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 Post subject: Arc Flash Analysis vs. NFPA70E Table 130.7(c)9
PostPosted: Mon Nov 02, 2009 1:32 pm 

Joined: Mon Nov 02, 2009 1:19 pm
Posts: 3
OK, we have our analysis complete, but I am stumped on an issue. Our 600V MCCs are rated at HRC0 by the analysis. To complete the task "work on energized parts, including voltage testing" which the NFPA table lists as HRC2, will I be able to do this task with Cat0 PPE as long as I have adequate shock protection?


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PostPosted: Mon Nov 02, 2009 1:46 pm 
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Location: Charlotte, NC
lcrowley wrote:
OK, we have our analysis complete, but I am stumped on an issue. Our 600V MCCs are rated at HRC0 by the analysis. To complete the task "work on energized parts, including voltage testing" which the NFPA table lists as HRC2, will I be able to do this task with Cat0 PPE as long as I have adequate shock protection?


If you did an analysis why are you even looking at the tables and HRC's?


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PostPosted: Mon Nov 02, 2009 2:15 pm 
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Location: Lawrenceburg KY
As for me that’s a common issue I see in a lot in our facilities. I have personally been involved in an incident. A fuesed 600A MCC 480v met my screwdriver with a short circuit on the line side of a breaker (young and dumb). :cool: The IE calculates as a 1.0 Cal/cm2 HRC0. However, since I know now what a 1.0 cal/cm2 looks like I ask companies to increase the HRC from #0 to #1 so ER face shield and hard hat is used while interacting with LV MCC's.

My safety glasses were smoked, eyebrows reduced in length, hair burn, temporary hearing and vision loss. Some say, I looked like a raccoon, other then that I did not get any bad burns. Maybe a little red in the face but that may have been from embarrassment from so many people running to watch me smoke. :o The incident happened around 20 years ago when electrical safety was more a cowboy approach.

The problem is most programs can calculate the IE in a box but the focus of the SC fault can sometimes be intensified by small MCC buckets and most of the time that focus is at eye level therefore the face will normally take the blow.

When interacting with LV MCC's personally, I would never recommend anyone using a HRC0 without an ER face shield and hard hat.


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PostPosted: Mon Nov 02, 2009 2:18 pm 

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My boss does not believe that the arc flash analysis overpowers the tables if working on live voltages. Just wanted to make sure I am right.


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PostPosted: Mon Nov 02, 2009 2:26 pm 
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lcrowley wrote:
My boss does not believe that the arc flash analysis overpowers the tables if working on live voltages. Just wanted to make sure I am right.


You never mix the tables with the results of an analysis. Several times the 70E refers to "In leiu of the analysis" and vice versa. 130.7(9) for example. He should also read 130.3 (B)

Now the real question here is are you doing testing or "Energized work", if the latter is the live work justified per 130.1?


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PostPosted: Mon Nov 02, 2009 2:28 pm 
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THE CABLE GUY wrote:
However, since I know now what a 1.0 cal/cm2 looks like I ask companies to increase the HRC from #0 to #1 so ER face shield and hard hat is used while interacting with LV MCC's.


Amazing isn't it? First time a saw a 1 cal/cm2 arc in a test lab my jaw dropped. I think most people think it is nothing, but a 1 cal/cm2 arc will put you in a burn unit in about 0.12 seconds. Burn units suck.


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PostPosted: Mon Nov 02, 2009 5:08 pm 

Joined: Wed Nov 26, 2008 7:55 am
Posts: 44
Location: Connecticut
Incident Energy Calculations

What we all forget is that when we calculate the incident energy, whether via software or long hand, the time to clear the fault can sometimes, more like always, will be a variable. A circuit breaker will open in ? seconds the day it was installed. What is the time after years of sitting out on the factory floor, in a dusty grimy environment? Maintenance requirements is a big part of 70E, and should not be ignored. Many facitlities do have service performed on their switchgear and other critical components, but rarely do I see them fully tested. Some yes, but not always.


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PostPosted: Tue Nov 03, 2009 6:40 am 

Joined: Mon Dec 08, 2008 7:13 am
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Location: Quebec, Canada
John Perrotti wrote:
Maintenance requirements is a big part of 70E, and should not be ignored.


That's an excellent point and something about that maintenance requirement should be written in every arc flash report.

That's also why we have to be careful with AF results that are near a HRC limit. I know some say we should not talk about calculations and HRC, but HRC's are still how we choose PPE right? ;)


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PostPosted: Tue Nov 03, 2009 7:18 am 

Joined: Mon Nov 02, 2009 1:19 pm
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Well, from a simplistic view, utilizing the HRC categories is the simplest way for PPE selection based upon caloric values from the analysis. For this company, we had an outside contractor, let us call them "Round E" for privacy purposes, rendered our results in HRC tables. As long as your PPE selection is adequate for the caloric value, what's the big deal? Use the HRC table...


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PostPosted: Fri Nov 06, 2009 1:24 pm 
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Location: St. Louis, MO
lcrowley wrote:
As long as your PPE selection is adequate for the caloric value, what's the big deal? Use the HRC table...


Because this could result in the electricians being over dressed, and unable to perform the tasks assigned safely.

If I mix the tables and analysis, and I calculate the energy level at 27 cal/cm at 18" on a piece of equipment, then show that as CAT 4, I may have hindered my electricians.
Now, in order to do any troubleshooting on the gear, he may put on a full bear suit, when he could have worn his normal 20 cal/cm ATPV rated jeans with an 11 cal shirt, with 8 coverall over the top, and a work jacket. He would still have to wear the hood, but it is much more comfortable when you don't have the rest of the outfit on.
With the PPE necessary for the calculated hazard, the electrician is more comfortable, happier, and better able to perform the work. He will likely do a better job, and safer, then if he rushed it to get out of that [email protected]&$ suit.


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PostPosted: Wed Nov 11, 2009 1:37 pm 

Joined: Fri Sep 05, 2008 8:10 am
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[quote="WDeanN"]Because this could result in the electricians being over dressed, and unable to perform the tasks assigned safely.

If I mix the tables and analysis, and I calculate the energy level at 27 cal/cm at 18" on a piece of equipment, then show that as CAT 4, I may have hindered my electricians.
Now, in order to do any troubleshooting on the gear, he may put on a full bear suit, when he could have worn his normal 20 cal/cm ATPV rated jeans with an 11 cal shirt, with 8 coverall over the top, and a work jacket.QUOTE]


I'll admit I'm no expert in PPE selection or layering but how could the clothing you mention here be adequate with a 27 cal/cm2 IE? Neither the "jeans" or the clothing on the torso meet 27 cal/cm2.

Thanks,


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PostPosted: Thu Nov 12, 2009 11:34 am 
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MIEngineer wrote:
I'll admit I'm no expert in PPE selection or layering but how could the clothing you mention here be adequate with a 27 cal/cm2 IE? Neither the "jeans" or the clothing on the torso meet 27 cal/cm2.
Thanks,


Ok, bad example, and I left out the part about the jacket being rated 20 cal/cm.

The point that I was (badly :confused: ) trying to make, is that most daily wear arc flash clothing is much more comfortable, even when worn in layers, then most of the flash suits available. When worn in layers, although there is some debate on the additive properties of the layers (unless tested together), they may not add up to the 40 cal minumum of a Class 4 rating, but could easily be sufficient for the calculated energy level.


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