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 Post subject: Question on NFPE 70E Arc Flash Calculation for Low Voltage
PostPosted: Thu Jun 12, 2014 8:22 am 

Joined: Sun Oct 07, 2012 9:22 am
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We have different type of old 480V switchgears (GE, ITE, Westinghouse...) and these switchgears feed different type of Motor Control Centers (MCCs). We performed an arc flash calculation for these MCCs using NFPA 70E table D.7.7 2012 edition for low voltage circuit breaker. However, reading Jim Phillips "Arc Flash Hazard Calculation Studies" book in Arc Flash Duration and time current curves stated that "For specific equipment such as a panel, switchboard, or motor control center, the device that should trip and clear the arc flash is usually considered the next device upstream in a separate enclosure...". and shows a Figure 9 -1 as an example, since the copy pdf of the article from the book is too big I could not attach the article from Jim Phillips book but please refer this book page 113 - 114. So, the question we have is that:
1) The arc calculation that we performed using NFPA 70E table D.7.7 for MCCs is not correct? If Yes, why?
2) Since these MCCs are old early 1980s and even older, we don't thing they are arc resistant MCCs. So, Should we re-due the arc flash calculation with consideration and using the tripping time of upstream device (480V switchgear breaker)?

Please give us some feedback if we were doing it wrong or right.


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 Post subject: Re: Question on NFPE 70E Arc Flash Calculation for Low Volta
PostPosted: Thu Jun 12, 2014 2:31 pm 
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Joined: Mon Sep 17, 2007 5:00 pm
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Location: Scottsdale, Arizona
It is all a matter of correctly selecting the protective device that will determine the arc flash duration. With a motor control center, it depends on the design. Some believe if there are many vertical sections, an arc flash at the farthest distance from the main breaker can not propagate back to the main breaker. Others disagree.

A lot of it comes down to the MCC design. You are correct that your MCC is most likely not arc rated due to the age. The question that needs answered is: if an arc flash occurs, can the arc/plasma reach the main section and propagate ahead of the main breaker. That is a judgment you will need to make.

A couple of years ago the former Chairman of IEEE 1584 asked a hypothetical question: do you use the main of an MCC for the calculations. The possible answers were Yes/No/It Depends. The overwhelming answer was "it depends". I asked a similar question in the "Question of the Week" section of the forum a year or 2 ago. Perhaps you can take a look at that area.

I did some testing about a year ago that involved one MCC section. The arc ran to the bottom (running away from the source) and we thought it went out. Then we saw it restrike at the top of the MCC via high speed (slow motion) video. The thought was the plasma made its way to the top and re-initiated the arc.

Hope this helps shed a bit more light on the subject. btw, hope you enjoy the book!

Best Regards,

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