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 Post subject: Hazard Category vs PPE level
PostPosted: Fri Apr 25, 2014 12:29 pm 
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Joined: Mon Feb 03, 2014 7:06 am
Posts: 7
Hi, I would like to understand the "scale" of Hazard category for PPE vs the energy level.

CSA Z462.12 specifies Cat 0, then Cat 1 for min arc rating of 4 cal/cm2, ... Cat 4 for min arc rating of 40cal/cm2

I have read that beyond 40cal/cm2, basically no PPE is capable to protect the arc flash.

Should Cat 4 used for arc energy from 25-40cal/cm2 instead? Similarly for Cat 0, should it be for any arc energy <4cal/cm2?

Thanks


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PostPosted: Sat Apr 26, 2014 5:48 am 
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Location: North Carolina
1. H/RC levels are strictly for use with the tables. If you don't follow the tables, then the H/RC levels do not matter.
2. If you have determined incident energy then you wear PPE to equal or exceed that value. 70E lists PPE required for various levels. For instance below 1.2 cal/cm^2, non-meltable clothing is acceptable. The reason is because the threshold (2nd degree burn) is 1.2 cal/cm^2 so as long as the clothing doesn't stick/melt/burn in and of itself it will not increase the injury. Below 4 cal/cm^2, wearing appropriate shirts and pants or coveralls with a rating of 4 ATPV or higher is acceptable. Above 4 cal/cm^2, the issue then becomes the face so a face shield is necessary along with a balaclava to protect the sides and back of the head. This works up to 12 cal/cm^2. Note that there is an H/RC level specifying 8 cal/cm^2 PPE but most treated cotton industrial work clothes end up rated around 10-13 cal/cm^2 in practice. Above this point multilayer clothing and a full hood becomes necessary. This is all quite well detailed in not only the text but in the annexes. Some practitioners suggest that you rely on the clothing manufacturer to specify a clothing system to shift liability off of yourself.
3. At one time the highest available PPE was 40 cal/cm^2. There used to be a note in 70E that alluded to the idea that 40 cal/cm^2 was some sort of ceiling. It referred vaguely to an arc blast concern. However arc blast and incident energy are completely unrelated to each other. Thus the footnote was removed. There is PPE on the market available up to 100 cal/cm^2 which is in use by some utilities among others. So the old 40 cal "cutoff" is really no longer. At this point there is an informational note in 70E (informational only, not "Code") which suggests that a heightened awareness should be used above 40 cal/cm^2 but it gives absolutely no guidance as to what that means.

In practice where I work, we've put things in place to make it no longer necessary to work above 40 cal and I'm slowly working on strategies to drive this down as low as 10-12 cal/cm^2. It's a slow process since at this point all the "easy" changes have been made. The only practical solutions now require some design changes. One of the worst culprits is very large (in excess of 1500 kVA) 480 secondary transformers. The most practical solution here seems to be to install a breaker on the upstream side with bushing CT's on the secondary side. The primary side is frequently 23 kV though so the trick is coming up with a reasonably priced alternative to fuses. I have found a solution though. There are several reclosers on the market that aren't much more expensive than fuse cutouts and the advantages tremendously outweigh the costs. The only real trick to any of it is to remember to upgrade the LA protection, provide some sort of cutouts for isolation anyways, and figuring out how to disable 99% of the features inside the recloser relays to "dumb them down" to a simple breaker. The latter isn't easy...the manuals for the 651R that I'm working with right now are hundreds of pages.


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PostPosted: Mon Apr 28, 2014 4:04 pm 
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Billy,

See http://www.resourcesandenergy.nsw.gov.au/__data/assets/pdf_file/0008/492722/Matt-Brazier.pdf. This was presented within the context of an Australian mining electrical safety conference but explains some concepts that might be useful.

Matt


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