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 Post subject: 600 volt MCC Electrically Safe?
PostPosted: Mon May 04, 2009 10:21 am 
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Joined: Mon May 04, 2009 10:15 am
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Is a 600 Volt class MCC really considered Electrically Safe per NFPA 70E once you verify the 6 steps outlined in regs i.e. check documents, verify stabs disconnected, check voltage, etc? I think this strange since you will always have line voltage on the line side of the switch and "NEAR" where you are working? The only time it seems it would be electrically safe is with the bucket pulled out of the line or the entire MCC de-energized? Please help with this dilemma.
Thanks!
Mark


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PostPosted: Mon May 04, 2009 1:27 pm 
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Joined: Fri Jan 16, 2009 1:07 pm
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Location: North Florida
Great question Techman. I've had this same discussion with others, and here's what we came up with. It depends in part on the construction of the MCC. Newer MCCs can be bought with a plastic shield that covers the line side of the breaker (or switch) essentially isolating it from contact, e.g. you cannot get a tool or other conductive item into the energized areas. I've also observed that quite a bit of equipment coming of of Europe has similar barriers. The key is to provide some form of isolation so that a conductive object cannot get into the energized line side and cause an arc. Then once you have cleared the switch, you're ok to work.

Older equipment is more of a problem. You often have exposed screws and sometimes even small amounts of stripped cable exposed. In that case, you must protect against both shock and arc flash hazard, so our guys had to do a full Cat 2 suit up to do work in those buckets (that was based on the i.e.). A real pain but necessary since a tool could inadverdently get into the energized equipment and cause a flash. Pulling the bucket while energized is much more risky than wearing the Cat2 gear and providing some type of insulating barrier when (IMHO) when doing troubleshooting or minor repairs. But if you are replacing components like a CPT, it would make more sense to pull the bucket on a down day when you can de-energize the entire MCC to pull the bucket.


TxEngr


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PostPosted: Mon May 04, 2009 7:15 pm 
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Joined: Thu Jan 10, 2008 8:49 pm
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I generally agree with Tx. If the line side of the disconnect is protected with no exposed conductive parts, then you are good to go. You need to be in full PPE until you verify voltage is removed on all three legs. If you have an older design with exposed parts then whole bucket is considered live. Unless of course it is so large, like two or three buckets high, that you are outside of the Restricted Approach Boundary, then I might consider doing it. Not technically correct as you have exposed parts and you are within the AFB, but maybe with Level 2 and some quick calcs for increased distance it might me doable.


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PostPosted: Tue May 05, 2009 9:09 pm 
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Joined: Thu Oct 18, 2007 7:10 pm
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Location: NW USA
It is convenient if you have a main breaker separate from the MCC, such as a transformer secondary main.


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PostPosted: Wed May 06, 2009 11:53 am 
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Seems to be a common sense consensus to the issue. Upon further review I have found our GE 7700 & 8000 series MCC switches in the buckets are isolated/protected from inadvertant contact, as well as our Newere AB centerline MCC's, but our older Allis Chalmers are deinitely a hazard. Now to decide to have a blanket policy to wear the gear all the time doing the tasks or do it on a case by case scenario?

Thanks


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PostPosted: Tue Jun 02, 2009 3:38 pm 
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Since you are going to have the PPE on until you have determined that the equipment is electrically safe why not keep a blanket policy so you are always covered and there is no confusion about the required PPE. Our buckets have a barrier over the line side of the disconnect warning about them being live and preventing direct contact but they don't protect you from dropping tools, bolts and such into the MCC and starting a problem in another bucket (something to consider since people are always dropping things into the MCC). You are also NOT protected against a failure in another bucket, blowing out the door you are working in (I saw a 8ft high fireball shoot out the open door of an MCC when a 75 hp motor starter failed on a buss stab when the motor started).


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