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 Post subject: What if the working distance exceeds the flash boundary?
PostPosted: Wed Jul 01, 2009 2:53 pm 
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The engineering company that did our arc analysis used 18 inches for the working distance for most of the calculations. Many of the calculated arc flash boundaries are 4 to 8 inches. Most are hazard catagory 0. If the workers body is 18 inches from the arc source, and outside the flash boundary, are we required to wear arc ppe?

All of the information I've been able to find on the net is related to much larger flash boundaries and higher risk catagories. Its the low end of the flash risk that I'm having trouble with.


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PostPosted: Wed Jul 01, 2009 5:47 pm 
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Are you using the Incident Energy method or the Table method?


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PostPosted: Thu Jul 02, 2009 6:13 am 
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The numbers make sense. The Arc Flash Protection Boundary (AFPB) is where the incident energy drops to 1.2 cal/cm^2 so if the AFPB is very small i.e. just a few inches, that is where you hit 1.2 cal and moving back to 18 inches for the working distance it will be even less.

Except.... that assumes you are really 18 inches away. 70E makes the statement about parts of the body closer than the working distance may need additional protection. This could be interpreted as the hands and perhaps forearms need protection but assuming they have on gloves and leather protectors, that is ususally the norm for protection of the hands.

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PostPosted: Thu Jul 02, 2009 6:25 am 
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By table, I assume you mean the table in NFPA 70E? Then no, the analysis actually calculated the incident energy. I've got a number of pieces of equipment that are in the 0.1 to 0.5 cal/cm2 range. The required PPE would be a long sleeve cotton shirt IF the equipment wasn't in a clean room. Non-melting clean room smocks are about $400 a piece. I've got 20 mechanics that do work in the clean room. If there is a real arc risk, we'll comply with the NFPA rules but I think my situation is quite different than the common image of the exploding service panel.

Any links to discussions of low arc risk examples would be greatly appreciated.

And yes, we are using rubber gloves with leather over gloves. What I'm hoping to avoid are the speacial smocks, head covers, beard covers and shoe covers.


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PostPosted: Fri Nov 27, 2009 9:38 am 
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Arc Flash Protection Boundary - beyond the calculation

Having spend a lot of time around the high current lab, I cringe at the idea of basing the arc flash protection boundary on a calculation that only takes into account radiated heat from a stationary arc. I'm perfectly willing to believe that if you cover up with gear that matches the ATPV to the calculated heat flux that performance is good. But I can't bring myself to put an unprotected face at a distance calculated to be 1.2 cal/cm^2, like 33 inches, when I've seen the testing and know very well that balls of plasma and molten metal can shoot out 10 feet. Good examples of this are shown in Westex's Kema testing video. I wouldn't want to be at 1.2 cal/cm^2 line unprotected!

So I think the idea is to make the effective arc flash protection boundary conservative enough to easily take into account the concentrations of hot, thrown material, and then you don't have to worry about whether someone's hand is sticking out.

I put a document together for our higher energy metalclad installations (600 V -28 kV) that don't have specific calculations yet. It says that if anyone is performing an activity with an arc flash hazard, that other workers are expected to suit up or leave the room, and if it's a large room, then you can stay in what we're calling Cat 0 (still rated clothing, but covering up all exposed skin not required, so no face/hand protection) at 7 metres away (or 20 feet). The idea is that then they don't need a tape measure and it doesn't matter where their hands are. When we get to the lower energy stuff like you're talking about, I'll likely say something like calculated + 2 metres, and something similar for outdoor air insulated stations where you don't get as much concentration of material blown around.

still working on it...


Jody Levine
Hydro One (transmission part of the old Ontario Hydro)
Toronto


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PostPosted: Sat Nov 28, 2009 9:19 am 
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Check on the boundaries

Check the boundaries. If I'm not mistaken they are usually overstated because the boundaries use the Lee equation which had no supporting data (this is fine for a rough distance like the boundaries but not accurate enough to get too concerned pragmatically). The 18 inch working distance uses the actual data from IEEE 1584 testing which has adequate data for three phase arcs at 480V. This is the difference. So the arc flash boundary is overly conservative. See Duke Energy's testimony to OSHA about their 500kV lines having arc flash boundaries of 46 ft. They are only 40 ft. off the ground so the worker would have to work them from 6 ft under (pun intended). The question from Duke was are these lines inherently unsafe to the public? The answer is no. The issue is there has been little research for realistic medium voltage arc flash boundaries and the low voltage boundaries contain a safety measure from the Ralph Lee equations for good measure. I'm not concerned with the safety measure but folks doing assessments and making decisions should know what the arc flash boundary means.

Hope this helps someone.


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PostPosted: Sun Nov 29, 2009 3:59 pm 
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jody wrote:
So I think the idea is to make the effective arc flash protection boundary conservative enough to easily take into account the concentrations of hot, thrown material, and then you don't have to worry about whether someone's hand is sticking out.


Jody,

You certainly have the right idea! I have been advocating for years to select uniform arc flash protection boundaries. Here is an article I wrote that talks a bit about this approach. I also talked about it in the three part arc flash guide that is listed elsewhere in this forum.

[url="http://www.brainfiller.com/documents/Arc%20Flash%20Protection%20Boundary%20Jim%20Phillips.pdf"]Arc Flash Protection Boundary[/url]


Bottom line, if you have numerous boundaries someone will get it mixed up. Pick the largest boundary within reason (discussed within reason in the article) round it up and use it for each location.

I agree with you about the ejected material. That has always been a problem. I have staged some arc flash tests that resulted in incident energy below 1.2 cal/cm2 that I would not want to be front of. I managed to throw a small bus bar laying on stabs of a padmount transformer with low incident energy <1.2 cal/cm2.

Keep in mind the AFPB is for people that are not suppose to be in the area - move them far out of the way and don't split hairs about 4.6 ft. 6.7 ft. 3.5 ft etc. Just round it up and keep them all away. If they need to be in the "battle zone" then dress them up in PPE and make sure they are trained, qualified etc.

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PostPosted: Sun Nov 29, 2009 8:22 pm 
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I have been following Jim's method also. This is not about the science of arc heat calculations, its about fundamental electric safety. The shortest arc flash boundary I would accept is 4 ft. But this still permits a person in street clothes to be basically hanging over the shoulder of the electrician performing live work. So pick something more like 6, 8 or 10 feet. If you can get one distance for the entire complex that makes sense, use that. Same goes with the IE calcs. You don't have to list PPE 0 and IE of 1.0. You can elect a higher rating, such as PPE 1 and IE of 4.0.

Don't think in terms of calcs and numbers, think in terms of a program that makes sense, is easy to understand, and doesn't require a lot of detail understanding.


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PostPosted: Mon Nov 30, 2009 8:19 am 
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haze10 wrote:
I have been following Jim's method also. This is not about the science of arc heat calculations, its about fundamental electric safety. The shortest arc flash boundary I would accept is 4 ft.


I also use a version of Jim's method. The shortest arc flash boundary that I post is also the Limited Approach Boundary. I also round all of my AF Boundaries up to the nearest foot. This makes it easier to visuallize for the electricians. Instead of having to figure out and measure 64 inches, they can usually picture 6 feet pretty easily.

60wag wrote:
I've got a number of pieces of equipment that are in the 0.1 to 0.5 cal/cm2 range. The required PPE would be a long sleeve cotton shirt IF the equipment wasn't in a clean room. Non-melting clean room smocks are about $400 a piece. I've got 20 mechanics that do work in the clean room.

And yes, we are using rubber gloves with leather over gloves. What I'm hoping to avoid are the speacial smocks, head covers, beard covers and shoe covers.


I would advise getting a couple of sets of the FR clean room gear. There should only be one or two working on a piece of energized equipment at a time. They can trade out as needed.
What you've got to look forward to is training, training, training.


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PostPosted: Mon Nov 30, 2009 4:00 pm 
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elihuiv wrote:
Check the boundaries. If I'm not mistaken they are usually overstated because the boundaries use the Lee equation which had no supporting data (this is fine for a rough distance like the boundaries but not accurate enough to get too concerned pragmatically). The 18 inch working distance uses the actual data from IEEE 1584 testing which has adequate data for three phase arcs at 480V.


We used Arcpro for the medium voltage stuff. I intend to use it for HV also. But my point was is that I wouldn't want to have an unprotected face or hand even 4 feet away from an arc-in-a-box that's 8 cal/cm^2 at 18 inches. I couldn't imagine standing there during a test!

Looking at the nature of arc flash incidents we've had, I'm hoping I can find an excuse such that no face protection is required at the usual stick lengths for outdoor open air in both medium voltage and high voltage. I'm expecting a possible exception with some fuse protected distribution stations with long clearing times (seconds). (Getting off topic here, sorry!)

Jody Levine
Hydro One


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PostPosted: Mon Nov 30, 2009 7:50 pm 
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Standard Boundaries

Thanks Jim and Jody. I always recommend a standard arc flash boundary too. It is crazy not to have a simple easy to follow standard safety policy. I recommend making this boundary the shock boundary too so all the PPE is in place before crossing this boundary.

We have usually set it at 5 ft to make it simple and slightly more conservative than the min 4 ft., unless the calculations show this is grossly conservative, then we recommend 4 ft.

I wish all equipment made this a safe distance.


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PostPosted: Wed Dec 09, 2009 11:48 am 
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Be Realistic

I'm not sure what was meant by "the AFPB only applies to those that aren't supposed to be in the area". That implies that it is only applicable to unqualified employees. My recommendations would be to follow the analysis results and, as others stated, ensure the hands, forearms, and eyes are protected. In reality, most of the electric enclosures (<600V) on the plant floor are Cat #0 with a very short (<6") AFB. Adding 6 feet to a 6" AFB will not always be prudent or accepted. Follow the NFPA 70E. Keep unqualified people out of the Limited Approach Boundary and the Arc Flash Protection Boundary. Do not work on energized equipment except for diagnostics or when it is infeasible (impossible) to deenergize.


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PostPosted: Wed Dec 09, 2009 12:15 pm 
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viper57 wrote:
I'm not sure what was meant by "the AFPB only applies to those that aren't supposed to be in the area". That implies that it is only applicable to unqualified employees.


...and unprotected people.

You are quite correct, if you have a system that has low incident energy and small AFPBs then a large boundary may not make sense. However, many sites are quite large with boundaries spanning quite a range of distances. Using a single larger boundary keeps it simple for those that have to use it.

Qualified and protected people can perform the work and unqualified and / or unprotected people keep out of the way.

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