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 Post subject: Overdutied Breaker and Arc Flash
PostPosted: Sun Nov 23, 2008 10:34 am 
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I have run into the following situation and was wondering if other people have and what they did.

In an industrial facility, on a 480V system, I have found some breakers that are overdutied for the bolted short circuit. Obviously, these need to be replaced but in the meantime this is not something that will happen in the short term. The AFH for the equipment feed by these breakers is a Category #1, due to the fact that the arcing current is less than the bolted fault current. This bolted fault current is less than the breaker withstand rating.

So, what to label the AFH as? Using the arcing current for the AFH, the breaker can safely interrupt the current, but if it is a bolted fault the breaker will be overdutied and not interrupt the current.

I have my thoughts but would like to hear everyone else's opinions.

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PostPosted: Sun Nov 23, 2008 2:22 pm 
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wbd wrote:
The AFH for the equipment feed by these breakers is a Category #1, due to the fact that the arcing current is less than the bolted fault current.


Huh?, it always is and has nothing to do with the catagory. Maybe I am not following what you are trying to say but that dosent make any sense. Sometimes lower arcing current result in higher AFH's due to the incleased clearing times.


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PostPosted: Sun Nov 23, 2008 2:31 pm 
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Sorry for the confusion. The point I was trying to make is that the bolted fault current is greater than the short circuit withstand rating of the breaker. However, the actual arcing current, used for the AFH calculation, is less than the short circuit rating of the breaker. Using this breaker, even though for a bolted fault is overdutied, the AFH is Category #1. If this breaker has to be ignored, due to the overdutied bolted fault condition, the AFH will be a Category #4.

So my question is, can this breaker be used for the AFH since the arcing current is within the breaker short circuit withstand, even though it is overdutied in a bolted fault condition?

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PostPosted: Sun Nov 23, 2008 4:16 pm 
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I do believe I understand what you are asking and I would think that the 2 operating conditions are normally separate issues. One is a design issue and the other is a maintenance safety issue with the AFH. I would think that you must assume that the upstream protective device operates as designed, but this one is being asked to do more than that.

That being said, in this case I think you face a more serious issue with a circuit breaker that is exposed to fault duties in excess of its rating. We probably do not know how long this has existed and how many rated or over rated operations this equipment has been subjected to and managed to survive (to this point). I would be concerned about its immediate suitability for continued service and would make sure that the owner is advised of the danger of not finding the funds to correct the problem immediately. In fact I would not wait to complete the report, but would notify the owner in writing immediately.

After all, if you cannot count on a device operating properly to limit the energy, what is the study worth? I know you say that the arcing current is less than rated for the device, but its capability could very well have been comprimised by now.

I would certainly not close that breaker locally at this point!

Going back to the next device upstream could certainly help convince the owner to correct the problem as well as detailing it in your report.

Alan


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PostPosted: Mon Nov 24, 2008 8:06 am 
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wbd wrote:
Sorry for the confusion. The point I was trying to make is that the bolted fault current is greater than the short circuit withstand rating of the breaker. However, the actual arcing current, used for the AFH calculation, is less than the short circuit rating of the breaker. Using this breaker, even though for a bolted fault is overdutied, the AFH is Category #1. If this breaker has to be ignored, due to the overdutied bolted fault condition, the AFH will be a Category #4.

So my question is, can this breaker be used for the AFH since the arcing current is within the breaker short circuit withstand, even though it is overdutied in a bolted fault condition?


Ok, now I get what you are asking, and I would have said the same thing Alan did.


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PostPosted: Mon Nov 24, 2008 9:48 pm 
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The overdutied breaker needs replaced. Do not rely on it to clear the fault. Recommend a repalcement breaker w/ proper interrupting rating. Run the calcs. w/ the new model and tell the customer to install this new breaker to achieve these results.


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PostPosted: Tue Nov 25, 2008 5:25 am 
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Thanks all. Those were my thoughts. It may not be as easy as replacing a breaker as I am trying to verify the s/c rating of the panelboard If the client has to replace a panelboard with an extended outage, it may be a while, especially with capital projects getting delayed/canceled due to the economy. I know that sounds lame but it is reality in the business world.

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