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 Post subject: MCC Wireways
PostPosted: Mon Jun 02, 2014 6:52 am 
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Joined: Tue Nov 12, 2013 6:31 am
Posts: 238
Location: Port Huron, Michigan
The question has come up in our facility about how to treat the wireways in motor control centers. Most specifically, the vertical trough to the right of the buckets, but also the trough at the top and bottom of the MCC.

We have two different cases in this facility. One is older GE MCC's. In these, there are terminal strips for the starters in the wireways. The other case are new AB MCC's. These isolate the starters from the wireway with plastic shields.

I have not been able to find any mention of this topic in 70E, and an inquiry sent to NFPA has gone unanswered. What do other facilities do, and how should we handle this? Our current policy is to treat the wireways as having the same incident energy available as anywhere else in the MCC. But for a section with nothing exposed this seems a little excessive.


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 Post subject: Re: Arc Flash Considerations & PPE Requirements for MCC Wire
PostPosted: Mon Jun 02, 2014 5:50 pm 
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Joined: Tue Oct 26, 2010 9:08 am
Posts: 2174
Location: North Carolina
This is where a risk assessment comes in. If you are not interacting with the equipment in such a way as to CAUSE an arc flash, and outside the restricted approach boundary or there are no exposed conductors, there is no arc flash hazard.


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 Post subject: Re: Arc Flash Considerations & PPE Requirements for MCC Wire
PostPosted: Tue Jun 03, 2014 5:47 am 
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Joined: Tue Oct 26, 2010 9:08 am
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Location: North Carolina
Keep in mind that this may not be a "rigid" rule. For example if you are planning on sticking fish tape into the wireway or fishing wires through it and there is any risk that it could stick or go down into a blind hole somewhere, especially back into the bus bars, then that would definitely change the assumption about "not interacting". Similarly if you have any cables that have overheated or might be overheated, or are old/decayed to the point where the insulating might have gotten so stiff that it just crumbles and falls off, again obviously that changes things. Same with the possibility of dirt/mud/water, especially in an area with grinding or cutting operations where metal powders or flakes are present may change your assessment considerably.

This is why OSHA is suggesting in the example risk assessment in the new 1910.269 rules, and the 70E committee has similar language in the draft 2015 version, that you need to consider equipment condition as part of the assessment. As an example, OSHA has the following criteria for "normal operation" of breakers:

1. The equipment is properly installed and maintained.
2. There is no evidence of arcing or overheating.
3. Parts of the equipment are not loose or sticking, and the equipment does not otherwise exhibit any signs of lack of maintenance.

In contrast, 70E-2015 draft's list of criteria are:
1. All of the equipment is properly installed and maintained.
2. All doors are closed and secured.
3. All covers are in place and secured.
4. There is no evidence of impending failure.

Personally, I recognize that the first criteria (properly installed) can be determined ahead of time. The "properly maintained" question could also be determined ahead of time, except that the available "definitions" for this (NFPA 70B and NETA MTS) have a lot of maintenance items that are unrelated to safety aspects of the equipment. Item #2 and #3 from OSHA's list are very specific and clear. I like them much better than item #4 in 70E's list which is pretty vague and subject to interpretation. Also 70E is consumed with the idea that all the doors have to be latched for breakers in spite of the fact that IEEE 1584 does not give "credit" for latched doors. Although 269 rules include a lot of "open" outdoor style equipment, I don't see where there is much difference from a functional point of view. The definition of "exposed" would come into play in my opinion in triggering a need for shock (and arc flash) PPE.

I would also suggest three more critiera that could be used and probably should be in the list that I use on my list:
1. No visible evidence of environmental contamination such as visible water, mud, oil, or signs of corrosion.
2. No visible evidence of mechanical damage such as bent doors, failed support structure,
3. Equipment has not previously faulted and been released back to service.


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