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 Post subject: Molded CB Exposed Face
PostPosted: Fri Dec 19, 2008 2:45 pm 
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I'd be interested to know how others are handling molded circuit breakers (150 - 250 amp range), mounted in a switchboard. Load centers are common, where the CB face is partially exposed. The CB can be manually operated without taking off any panels. Technically, there is no exposure to energized parts. However, the CB can fail and the probability of such would be greater when the CB is manually operated. It would seem that an arc flash could propogate through the face of the CB. However, I would like some feedback to know if I'm making more of this, than is reasonable.

Thanks,
Rob
robert.gray@avistacorp.com


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PostPosted: Sat Dec 20, 2008 5:47 am 
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The following link goes to a letter from OSHA and Question 2 in the letter directly answers your question. [url="http://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=INTERPRETATIONS&p_id=25973"]OSHA LETTER[/url] As you correctly point out, there is not shock hazard but they have an interpretation that there is a remote chance of an arc flash.

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PostPosted: Sat Dec 20, 2008 7:11 am 
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Rob Gray wrote:
I'd be interested to know how others are handling molded circuit breakers (150 - 250 amp range), mounted in a switchboard. Load centers are common, where the CB face is partially exposed. The CB can be manually operated without taking off any panels. Technically, there is no exposure to energized parts. However, the CB can fail and the probability of such would be greater when the CB is manually operated. It would seem that an arc flash could propogate through the face of the CB. However, I would like some feedback to know if I'm making more of this, than is reasonable.

Thanks,
Rob
robert.gray@avistacorp.com


Well if you are using the tables, easy one, they have a task listed for xactly what you are talking about. If you did an analysis, then it is a little fuzzy. Look at the definition of arc flash hazard in the 2009 70E, that should answer your question.

OSHA addresses this in 1910.269, has been there for 27 years, if there is a potential arc flash hazard the employees shall not wear meltable fabrics, so at the very least the person operating that MCCB should be wearing 100% cotton clothing.


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PostPosted: Mon Dec 22, 2008 6:44 pm 
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How do we confine this to a usable guide. With electricity, there is always the possibility, as remote as it may be, that an arc fault is possible. So wear do we draw the line?

Is operation of a single gang light switch require non meltable fibers.
How about the 208 3p safety switch, doors closed.

IEEE 1584 has basically stated that arc faults below 240v are statistically remote. So how come in the CB operation, on a less than 240V breaker we can't assume that arc faults are remote and require no restrictions on clothing type.

What about 480V MCC Buckets and Safety Switches, doors closed. Is OSHA saying that any arc source requires PPE, or only those faults that subject the employee to a 1.2 Cal/cm2 blast. In other words, if you can calculate that there is insufficient energy to blow the door open, would a low energy arc inside a MCC bucket or Safety Switch, that does NOT open the deadfront, really cause a thermal risk to the employee.

How far do we take this. Does merely walking into an electric room require thermal PPE?


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PostPosted: Tue Dec 23, 2008 9:41 am 
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I think this interpretation seemed to be heavily weighted on liability rather than reasonable risk.

Using this kind of logic, there is a very very remote chance that a wheel might fall off of my car while driving (there is documentation of this actually occuring to people so it is a real hazard). Based on this hazard I should consider driving at 5 mph. I considered it and I don't drive at 5 mph.

Right now many of the interpretations seem to be made by people covering their legal bases creating some difficult situations for end users in the "real world".

I can't imagine the first person to arrive at the office in the morning that happens to be wearing a polyester shirt deciding not to turn on the lights at the lighting panel since he/she is not wearing non melting fabric. Which is worse, the trip hazard from being in the dark or the remote chance of an arc flash?

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PostPosted: Tue Dec 30, 2008 5:55 pm 
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brainfiller wrote:
I think this interpretation seemed to be heavily weighted on liability rather than reasonable risk.

Using this kind of logic, there is a very very remote chance that a wheel might fall off of my car while driving (there is documentation of this actually occuring to people so it is a real hazard). Based on this hazard I should consider driving at 5 mph. I considered it and I don't drive at 5 mph.

Right now many of the interpretations seem to be made by people covering their legal bases creating some difficult situations for end users in the "real world".

I can't imagine the first person to arrive at the office in the morning that happens to be wearing a polyester shirt deciding not to turn on the lights at the lighting panel since he/she is not wearing non melting fabric. Which is worse, the trip hazard from being in the dark or the remote chance of an arc flash?


So Jim, as part of the IEEE committee on these matters you cannot wrestle them into a corner and force the other members to come up with something workable? :D


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