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 Post subject: 208V, 1600A switchgear
PostPosted: Fri Apr 25, 2014 10:22 am 
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Joined: Mon Apr 14, 2008 7:10 am
Posts: 142
There is a 208V switchboard in the system that was rated at dangerous (AHC) due to the high incident energy on the line side of main breaker which is direclty fed from a big transformer. The contractor is to replace some breakers on the load side of main breaker (branch circuit with some sort of barriers). They will do rack in and out the breaker when the board is energized, however, there is no 2nd lable on that switchboard indicating the hazard level. Since that was a hospital project, it is almost impossible to shut down the service to do this work. We tried to follow the table 4A to find out if there is any Hazard category we can choose for this task, however, other than "panelboard or other equipment rated 240V and below", there is nothing for the 208V switchgear specifically. Can we just use the table for the panelboard rated at 240V and below for the 1600A, 208V switchgear instead? How come the table doesn't have the info. for the 208V switchgear?


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PostPosted: Sat Apr 26, 2014 7:16 am 
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The intention is that the table would apply to anything rated 240 V and below because it took some conservative ratings. Note however that you probably can't use the table anyways. Look at the maximum short circuit rating and the trip time on the table. Since it is a switchboard/panelboard, you can't rely on arc propagation not to occur and carry over to the transformer secondary side where your only protection is from the high side of the transformer so this is the correct way to model it. Still since at 480 V, I've found that generally speaking over 1800 A it is prohibitively difficult to reduce arc flash ratings to something reasonable even with breakers, fuses, etc., what you are describing is certainly going to have an arc flash rating so high that it is not practically possible to do much with it other than just accept it. Couple this with the fact that you are talking about "racking" breakers. 80% of draw out breaker failures occur while inserting or removing the breaker from the cell and since you also said it is a switchboard by this I'm assuming you are talking about physically bolting/unbolting them which is to say the least, the whole reason that the EEWP paragraph was created in the first place...to inform you that this is so dangerous that unless someone can show a greater hazard exists by doing it another way, you de-energize and in this case, de-energize on the primary side of the transformer, never the secondary, unless the equipment has been regularly serviced and maintained, which I highly doubt has been happening. If you search through the data and statistics on the American OSHA web site ([url="http://www.osha.gov"]www.osha.gov[/url]), you will find a nearly identical case that occurred at a hospital where there were multiple fatalities and hospitalized injuries from doing almost exactly the same tasks you are describing.

The energized work permit MUST be signed by the owner/manager and they need to be informed that basically if anything happens, anyone involved will likely be either spending several months in a burn unit at the very least and it is potentially fatal. The person signing off needs to provide adequate justification that OH&S would accept as a greater hazard compared to de-energizing.

And while you are at it, I would strongly suggest finding a way to reconfigure the equipment to get the ratings more reasonable such as replacing the monster transformer with smaller ones, installing arc flash relays, installing arc termination devices, going to high resistance grounding, differential protection, etc. There are technological solutions to this and not all of them are impossibly expensive.

The one profound difficulty in all of this is that calculations at low voltages is still not that great so the real ratings will be less than predicted by currently available fault models but no one knows by how much and given the amount of current (you haven't even mentioned the fault current level), the model discrepancies probably don't even matter.

If you use the tables anyways, be aware that a recent historical study based mostly on injury data from Dupont found that the tables work only about 50% of the time in the event of an arc flash. They are better than nothing but you have enough information to go well beyond the tables and obviously someone previously already has done so, hence the "prohibitive" rating (whatever that means). Almost all the incidents that occurred at my current employer (Potash Corporation of Saskatchewan, PCS) occurred while working on either MCC''s or switchgear and involved racking things on/off buses or operating breakers that had not been maintained in years.


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PostPosted: Mon Apr 28, 2014 6:17 am 
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Very helpful! Thanks, Paul!


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PostPosted: Thu May 01, 2014 7:29 am 
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I think that the safety is very important, however, too conservative is not a good practice either. To me it makes sense to shut down the switchboard if you want to do racing or do any work when chance is that you may exposed to the bus. However, to operate a branch breaker (well maintained) in that kind of switchgear should be less hazard even the line side of the main breaker is rated at dangerous. The code should provide some instruction on this.


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PostPosted: Fri May 02, 2014 4:46 pm 
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The 2015 edition of the Code addresses this. It specifically will state that arc flash PPE is not needed for breakers that are being operated that have been properly installed and maintained and don't show any signs of impending failure. State-site, OSHA includes specific signs to look for (signs of overheating or environmental damage) beyond a vague "impending failure" statement. These terms will be part of an appendix to their update to regulations for utilities starting in 2015 as well.


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