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 Post subject: Does an arc flash hazard exist at less than "HRC 1"
PostPosted: Fri Feb 06, 2015 10:36 am 
Sparks Level

Joined: Mon Jul 26, 2010 10:55 am
Posts: 65
Had an interesting conversation the other day. The debate is basically my subject line ---

Does an arc flash hazard exist at less than "HRC 1"?

The answer I got from the powers that be surprised me. Maybe I'm missing something though. Thanks for any input.


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 Post subject: Re: Does an arc flash hazard exist at less than "HRC 1"
PostPosted: Fri Feb 06, 2015 11:24 am 
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Joined: Tue Jan 13, 2009 5:00 pm
Posts: 524
Yes.


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 Post subject: Re: Does an arc flash hazard exist at less than "HRC 1"
PostPosted: Fri Feb 06, 2015 12:04 pm 
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Joined: Mon Sep 17, 2007 5:00 pm
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Location: Scottsdale, Arizona
stevenal wrote:
Yes.

Adding to Stevenal's "yes" here is a bit more.

I have heard many people through the years (some that would surprise you) say that an arc flash hazard does not exist for:

Category 0
HRC 0
Less than 1.2 cal/cm2
Circuits using the 125 kVA exception

As Stevenal pointed out "YES" an arc flash hazard still exists, even for the cases listed above. The severity of the arc flash is less for these cases but there is still a hazard.


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 Post subject: Re: Does an arc flash hazard exist at less than "HRC 1"
PostPosted: Fri Feb 06, 2015 12:11 pm 
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Joined: Mon Jul 26, 2010 10:55 am
Posts: 65
Thanks for the input. Now I just need to nudge others into our understanding. Unfortunately we dove head first into 70E a few years ago and continue to deal with difficulty on keeping a common understanding of things. Human nature is a good and bad thing all at the same time. :D


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 Post subject: Re: Does an arc flash hazard exist at less than "HRC 1"
PostPosted: Sat Feb 07, 2015 6:33 am 
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Joined: Tue Oct 26, 2010 9:08 am
Posts: 2174
Location: North Carolina
70E is based on a somewhat arbitrary standard. Based on burn data the threshold for fatal burn inuries occurs with 2nd degree or greater burn injuries to the face and chest area. This occurs at 1.2 cal/cm^2 at 1 second. PPE is measured using cloth samples facing a test arc and looking for the threshold where it either breaks open or the measured heat flux matches the Stoll curve which is 1/2 again at 1 second. 70E standard for arc hazard boundary is thus 1.2 and the IEEE 1584 calculation estimates the incident energy at the face/chest area which is why although human ergonomics says there is about 15 inches from the work area to the body, the face/chest is elevated a little and farther away at 18 inches.

Hands are clearly much closer. And even if wearing the PPE specified,a first degree burn to the chest/face may still happen. And the IEEE estimate only givesa numerically determined 95%success based on the test data that was used in developing the IEEE 1585 empirical equation. So in summary the 70E standard is a 95% chance of avoiding a fatal injury. It in no way is based on avoiding minor or even serious burn injuries.

Second issue is that the 1.2 number is basedon on bare skin. Natural fiber clothing chars which is relatively harmless but synthetics melt and can ignite which tremendously increases the injury. So avoid “work dry“poly blend shirts, Under Armour, nylon insulated jackets and overalls, and most of the Hi His stuff. Cotton, wool,silk, and leather are fine. This means gloves, winter wear, hi his, and a lot of shirts should be checked very carefully. There are even cases where workers have been severely burned by wearing this stuff under or over arc rated PPE.

So in summary, less than PPE 1 aka HRC 1 aka 1.2 cal/cm^2 is no guarantee of no burn injury.


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 Post subject: Re: Does an arc flash hazard exist at less than "HRC 1"
PostPosted: Mon Feb 09, 2015 1:52 pm 
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Joined: Mon Jun 07, 2010 9:40 am
Posts: 119
Since you used it more than once, I assume that "Hi His" isn't a typo, but what the heck is it.


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 Post subject: Re: Does an arc flash hazard exist at less than "HRC 1"
PostPosted: Tue Feb 10, 2015 1:22 pm 
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JKlessig wrote:
Since you used it more than once, I assume that "Hi His" isn't a typo, but what the heck is it.


Its a spell checker in Android automaticlaly correcting me without even asking. "Hi vis" or high visibility clothing is the correct term.


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