance of zero risk, or absolute safety.

Having said that, we choose ONE utility available fault current, choose ONE distribution system configuration where there are alternate feeders, and choose to NOT recalculate results for emergency generator output. All assumptions of this nature are shared with the site owner and documented in the reports. We simply would not ever get done if not for these simplifications.


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