# Arc Flash PPE Category



## Wirenuting (Sep 12, 2010)

Welcome to ET

Please finish filling out your profile,, “About Me” section. 


AF protection??? Kinda like a well done hotdog amount?


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## Zog (Apr 15, 2009)

cbeckmeyer said:


> New to the forum... Have a question about sizing PPE per NFPA 70E. Working in a commercial building that doesn't have an Incident Energy Analysis completed. Now I know there is the other table category method for sizing PPE, but that also requires knowing the available fault current and clearing times. The panel that we are working in is fed from a insulated case circuit breaker with a bunch of adjustable trip ratings. My question is, shouldn't i be able to determine the maximum available fault current that said breaker will allow through by looking at one of the settings on that breaker? Therefore allowing me to size my PPE using the Arc Flash category method of NFPA Table 130.7(C)(15)(a)?


No, you need to know the available fault current, that has nothing to do with the trip settings on the breaker. The facility may not have an arc flash analysis done but I am sure they have a short circuit study somewhere??


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## Zog (Apr 15, 2009)

Also, what work are you doing that you need PPE for?


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## paulengr (Oct 8, 2017)

Zog said:


> No, you need to know the available fault current, that has nothing to do with the trip settings on the breaker. The facility may not have an arc flash analysis done but I am sure they have a short circuit study somewhere??


Short circuit study is not what you need. The coordination study is what is needed.

The tables are there in case you don't have any other information. Realistically the claims about available fault current and opening times come from a coordination study, not just breaker settings but transformers and wiring too. It's hard to do anything quick and dirty. There are some ideas how to do it but nothing concrete yet.

If you knew that information you wouldn't bother with the tables because you can even get a spreadsheet that IEEE put out that calculates incident energy for you. So if you don't know, use the tables. That's what they are for.

The footnotes are there to make the academics happy. And I've got news for you...The engineering method has serious problems, too. Don't let some engineer fool you into thinking they know what they are talking about. Want a simple example? So the incident energy is the calculation at the face/chest. The PPE rating matches that. Incident energy roughly doubles if we cut the distance in half. So what is the rating on the arms? What about the hands? The whole thing is a totally fictitious and questionable model that just happens to work. The track record is really good and that's what we're after.

Let's consider reality for a minute. Several years ago some guys at DuPont collected a bunch of arc flash cases, almost 60 of them. Out of those in cases where the proper PPE was worn it worked 100% of the time. In cases where they still used arc flash PPE but it was underrated, it still worked 50% of the time. In cases where nothing was used 15% of the time they still walked away. So if you follow the tables your odds are somewhere between 50 and 100%, and tending more towards the 100% side of things. And this is when shock injuries are twice as likely and all electrical injuries make up 0.4% of industrial accidents. So you can see why the table based method is far better than nothing and overall realistically not really all that bad. When those cases were collected we still used the H/RC tables that have some glaring issues so things have gotten much better since then.

As to why settings don't "matter"...when you have an arcing fault the current is what creates arc power. The breakers don't limit the current. Breaker settings only determine how long it can arc before the breaker shuts it down. You know the relationship (time to trip if you know the current) but not the amount of current. And the short circuit study just does a gross approximation to make sure the equipment will withstand a short circuit. It does not take into account line resistance so it is much higher than the real world. Curiously this might seem like it would give you a high but safe incident energy but it doesn't! It actually underpredicts because as the current goes down because of the properties of inverse time curves in breakers the time to open increases at a much faster rate so the energy (arc power times seconds to open) actually goes up.


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## joebanana (Dec 21, 2010)

Priorities matter. By now an IEA should have been completed. There's no good excuse not to. You have no reason to work on that equipment, until you can do it safely.
I'm pretty sure OSHA will back you on that.


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## MechanicalDVR (Dec 29, 2007)

Welcome aboard @cbeckmeyer!

What work are you looking to do there?


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