Arc Flash clothing class

[lile001]

> 40 Cal/CM

> 40 Cal/CM

I see 100 cal/cm? clothing advertised, but I don't see how that is possible.

You could probably make an arc flash suit that could be rated to 1000 cal/cm, however an arc flash also produces blast effects, and possibly shrapnel. NFPA's resoning was that above 40 cal/cm, there is likely to be shrapnel, blobs of molten metal and blast effects, and the poor guy might come out unburned but fulla holes.

 

[zog]

Wearing a 100cal suit in a 100cal arc flash would probally only allow for an open casket instead of a closed casket.

 

[CarRamrod]

The 2006 Canadian Electrical Code requires field markings for arc flash hazards. It also refers the reader to NFPA 70E and IEEE 1584 for addressing the arc flash hazard. Thanks to the Americans for putting out NFPA 70E.

I have recently learned how to do arc flash studies using SKM. Depending on the situation, 600 V buses can be the most dangerous areas of the plant by far. The problem is that arcing faults do not necessarily have the the high currents a solidly connected fault would have, particularly at lower voltages. Also, the length of cable from the supplying transformer will bring the fault current down too. So, you get fault currents which are still quite high, but not high enough to instantaneously trip the breaker feeding the bus. When the arc time starts to get up in the hundreds of milliseconds, the incident energy level will be high enough to require Category 3 or 4 PPE. Equipment at ~600V which is upstream of the bus breaker but downstream of a transformer should never ever be worked on live.

Can there be dangerous levels of incident energy on higher voltage levels? Of course. But how often do you work on 13.8 or 25 kV equipment when it's energized? Hopefully not too often. So 600 V and below is the stuff to watch out for, because of the misconception that it can't hurt you as much.

 

[kingpb]

Whats interesting is that in all this discussion of Arc Flash, I don't recall reading anybody making a single reference to IEEE 1584.

Everyone who is involved with the electrical calculations, or required to interrupt the calculations needs to read this standard.

Currently, IEEE and NFPA are working together to help NFPA remove some of the discrepancy between the two documents.

Also, a little history; NFPA felt that although a higher PPE suit may be available, it was deemed to be to much of a hazard because of how restrictive and bulky it was, and therefore the felt workers would not be inclined to wear it. Therefore, they stopped at 40. NFPA does not address arc blast and therefore assumptions made herein regarding arc blast killing you when going over 40 cal/cm^2 is conjecture, and mis-leading.

 

[zog]

kingpd wrote "Also, a little history; NFPA felt that although a higher PPE suit may be available, it was deemed to be to much of a hazard because of how restrictive and bulky it was, and therefore the felt workers would not be inclined to wear it. Therefore, they stopped at 40. NFPA does not address arc blast and therefore assumptions made herein regarding arc blast killing you when going over 40 cal/cm^2 is conjecture, and mis-leading."

I disagree with this statement, here is a quote from the 2004 NFPA 70E Handbook 130.7(C)(5) "Some FR clothing is rated for use with incident energy exposures up to 100cal/cm2. The 70E technical committee determined that although it might be possible to protect a person from such extreme thermal exposure, the clothing would be unlikely to protect the worker from the effects of the accompanying pressure wave."

Your statements on the 1584 are right on the money, I believe the 1584 method is superior to the 70E tables and equaltions for most instances, the 70E methods are just easier.

 

[zmikc115]

I have been extremely busy with a new project and havent been able to visit the site. Thanks to everyones reply and I appreciate the help. I probably should start a new subject but if anyones looking we have just completed a electrical safety policy and I would like someone with some expirence in this area to critique the policy. I am not sure how bring the policy over to this site or if you can do that I am going to look and see but if I can I would welcome any thoughts to make this a safe and workable policy. Most of it references the 70E and some we have customed.

 

[coulter]

Its okay to read all of 1584 and 70E

Its okay to read all of 1584 and 70E

Lots of statements made here where the reasoning eludes me.

1584 talks about not being able to get an arc to reliably stay lit below 240V. 1584 also says one can discount any 240V (208V?) panels fed by transfromers less than 125kva. Also see annex B about the two second rule.

I'm fighting one right now where the tag says 34cal/cm^2. The panel is 240V, 1ph, fed with a 37.5kva, 5.7%Z. The book says the electricians have to use a moon suit, haz cat 4, for the dead bus verification - which doesn't please the electricians a lot.

So why is the tag so high? Well, with a low available short circuit current, the secondary cb takes a long time to trip - maybe 10s of seconds - I couldn't find a trip curve. So, if one could get an arc to sustain after one removed the errant scewdriver, then even though the rate of energy release is low, the time is long, which makes the cal/cm^2 high.

Dangerous? I think less risk than driving to work. My calcs say it is a haz cat 0 or a haz cat 1. 70E defaults to a haz cat 0. However, until we can get the engineering company to fix the model to include the 125kva rule and the two second rule, we are stuck with the tag requirments. Our management did agree to send me the model to review - maybe they will.

So where am I going with this?

70E defaults work fine, but, you have to read the notes, and do enough investigation to apply the 70E defaults with in the limits of the model - in which case they work good.

Above 40 cal/cm^2 the pressure wave may mangle you. It depends on the SSC and the model. 40cals over 2 seconds is much less of a blast compared to 40cals in 50 ms. However, other than doing a dead bus verification, or ground clamp application on MV, why would one be in high SCC equipment. Yes, I know that production always has reasons.

Don't discount that low voltage (240V), low SCC equipment with high cal ratings may well be the result of poor modeling.

carl

 

[jim dungar]

The panel is 240V, 1ph, fed with a 37.5kva, 5.7%Z. carl

I am not aware of any method for calculating arc falsh incident energy on any single phase system.

 

[coulter]

Jim -

In one way that's too bad. Cause if you did, you would be able to educate me and I always like that. However, you're right, 1584 says it is for 3 phase. But then again, I haven't seen the calculator that can be had for the 1584 model.

Could make one wonder how the engneering firm, filled with illustrious PEs, came up with a number.

As I said, maybe I'll get to look at the model.

carl (PE, EA, other alphabet soup)