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 Post subject: Back of head
PostPosted: Fri Sep 26, 2014 10:01 am 
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Location: North Carolina
70E used to require a face shield only starting at 4 cal/cm^2, and a balaclava was required only starting at 8 cal/cm^2, finally requiring a hood at 12 cal/cm^2. As of 2012 edition this reduced to a face shield starting at 1.2 cal/cm^2 and a balaclava starting at 4 cal/cm^2. Now with the 2015 edition this has reduced further to a face shield and balaclava for anything over 1.2 cal/cm^2 up to 12 cal/cm^2.

The concern is protecting the "back of the head". My simple question is...does this make sense?

So I tried a little numerical experiment. Now using IEEE 1584, this gets a little confusing because the standard working distance and an exponent depend on the type of equipment and the voltage, but there are only 4 combinations of exponents and working distances (610 mm, 1.473; 910 mm, 0.975; 455 mm, 2 and 1.641). Data from the RAF manual on anthropometric data gives a head "length" (from the base of the nose to the back of the head) of 7.67 inches or 195 mm. I'm assuming that the real geometry is larger on some kind of diagonal but I can't honestly find any information about the assumed working distances so thus can't apply anthropometric data to make this more accurate.

If we then assume 1.2 cal/cm^2 at the back of the head and work forwards to the nose as the assumed working distance, we get 1.45 cal/cm^2 for medium voltage switchgear, 1.81 cal/cm^2 for low voltage switchgear, 2.15 cal/cm^2 for MCC's and panelboards, and 2.45 cal/cm^2 for open air conditions.

Using the greatest "energy drop" and working backwards from an assumed 4 cal/cm^2 at the nose to the back of the head with the greatest decrease in incident energy (open air) to determine a "head size" that would equal 1.2 cal/cm^2, I get a head 375 mm across, or 14.8 inches. This seems a little preposterous to me unless we are assuming a diagonal profile such as an arc flash initiating say at chest level instead of at head level. In this case we can look at it from a geometrical point of view. This would mean that the angle would be the arc tangent of (14.8/7.67) = 63 degrees. This doesn't really make a lot of sense either.

Is there some test data or something validating the idea of not requiring a balaclava until 4 cal/cm^2, as per the 70E-2012 standard?


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 Post subject: Re: Back of head
PostPosted: Fri Sep 26, 2014 10:25 am 
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Location: Port Huron, Michigan
Are you assuming that the person would always be facing the equipment, and the arc flash would travel past the front of the head to get to the back of the head? If so, I think that's a bad assumption. It is very possible someone could have their back to the equipment when the arc flash occurs, in which case the back of the head might be the closest part of the body.

One scenario is that electricians are working in pairs. One trigger the arc flash just as the other has turned to walk away from the equipment.


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 Post subject: Re: Back of head
PostPosted: Mon Sep 29, 2014 7:46 am 
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Voltrael wrote:
Are you assuming that the person would always be facing the equipment, and the arc flash would travel past the front of the head to get to the back of the head? If so, I think that's a bad assumption. It is very possible someone could have their back to the equipment when the arc flash occurs, in which case the back of the head might be the closest part of the body.

One scenario is that electricians are working in pairs. One trigger the arc flash just as the other has turned to walk away from the equipment.


Reading the commentary from OSHA for the 1910.269 update (there is so darned little information on justifications for any working distance models)...

Anthropometric data suggests a hand-to-chest distance of about 15". This number is repeated from multiple sources so is not controversial. OSHA then projects around 10" from chest to nose and then takes the distance to the face from those (pythagorean theorem) to estimate 18" to the face as a working distance. I'm simply projecting from there. Now if you turn since the neck is not capable of rotating 180 degrees the trunk has to be involved and without getting into a really awkward position usually the feet would move to position the whole body further away. Try it and you'll see what happens as you take a half step (T-step) away.

With a two-man scenario there really isn't enough room in front of a lot of equipment to accomodate 2 men (20" width is common) without getting in each other's working space, unless necessary in which case all 4 hands will be about the same distance away. When I observe a two-man scenario, one is generally holding and reading a meter a few inches farther away from the first man. Shoulder-to-shoulder is really awkward (bumping each other).

If one is facing away from equipment and is closer than 18" then there is a high likelihood of a shock, not just an arc flash, from backing into live electrical equipment. The scenario of facing away from equipment while switching as a means of reducing possible danger from arc blast does not really apply because in that case the hand(s) are at the plane at the face of the cabinet, a considerable distance away from the area most likely to experience an arcing fault (busbar or cable connections). For instance MCC buckets are typically about 16" deep (20" total cabinet depth) so the wrist is most likely located at the 18" working distance even with rotary disconnect handles, placing the entire body much farther away than 18".


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 Post subject: Re: Back of head
PostPosted: Mon Sep 29, 2014 8:03 am 
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It doesn't matter if the back of the head is 18" away. It just matters if the back of the head is within the arc flash boundary. Anywhere within that boundary is by definition greater than 1.2 cal, and depending on where you are, you could have a balaclava on and have the back of your head exposed to any incident energy value less than 12 cal/cm^2.


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 Post subject: Re: Back of head
PostPosted: Mon Sep 29, 2014 11:50 am 
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Joined: Mon Sep 26, 2011 2:17 pm
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Location: Waggaman - Cornerstone Chemical Company
Good discussion!

Has anyone heard of any incidents where face shield was worn (without bacalava), arc flash occurred and worker was burned in upprotected areas on the side of the head? Were they facing equipment or did they turn their head at an unfortunate moment?


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 Post subject: Re: Back of head
PostPosted: Tue Nov 18, 2014 5:38 am 

Joined: Tue Nov 18, 2014 5:37 am
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thanks for sharing. good luck


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 Post subject: Re: Back of head
PostPosted: Mon Dec 15, 2014 9:38 pm 

Joined: Tue Nov 18, 2014 5:37 am
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thanks for sharing. good luck




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