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 Post subject: Draw Bench PPE
PostPosted: Mon Feb 24, 2014 9:45 am 
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Joined: Tue Jul 07, 2009 1:20 pm
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Location: Lima, OH
Hi all,
I'd like to get your opinion on a question asked of me by a client. They have a draw bench type enclosure that houses a molded case circuit breaker and a number of VFDs. They have need to periodically access the VFDs for adjustment. To get at the drives, you have to defeat the interlock on the door covering the circuit breaker before the doors to the drives can be opened. At the current time, (we are working with them at to try to reduce the arc flash) the equipment requires level 3 PPE (8.7 cal/cm², 5 foot arc flash boundary). Some of these draw benches are on a mezzanine level that can only be accessed via ladder. The arc flash labels are representing the hazard level only and not risk. Adjustment of the VFDs appears to them to be a low risk operation. They are wondering if the lower risk level would enable them to access the panels with less than an arc flash suit, since the arc flash suit would pose a certain level of risk itself. So what do you all think?
Roger


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PostPosted: Mon Feb 24, 2014 10:20 am 

Joined: Sat Feb 19, 2011 3:44 pm
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I would look into remote mounting of the drive keypad. Put it in an easily accessible location.


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PostPosted: Mon Feb 24, 2014 11:59 am 
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70E specifically talks about 600 V class MCC's. Two tasks that are mentioned with an H/RC 0 rating are reading a panel meter while operating a meter switch. In the section on panelboards and other equipment rated 240 to 600 V, it states H/RC 0 when opening and closing hinged covers. There were about 8 cases that OSHA investigated from 2007-2011 which involved removing or installing bolted covers (not hinged) which resulted in an arc flash, but none involving opening and closing hinged covers.

In my mind without seeing what you have, it would be a judgement call. I've seen similar equipment but not sure what a "draw bench" refers to. This sounds like an industrial control panel. Many older panels have exposed conductors, frequently even outright exposed bus. Others, especially newer types, are specifically designed with almost no exposed conductors. Most buses are covered and energized screws and such are recessed (guarded). It is frequently possible to add fiber board or Lexan or similar materials to an older panel to eliminate exposed bus work.

If there are exposed conductors then there is a risk of creating an arc flash, most likely by accident. Thus even if the exposure is small, I'd definitely be in the camp of looking at arc flash PPE. If however this is not the case then it becomes a judgement call. It should be solved via a team which does the risk assessment including safety department, electrical department, production, management, etc. There are standardized risk assessment methods out there. I'd suggest looking into the RIA (Robot Industries Association) risk assessment methodology for four reasons. First because it is very easy to use. Second because it doesn't leave a lot of items open for interpretation. Third because it is a very good fit with industrial machinery analysis (it is not limited to just robots). And finally because it is an ANSI standard and thus a consensus safety standard.


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PostPosted: Tue Feb 25, 2014 7:05 am 
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Location: Charlotte, NC
You need to remotely mount the keypads, preferably not on the door of the enclosure in front of the drive.

I'm not sure whose drives you are talking about. We generally use Rittal enclosures on our systems which are vented for the filters and cooling fans. Remember that enclosures only provide a shock barrier, and do not provide protection in regards to Arc Flash.

You might want to look into an HMI for the system that can be remotely mounted at the factory floor level and provide access to all the parameters on the all of the drives you might want or need to adjust.


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PostPosted: Fri Feb 28, 2014 8:50 am 
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Thank you all so much for your responses! What I was trying to communicate was that although there are many different types of mitigation strategies available (we are working on that) all that takes time and money. In the meantime, their workers still have the necessity to regularly adjust the drives. So we have to come up with a strategy that allows them to do that safely until such time as other mitigation strategies can reduce the arc flash exposure through setting changes, maintenance switches, remote displays, etc. What concerns me, is having the workers either wearing an arc flash suit or dragging the arc flash suit bag through the ladder access to the mezzanine. A possible increase in hazard situation I would think. I've attached pictures of the draw bench. The first one shows the general organization with the right most door containing the interlock safety. The second picture shows the line side (behind the rightmost door) with circuit breaker and fuses. The third picture shows the leftmost two doors containing the actual Emerson drives. Thanks again.
Roger


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PostPosted: Fri Feb 28, 2014 8:58 am 
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Those pictures look pretty standard for drive enclosures to me. If the calculation is for 8.7 cal/cm2, than that is the level you should be protecting to if you are opening the cabinets while they are energized. Frequently companies will round anything over 8 cal up to a blast suit, but that isn't necessarily necessary. It's possible to get shirt/pants (or coveralls) and balaclava/face shield combinations that all exceed 8.7 cal.


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PostPosted: Fri Feb 28, 2014 1:10 pm 
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Other than the phone of that little Omron unit, I don't see anything that I would consider exposed in the photo of the drives and the PLC. I still don't see anything here though that would concern me as interacting with equipment in a way that would cause an arc flash unless there is something exposed (not guarded, isolated, or insulated) that is not visible in the photos. If it is, then a little creativity with some lexan, insulating fiberboard, or rubber blanket material should easily take care of that. There is clearly nothing to get pinched in the doors while opening or closing them. That just leaves the possibiity of bumping into something.


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PostPosted: Mon Mar 03, 2014 8:52 am 
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Thanks. That's pretty much what I was thinking


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PostPosted: Mon Mar 03, 2014 9:12 am 

Joined: Fri Aug 10, 2012 11:47 am
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Also, I'm guessing that the 8.7 cal/cm2 is only at the line side conductors on the main circuit breaker. If you do the calculation taking into account the main circuit breakers and/or branch fuses I'm betting that your incident energy will be far far less for any downstream components. Assuming the line side conductors/terminals are well insulated and no work is being done in that vicinity I don't see much of an arc flash hazard. I would be more concerned about the shock hazard, some lexan covers over the fuse blocks would be nice.


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