Arc Flash Boundary Calculation Question

[ThePunisher]

In the calculation of the Arc Flash Boundary, an incident energy of 1.2 Cal/cm2 in the equation as the arc flash boundary is based on the incident energy that would result to 2nd degree burn.

Now, how about if the say a client specifies that all their PPE requirement is Category 2 (8.0 Cal/cm2). Can we change the arc flash boundary based on 8.0 Cal/cm2. I know it is little bit dumb question but a clarification would help me (and others) understand the proper application of the equation.

Thanks

 

[Zogzog]

AFB is by definition where you need to where PPE so I would not re-define it. However, you can do whatever you want, so you could create a "HRC2 boundary" to tell the guys how close they can get wearing only HRC2 PPE. Don't forget HRC2 is not just the 8cal clothing, you need faceshield, gloves, etc...

 

[dpc]

The AFB is clearly defined in NFPA 70E and not really subject to interpretation. The intent is to protect someone who is not wearing PPE.

 

[Skythian]

The infamous 1.2 cal/cm^2 threshold incident energy for a 2nd degree burn on bare skin cited in NFPA 70E and widely accepted in arc flash industry actually comes from misinterpretation of Alan Privettes' "Progress Report for ASTM Burn Study". As a matter of fact, Privettes' work is based upon tests where the test animals were shielded with flame retardant fabrics. Assuming Privettes' findings apply also to bare skin exposure is same as having a confidence you can safely touch hot glowing metal with bare hands as long as you have managed escaping damage by touching it previously while wearing protective gloves.

Indeed, incident energy does not have any risk associated with it. But incident energy coupled with time interval, within which the energy was delivered, does. For example, bare skin exposure to 1.2 cal/cm^2 incident is generally safe and oftentimes beneficial unless the energy was delivered within one (1) second time interval. In fact, only a fraction of 1.2 cal/cm^2 is required to cause the damage when delivered within shorter than 1 second time interval (see Evaluation of Onset to Second Degree Burn Energy in Arc Flash for more information). This may also explain the reason why NFPA group abandoned Category 0 and removed any references to it in new NFPA 70E year 2015 edition.