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 Post subject: 2 Second Cutoff time
PostPosted: Tue May 27, 2008 2:48 pm 

Joined: Tue May 27, 2008 2:42 pm
Posts: 10
We are currently doing arc flash studies for electric distribution utilities. We are trying to determine the cutoff time for fault currents that either have a really long interrupting time or none at all. It looks like historically 2 seconds was used (specifically in industrial settings). I would imagine most of this equipment is located on the ground therefore making it easy to get away in 2 seconds. In the utility setting some of the equipment will be up on a pole. Therefore the people working on it will either be in a bucket or on the pole using hooks. We feel that 2 seconds isn't a long enough time to get away and have been talking with our clients about 5 seconds. We would still use 2 seconds for underground equipment.

I have recently found out that the 2 second time is from IEEE 1584 and is based on the "typical" 2 second stopping distance / reaction time that you learn in driver's education.

Do you think this is reasonable? Have you heard of any other values that are being used and why? We can't find anywhere that it has to be 2 seconds.

Any help is much appreciated!

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PostPosted: Wed May 28, 2008 5:47 am 
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Joined: Sun Dec 23, 2007 1:44 pm
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Location: Charlotte, NC

We are doing these as well and I have the same problem for areas of the system where the max available is not high enougn to operate the inst. attachments and the inverse curve takes 8 to 10 seconds to trip. I have still not gotten comfortable with using a cutoff time, and if I did, what time it will be since it will be an arbitrary selection. I do realize that we need to make some assumption for these cases to be able to put linemen in clothes that will allow them to work, just haven't gotten comfortable with it yet.

I do agree that being strapped to a pole, instead of a bucket, is about the most restrictive case to consider and don't know if a person can clear themselves or the problem in 2 seconds.

The NESC folks did not give us much guidance for this issue, and I do believe that we are left with having to choose a cutoff time on our own for now.

Would be great to somehow come to some "standard" practice. With more input, maybe we can in this forum?


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PostPosted: Wed May 28, 2008 6:42 am 

Joined: Tue May 27, 2008 2:42 pm
Posts: 10
2 Second Rule


I agree that I don't feel comfortable picking some sort of arbitrary number. But if you don't pick something the incident energy and boundaries skyrocket. It makes a substantial difference between 2 and 5 seconds. We to are trying to gain some sort of consistency amongst utilities.

Thanks for your input.

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PostPosted: Wed May 28, 2008 3:52 pm 
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Joined: Mon Sep 17, 2007 5:00 pm
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I think I might have been the one to give Andrew the 2 second info off line. There really is not a good answer for this. Some will use 2 seconds if there is the possiblity of jumping out of the way but I am sure the lawyers will question why the decision is made. You could argue what if someone is knocked out, trips when being blown backwards, so on and so on. On the other hand, if you go with 20 or 30 seconds from time current curves the energy sky rockets as I think everyone is well aware and the Cat 4. / hard to work / heat stroke problem rises.

There is some limited / unpublished test data that shows lower level faults at lower voltages i.e. 208 and 480V have a very difficult time sustaining regardless of the time current curve. It was being discussed by a few people in the forum and I had a copy so I posted it in the forum.

This is one of the issues on the very long list of issues to be addressed with new testing. Until then, it's a lot of judgement and hopefully others can chime in here and give their opinion.

Jim Phillips, P.E.

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