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 Post subject: im a student, not an electricianPosted: Fri Jan 29, 2021 8:11 am

Joined: Fri Jan 29, 2021 7:55 am
Posts: 1
as it says, im a student, not even studying to become an electrician, and even though this is a blow off question on an assignment i think the nfpa 70e guide lines are something i should be somewhat knowledgeable on, (im going into wind tech, and i value myself) is there an equation or chart from osha or nfpa as to how the boundaries are obtained, and if so, are they always concrete, and this is slightly off topic, but im also having a hard time figuring out the specs for arc flash rated face shields/ helmets, whats the tint for every cat level, what differentiates a cat 1 shield from a cat 4, ive looked other places, but it seems like people trying to sell me their ppe, sometimes the ppe doesnt have a helmet so that seemed off, i just need some guidence and some terms i actually understand, thanks

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 Post subject: Re: im a student, not an electricianPosted: Sat Jan 30, 2021 10:38 am
 Plasma Level

Joined: Mon Sep 17, 2007 5:00 pm
Posts: 1700
Location: Scottsdale, Arizona
It is good to understand the hazards of electricity including the arc flash and electric shock hazards.

There are multiple boundaries involved.
For electric shock there are two boundaries - the Limited Approach Boundary (LAB) and Restricted Approach Boundary (RAB). These are fixed boundaries and defined in NFPA 70E Table 130.4(E)(a) and (E)(b). As an example, for 480 volts, the LAB is 3 ft. 6 inches for fixed equipment. For the restricted approach boundary at the same voltage, the RAB is 12 inches. Each boundary triggers specific requirements.

The Arc Flash Boundary (AFB) can vary depending on the severity of the arc flash which is defined in terms of incident energy in calories/square-centimeter. Incident energy equal to or greater than 1.2 cal/cm^2 is where arc rated protection is required.

The arc flash boundary is a distance from a prospective arc flash (when the hazard exists) where the incident energy drops to 1.2 cal/cm^2. Incident energy drops with distance. (NFPA 70E has a more formal definition) This a calculated value and the 2018 Edition of IEEE 1584 is used. It's not a straight forward calculation.

NFPA.org has a FREE online NFPA 70E viewer you can search for and use to view the document.

Brainfiller Technical Articles and Brainfiller Videos

Also, I have a few free classes where some of this is discussed at:
Brainfiller Online/On Demand Training

Other's here at the Arc Flash Forum can also be quite helpful in sharing their knowledge.

It's a great subject - good luck!

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 Post subject: Re: im a student, not an electricianPosted: Mon Feb 01, 2021 5:11 pm
 Sparks Level

Joined: Wed Jun 10, 2009 5:00 pm
Posts: 267
Location: Toronto
Jim Phillips (brainfiller) wrote:
Incident energy equal to or greater than 1.2 cal/cm^2 is where arc rated protection is required. The arc flash boundary is a distance from a prospective arc flash (when the hazard exists) where the incident energy drops to 1.2 cal/cm^2. Incident energy drops with distance. (NFPA 70E has a more formal definition) This a calculated value and the 2018 Edition of IEEE 1584 is used. It's not a straight forward calculation

It is important to note that the threshold incident energy to 2nd degree burn is not a constant 1.2 cal/cm2 value adopted by NFPA 70E and hard-coded in the most recent IEEE 1584 year 2018 guide for arc flash calculations but a variable function of heat flux. While it is equal to 1.2 cal/cm2 when the energy is delivered at the rate of 1.2 cal/cm2/sec for a one (1) second, it could be a small fraction of it when the energy is delivered at greater intensity which is usually the case with an arcing fault. Alice Stoll specifically warned against using the critical thermal load in heat damage studies but unfortunately this fact had escaped the attention of modern arc flash safety standards. When it comes to a numerical examination, ARCAD arc flash hazard analysis software is the only program capable of factoring in the variable nature of threshold incident energy when calculating arc flash boundary using the IEEE 1584 methodology.

_________________
Michael Furtak, C.E.T.

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 Post subject: Re: im a student, not an electricianPosted: Tue Feb 02, 2021 9:38 am
 Plasma Level

Joined: Mon Sep 17, 2007 5:00 pm
Posts: 1700
Location: Scottsdale, Arizona
It is important to note that the threshold incident energy to 2nd degree burn is not a constant 1.2 cal/cm2 value adopted by NFPA 70E and hard-coded in the most recent IEEE 1584 year 2018 guide for arc flash calculations but a variable function of heat flux.

Yes, there has been quite a bit of debate over the years about whether 1.2 cal/cm^2 is actually where a second degree burn injury begins. Because of this, the 2018 Edition of IEEE 1584 only references 1.2 cal/cm^2 and not the second degree burn injury.

The 2021 Edition of NFPA 70E also only references 1.2 cal/cm^2 in the arc flash boundary definition. However NFPA 70E does references the Stoll skin burn injury model and second degree burn but it is in an informational note that is not part of the mandatory part of the standard.

There are two carry over locations that still need changed (my opinion). They are 130.7(C)(6) Body Protection and a note in Table D.2.1 in the Annex (not a mandatory part of the standard) Hopefully these stragglers may change in 2024. Thanks for bringing it up!

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