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 Post subject: Stoll Curve And Incident Energy
PostPosted: Tue Jan 31, 2012 3:22 pm 

Joined: Wed Aug 25, 2010 7:15 am
Posts: 24
Location: St. Paul, MN
I need some help in understanding how incident energy is applied to the Stoll Curve. My understanding of the Stoll research and the resulting paper and curve is that the onset of a second-degree burn is a function of time and heat.

If you have heat flux (transfer of heat) at a rate of 1.2cal/cm2 per second and a 1 second exposure, you will have a total heat of 1.2cal/cm2 and have the onset of a second degree burn. But if the exposure time is 2 seconds, a heat flux of 0.73cal/cm2 for a total heat of 1.46cal/cm2 will will result in the onset of a second degree burn. Conversely, for an exposure shorter than 1 second, it would take a heat flux higher than 1.2cal/cm2 and a total heat of less than 1.2cal/cm2 for the onset of a second degree burn.

I think i understand that correctly, but my confusion comes in how we apply the results of an arc flash study. My understanding is that incident energy results from IEEE 1584 are the total heat. We use the incident energy to determine the proper PPE, but if we have total heat of 1.2cal/cm2 in a 0.1 second event that seems like a very different hazard than if we have the same total heat in a 2 second event.

How is the time element of the Stoll curve factored into the hazard determination from 70E, or isn't it?

Any help in understanding this would be really appreaciated.

Thanks.


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PostPosted: Tue Jan 31, 2012 3:32 pm 
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You don't calculate cal/cm² per second. You calculate cal/cm² which is the total heat for the arcing time. If you calculate over 1.2 cal/cm² incident energy, this will result in 2nd degree burns on bare skin.


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PostPosted: Tue Jan 31, 2012 8:25 pm 
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Answer to OP: It's not directly there like you expect.

Alicia Stoll's data is not defined at a POINT. It is a curve. As the time interval decreases, the energy required to induce a burn increases and vice versa. The magic "1.2 cal/cm^2" is only true at one second. There has been some follow up data that I've seen extending it down to around 100 ms but at the regions that are typically of interest for arc flash purposes, typically the time intervals are even shorter than that.

The ASTM method that relies on the Stoll curve on the other hand does actually look at time and looks to see if the energy absorbed by a calorimeter probe located behind the material under test exceeds the Stoll curve or not. The test is run at different cal/cm^2 values (roughly 30 tests) using some tricks so that several readings can be taken simultaneously. Usually about 10-20 data points close to the threshold value are found. The raw readings are then a particular cal/cm^2 and a pass or fail (0 or 1.0) score. These are fit to a sigmoidal curve and the 50% point on that curve becomes the ATPV. Although the sigmoid assumes some sort of gaussian type curve, the actual raw data that I've seen suggests that in a region of roughly +/- about 0.5 to 1.0 cal/cm^2, your chances of failure are possible. Below or above that point, the probability of failure or success is 100%. The sigmoidal curve works from a mathematical point of view only.

So...the PPE case is already covered. That just leaves the "no PPE" (HRC 0) case. When it comes to this case to be strictly accurate you should look up the required cal/cm^2 on Alicia Stoll's data based on the arcing time that the incident energy calculation indicated. However there is another problem with this. When Alicia did her experiments, she started at 1.0 seconds which is a very long time for most electrical gear. I've seen some "extended" data sets though where others have produced results over shorter time periods down to 0.1 seconds. Below that, there's simply no data available.


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PostPosted: Wed Feb 01, 2012 6:29 am 

Joined: Wed Aug 25, 2010 7:15 am
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Location: St. Paul, MN
I don't know much about arc rated clothing testing, and I am not sure I fully understand everything you said, but it sounds like the testing is designed so that an 8 cal/cm2 shirt will protect a person from a 2nd degree burn in an 8 cal/cm2 incident regardless of how long the incident lasts.

That helps a lot. Thanks


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PostPosted: Wed Feb 01, 2012 5:23 pm 
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The fact that you mentioned the Stoll curve sounded like you read the ASTM standsard for clothing which is a big part of how PPE is tested at Kinectrics.

You are correct that the Stoll curve is accounted for with clothing but not "without" it (HRC 0).


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