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 Post subject: PPE below 1.2 cal.
PostPosted: Tue Feb 16, 2016 5:08 pm 
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Do people need to use some kind of face shield while working below 1.2 cal.? NFPA 70E recommends a face shield for projectile protection. Can it be a general purpose face shield? Can it be discarded if using safety glasses? or we need both?


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 Post subject: Re: PPE below 1.2 cal.
PostPosted: Wed Feb 17, 2016 4:36 am 
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Since you are specifically asking about <1.2 cal/cm2, I assume that you have this value from a study that was performed as the Table Method in NFPA 70E-2015 only has a minimum of 4 cal/cm2.

Therefore, Annex H may provide some guidance. It lists Safety Glasses or Goggles for <1.2 cal/cm2. It also lists As Needed a face shield for projectile projection. Since glasses or goggles are required they would be worn under a face shield.

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Barry Donovan, P.E.
www.workplacesafetysolutions.com


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 Post subject: Re: PPE below 1.2 cal.
PostPosted: Thu Feb 18, 2016 2:07 pm 
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Agree with Barry.

Be aware that the doors CAN blow off even below 1.2 cal/cm^2. See "Report on Enclosure Internal Arcing Tests", IEEE IAS Magazine, Heberlein, Higgins, and Epperly, May/June 1996, pp. 35-42. They measured one down to 0.33 cal/cm^2. And even though incident energy and pressure are unrelated their tests and others have shown that there really is no "magic value" below which the doors will always hold. And given the pressures measured in that study a face shield really isn't going to stop much. The best approach is to stand to the side, the old tried-and-true recommendation.


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 Post subject: Re: PPE below 1.2 cal.
PostPosted: Mon Feb 22, 2016 9:39 am 
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PaulEngr wrote:
Agree with Barry.

Be aware that the doors CAN blow off even below 1.2 cal/cm^2. See "Report on Enclosure Internal Arcing Tests", IEEE IAS Magazine, Heberlein, Higgins, and Epperly, May/June 1996, pp. 35-42. They measured one down to 0.33 cal/cm^2. And even though incident energy and pressure are unrelated their tests and others have shown that there really is no "magic value" below which the doors will always hold. And given the pressures measured in that study a face shield really isn't going to stop much. The best approach is to stand to the side, the old tried-and-true recommendation.


Except on Arc Resistant switchgear or motor control center meeting ANSI/IEEE C37.20.7. By definition they should hold the door.

I would think 1.2 would rarely give a door blasting arc. It would have to be a VERY high fault current but there is NO good correlation to blast and incident energy since it is more a function of fault current and enclosure size.

Hugh Hoagland
e-Hazard.com


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 Post subject: Re: PPE below 1.2 cal.
PostPosted: Mon Feb 22, 2016 10:53 am 
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Hugh, I'd agree. However the testing has shown that this is not the case.

First we have the much older Heberlein, Higgins, and Epperly, "Report on Enclosure Internal Arcing Tests", IEEE IAS Magazine, May/June 1996, pp. 35-42.

Second, we have the more recent CIGRE modelling efforts supporting arc resistant gear that showed that effectively no matter what the conditions that they tried, the enclosures failed across the board at less than 1 cycle.

Third, we have the more recent effort, "Exposed to the Arc Flash Hazard", ESW2014-18, Lang and Jones. The paper confirms the conclusions from CIGRE (enclosures rupture within 1 cycle) and further shows that with arc energies as low as 33 kW-s and incident energies as low as 0.3 cal/cm^2, the enclosure ruptures. The only question I have after reading this one is that I'm not convinced that the "kw-s" unit is really the right one. I'd like to see what the results look like if we just took the IEEE 1584 model and removed the distance and time terms. Thus we get log(IE0)=K1+K2+log(Iarc)+0.00110g where g is the gap distance and:
log(Iarc)=K3+0.662log(Isc)+0.0966*V+0.000526*g+0.5588*V*log(Isc)+0.00304*g*log(Isc)

Granted this fixes the distance at 610 mm and the time at 2 seconds but should be proportional to the answer that we need. Otherwise, we could use the Wilkins simplified nonlinear model:
Rarc=1.757*Kw*Iarc^(0.1457-1)*g^0.2476*V^0.4166
Iarc=Vl_l/(sqrt((Rarc+Rsys)^2+Xsys^2))
Iteratively solve for Rarc and Iarc with the initial condition that Rarc=0
Then Parc=3*Varc*Iarc

Kw is a constant. I'm not sure what the value is. This is taken from one of Gammon's papers.


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 Post subject: Re: PPE below 1.2 cal.
PostPosted: Mon Feb 22, 2016 11:10 am 
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What fault currents were they using? Must be VERY high. We have a new blast paper but we were looking at pressure and door expulsion speed per unit area working on a model for predicting when blast will be harmful.

Hope you will be at the ESW. Doing it there. Love to have your input. I totally agree with "stand to the side" but haven't seen too many doors coming off on 1.2 cal equipment in the field. I'm sure it depends on equipment design and fault current.

Hugh


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 Post subject: Re: PPE below 1.2 cal.
PostPosted: Mon Feb 22, 2016 11:54 am 
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480 V testing.

Tests were supposedly run from 10 to 35 kA. However it's not a straightforward comparison because of course being Mersen they were promoting fuses so the actual peak currents were significantly different and the durations were similarly all over the place. The only way I can see analyzing it at this point is normalizing the data for enclosure size and using the peak currents rather than the bolted fault currents to derive some kind of estimate of arc power.


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 Post subject: Re: PPE below 1.2 cal.
PostPosted: Mon Feb 22, 2016 11:57 am 
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Agreed. At Kinectrics, we did peak and closed asymmetrically every time to assure maximum pressure. Interestingly little has to do with copper vapor. Super-heated air caused by the quick rise in fault current is the key.

Of course we aren't selling anything. I just try to do a project each year for the industry to produce a paper for IEEE-ESW to help understanding and give people ideas for research.

Blast was one we felt was getting overblown (pun intended).

Hugh


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 Post subject: Re: PPE below 1.2 cal.
PostPosted: Tue Feb 23, 2016 7:07 pm 
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My experience from working with thermal processes (kilns) is that conduction and convection have almost nothing to do with industrial heating processes. The processes are almost always driven by radiation and heat transfer is proportional to the 4th power of the temperature difference. In this case if we are dealing with a heat source that is somewhere in the neighborhood of 4000-6000 degrees, I'd expect radiation to dominate over just about everything else. This seems to be the case when considering arc flash except when the very thin arc column or superheated gas/plasma is ejected via magnetic propulsion.

The CIGRE studies confirmed that pressures developed by arcs are entirely due to heating the air within the enclosure. There are various "pressure wave" effects that can be detected in certain cases so blast pressure even in enclosed spaces is not uniform. CIGRE was going down this direction to try to confirm whether or not there were minimal room sizes for correct operation of arc resistant gear but the pressure fall off is so great that they never found any measurable hazard.


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