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 Post subject: Utility considerations
PostPosted: Thu Jul 26, 2012 1:06 pm 

Joined: Fri Jun 22, 2012 6:59 am
Posts: 8
I have recently been assigned to complete all arc flash analyses. Now this being said, I have a couple of beginner questions which I hope some of you can help me out with.

The main issue is concerning the utility considerations. According to IEEE 1584, the only information required is the min, max short circuit current rating and the X/R ratio.

However, I question whether the upstream utility protection and transformer capacity and impedence should be considered.

Ill give you two scenarios:

1. Basic overhead connection to main breaker (typical residential installation). Now if we were to only consider the kA and X/R ratio, with a max delay of 2s, the arc flash rating is going to be unreasonably high since no upstream protection is taken into account. At the same time, if given the upstream XFO and protection IS considered, their parameters would have to be adjusted in a way that the resulting kA at the main breaker would match that given by the utility company.

2. Utility supplied through padmount transformer. If transformer and protection parameters are available, the analysis should be straightforward, but the same adjusting would have to be done in order to match the kA at entrance.

All comments, suggestions would be appreciated.

C


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PostPosted: Thu Jul 26, 2012 1:57 pm 
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Joined: Tue Feb 24, 2009 7:05 am
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1. The utility protection shouldn't make a change in the kA at the utility connection point.
2. Given the Iarc, you can calculate the current seen by the utility protection across upstream the XFO, knowing the transformer voltage ratio. No need for XFO impedance for that. Then, the TCC will give you the max arc duration. It's possible the fault (arc) won't trip the utility protection.
3. The utility already accounted (even if they used infinite bus) for the transformer capacity and impedance in the kA they give you. If they haven't used infinite bus, they even accounted for the rest of their network.


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PostPosted: Fri Jul 27, 2012 6:51 am 

Joined: Fri Jun 22, 2012 6:59 am
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Thanks Vincent. That makes a lot of sense. Which program do you use to complete your studies? ETAP, SKM?

With the information given, I'm trying to find a way to force the short circuit current or trip time to no avail. You almost have to play with the transformer impedence in order to get the short circuit value that you want. Do you know of a simpler way?

Thanks again for your input!

C


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PostPosted: Fri Jul 27, 2012 6:53 am 
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Joined: Tue Feb 24, 2009 7:05 am
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Don't put it then. Start your one-liner at the LV with the kA you're given by the utility (knowing they probably quoted you an assumed max value, so it'd be wise to test your model if it's sensitive to a lower value or not).


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PostPosted: Fri Jul 27, 2012 7:03 am 

Joined: Fri Jun 22, 2012 6:59 am
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That makes sense. The only issue would be to include the interrupting time from the upstream utility fuse in order to determine the incident energy just above the main breaker. I believe this danger should be considered, since if a fault were occur on the bus bars just before the breaker, the electricien could still be within the arc flash boudary.


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PostPosted: Fri Jul 27, 2012 7:27 am 
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Joined: Tue Feb 24, 2009 7:05 am
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Paper wrote:
That makes sense. The only issue would be to include the interrupting time from the upstream utility fuse in order to determine the incident energy just above the main breaker. I believe this danger should be considered, since if a fault were occur on the bus bars just before the breaker, the electricien could still be within the arc flash boudary.

Sure! It could even happen that the utility protection device won't open because of it's TCC.


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PostPosted: Fri Jul 27, 2012 12:04 pm 

Joined: Fri Jun 22, 2012 6:59 am
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Thanks vincent. Input much appreciated.


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PostPosted: Mon Aug 06, 2012 10:28 am 

Joined: Thu Feb 04, 2010 5:00 pm
Posts: 36
Location: Camp hill Pa
ANSI C2, The National Electrical Safety Code, in general applies to utility work. Unlike the National electric code, the safety rules are included within this document. they are are tables in this code for safe working distances, and also other rules for safely working on utility lines. This code is generally adopted by the utility commissions in each state. These commissions oversee utility work. However I know of one state that adopts a fairly recent edition for electrical work and and a much older edition for communication work. Other then these lines being on the same pole, there should be no problem in any conflicts. The National Electrical Safety Code is published by the IEEE.


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PostPosted: Mon Aug 06, 2012 5:25 pm 

Joined: Sat Feb 27, 2010 4:14 am
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I get the fault level information from the utility where I can and also the protection details. This then allows me to consider the incoming terminals and if there is a higher risk at this point. If required I request changes to the protection to ensure coordination with the clients premises and the utilities protection. I would expect if there is a smaller substation e.g less than 1MVA that the level will be low enough to use PPE, but if it is a larger substation and close by then you may have dangerous arc flash levels.


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