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 Post subject: Calculating Utility Fault Current without Help from Utility
PostPosted: Fri Mar 09, 2012 12:43 pm 

Joined: Tue Feb 21, 2012 11:47 am
Posts: 2
Hello,

I am doing an arc flash study on a site where the utility is unable to give us any fault current or X/R rating.

What I do have, is all the information on the utility transformer, and the fuses connecting my site to the utility. Is there a way to derive "worst case" available fault current and X/R from these values ?

I tried looking on here and other sides, but could not find something similar to what I'm doing. Please be gentle, I am still new at all this! :)


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PostPosted: Fri Mar 09, 2012 3:08 pm 
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Joined: Mon Sep 17, 2007 5:00 pm
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Location: Scottsdale, Arizona
The short circuit current is very important. The X/R ratio only affects the way that impedances are added and in the absence of an X/R (common situation) many assume 12 or 15 for the utility. The X/R is the tangent of the impedance angle so Tan-1 (12) equates to an angle of 85.3 degrees and Tan-1(15) equates to an angle of 86.1 degrees - not much difference. You can assume numbers up to infinity and that will take you all the way to 90 degrees (a few numbers between 15 and infinity :) ) so 12 or 15 is usually fine.

For the short circuit current, the article at the link below might help. You can download it from the linked page. You want to be careful about using an infinite bus calculation with transformer data as you will read in the article. Good Luck!

[url='http://brainfiller.com/news/NewsDetails.aspx?PRId=62&Type=U']Article - Utility Data for Arc Flash Study[/url]

Let me know if you have any more questions.

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Brainfiller.com


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PostPosted: Fri Mar 09, 2012 8:20 pm 
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Joined: Sun Dec 23, 2007 1:44 pm
Posts: 348
Location: Charlotte, NC
Just have to ask, why can't the utility provide the info!


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PostPosted: Mon Mar 12, 2012 6:25 am 

Joined: Fri Sep 10, 2010 7:48 pm
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Location: Idaho
Assuming an infinite bus will not always produce the worst case, there are many instances where a lower arcing current will take longer to clear. This can result in higher energy and therefore higher PPE requirements.


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PostPosted: Mon Mar 12, 2012 7:24 am 
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Joined: Thu Oct 18, 2007 7:10 pm
Posts: 261
Location: NW USA
While Brainfiller is correct in stating this is important, it is equally or more important to understand that the utilities cannot provide an accurate available fault current. Transmission Lines and generators are switched and substations are paralleled on a daily basis; it is always fluctuating. This wasn't a problem when MAX AIC was considered worst case but as the utilities become aware of the liability of lower fault current I would be surprised if any render this information.

The work-round is suggested in Brainfiller's quoted article: assume a high plausible fault contribution and then repeat the calcs with a low plausible fault contribution. This is not horribly difficult if using computer analysis such as ETAP, but keeping track of the result is difficult as you will end up with multiple 'studies'.


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PostPosted: Mon Mar 12, 2012 8:04 am 
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Joined: Sat Nov 21, 2009 10:23 pm
Posts: 110
Location: Ohio
YoScott wrote:
Hello,

I am doing an arc flash study on a site where the utility is unable to give us any fault current or X/R rating.

What I do have, is all the information on the utility transformer, and the fuses connecting my site to the utility. Is there a way to derive "worst case" available fault current and X/R from these values ?

I tried looking on here and other sides, but could not find something similar to what I'm doing. Please be gentle, I am still new at all this! :)


There is still a bit of info missing, hope this helps:

1. The X/R ratio will also give the asymmetrical current. This is important when using a program that integrates the arcing current (that has now been accepted by the 1584 task force). In addition, all fuses, some relays and circuit breakers will respond differently to asymmetrical and symmetrical current, therefore, changing the heat enerergy of the arc.

2. In most cases with common fuses and circuit breakers, lower current will present more issues than higher arcing current. When you have overcurrent device with longer clearing times, then, higher curent may be the main issue. This is why bad utility data creates a problem, they are way too acustomed to giving unlimited fault current.

3. You will find that in some cases it is close to impossible to get the utility to respond. You must know one of the following to complete a conservative (not accurate) arc flash study:

A. An accurate short circuit current at the customers service entrance (load side of service transformer).

B. An accurate short circuit current on the primary of the utility transformer (you must do what-if's on the transformer impedance).

C. The name plate impedance on the service transformer (you must do what if's on the utility primary data).

D. If you do not fall into one of the above definitions, then, you must tell the customer that the study will have a high degree of inaccuracy.

As a caveat, always verify the transformer impedance, unless, you are 100% sure the transformer impedance data supplied by the utility is correct. Far too often they will quote you the ANSI standard or their stand impedance without verifying the actual %Z. I have caught them many/many time quoting me what they think, not what is factual. I completed a corporate study (multiple states) two years ago that included 25 utilities, they are all virtually the same with minor exceptions - buyer beware. Not one of those utility would I have graded A and several totally flunked when it came to knowing their own data.


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PostPosted: Mon Mar 12, 2012 9:55 am 

Joined: Wed Oct 20, 2010 11:46 am
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In our area, the local utility was not prepared for the housing boom of the late 90s and early 2000s and probably like most other utilities, worked feverishly not to get too far behind. While America was building houses and subdivision, the utility was laying off its engineering staff and oursourcing any industrial/commnercial design work to a local design contractor. After the housing bubble burst, even the contracted design company laid off half of its staff.

So, now we're left with a utility that doesn't know its own system so they tell us to ask their contractor. When we ask, we get very little data out of their design contractor, unless there's an active design and we're willing to cut them a check.

I don't see this process resolving itself anytime soon. So, we also are making assumptions and using the rules of thumb like the above commentors state.


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PostPosted: Mon Mar 12, 2012 11:54 am 
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Joined: Fri Apr 15, 2011 7:43 am
Posts: 177
Location: Colorado
I have seen different responses for different utilities. Some utilites will only give infinite high side fault data, other will give more detailed info. It seems to be either very large or very small utilites do not have the staff to give that data on thier distibution system. Generally it is easy to get data if connected to 69kV or greater. Sometimes it requires educating the utility (yes there are some that have not heard of arc flash).

Our stance is to ask for the greatest detail that we can get and document this in our reports - if we can we include the email. We must also accecpt that the info is only as good as when we get it so date everything.


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PostPosted: Fri Mar 16, 2012 12:15 pm 

Joined: Tue Feb 21, 2012 11:47 am
Posts: 2
acobb wrote:
Just have to ask, why can't the utility provide the info!


I am dealing with a utility company based in a third world country. When I attempted to get the data necessary, it took many weeks to get a response, and what I got was unsatisfactory for modeling in SKM.

What I do have... is nameplate on the utility transformer (15kV/400, 3000KVA %IZ=5.9 115max amps), and information about the fuses upstream from that transformer (Fuse markings denote I_n I_1, I_3 and U_n) .

I have been advised that without utility fault current, I should take roughly 90% of the capacity of what those fuses can handle as calculate the fault current based on that. I'm not sure this is the best method, as an exceptionally large capacity fuse, I would assume create a large amount of available fault current, and thus lower incident energy levels. If the incident energy level is actually higher and someone goes to rack out a breaker and its category 3, but he's only wearing a t-shirt because it says category 0, that could be problematic, eh?

What do you guys think about this 90%? am I correct that this is not a great way to approach calculating the utility fault current? Is there a method using the information above about the utility generator and fuses to estimate fault current? There are already so many assumptions going into my report, its ridiculous, but that's what they get.


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PostPosted: Sat Mar 17, 2012 5:12 pm 
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YoScott wrote:
I am dealing with a utility company based in a third world country. When I attempted to get the data necessary, it took many weeks to get a response, and what I got was unsatisfactory for modeling in SKM.

What I do have... is nameplate on the utility transformer (15kV/400, 3000KVA %IZ=5.9 115max amps), and information about the fuses upstream from that transformer (Fuse markings denote I_n I_1, I_3 and U_n) .

I have been advised that without utility fault current, I should take roughly 90% of the capacity of what those fuses can handle as calculate the fault current based on that. I'm not sure this is the best method, as an exceptionally large capacity fuse, I would assume create a large amount of available fault current, and thus lower incident energy levels. If the incident energy level is actually higher and someone goes to rack out a breaker and its category 3, but he's only wearing a t-shirt because it says category 0, that could be problematic, eh?

What do you guys think about this 90%? am I correct that this is not a great way to approach calculating the utility fault current? Is there a method using the information above about the utility generator and fuses to estimate fault current? There are already so many assumptions going into my report, its ridiculous, but that's what they get.

_________________
Jim Phillips, P.E.
Brainfiller.com


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