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 Post subject: Arc Flash Analysis for Existing System
PostPosted: Mon May 31, 2010 6:50 am 

Joined: Mon Feb 15, 2010 8:50 pm
Posts: 4
Hello there,

I am sure this issue has been addressed by this forum in some form or another.

Recently we are seeing an increase in jobs requiring us to perform Arc Flash Analysis even after a small modification to an existing system. The problem is the contractors / suppliers are asking us to do the analysis for just the new addition they are responsible for. My question is - is it appropriate (even correct) to do Arc Flash Analysis for just one (couple of) feeder?

Thanks for the help.


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PostPosted: Mon May 31, 2010 9:31 am 
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aregular wrote:
Hello there,
Recently we are seeing an increase in jobs requiring us to perform Arc Flash Analysis even after a small modification to an existing system. The problem is the contractors / suppliers are asking us to do the analysis for just the new addition they are responsible for. My question is - is it appropriate (even correct) to do Arc Flash Analysis for just one (couple of) feeder?


Good Question!

NFPA 70E 2009 Edition States:

130.3 Arc Flash Hazard Analysis ... The arc flash hazard analysis shall be updated when a major modification or renovation takes place. It shall be reviewed periodically, not to exceed five years, to account for changes in the electrical distribution system that could affect the results of the arc flash hazard analysis.

The intent is to catch any changes that could impact the results of the study. If there are system additions, they should be included in the study. Does this mean that you have to update the entire study? Probably not but it depends on when the original study was last performed and what types of additions are being made.

For example, if the change is only the addition of a panel or something similar and the original study is up to date (i.e. no other major system / utility changes) then I would just add the new data to the existing study.

However, if the new change includes significant motor load that could impact motor contribution, you will want to be sure that new motor loads do not affect the existing study. Also, changes that might require an increase in the size of upstream protective device settings would also be a good example where further review is necessary.

Hope this helps. Check back if you have any other questions.

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PostPosted: Mon May 31, 2010 12:00 pm 

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Thanks for the quick reply, brainfiller.

The scenarios that have been troubling us are when there is no study for the existing system to begin with and in cases where they can't provide us with the existing study?

The problem is some of the contractors / suppliers get into these contracts with the owners / consultants without clear understanding of the amount of time and effort required to perform some of the studies that are called for in the specifications.

Now in cases where there are no existing Arc Flash Studies, when we tell them that we need to go down to the site collect data of the existing system, model the system as accurately as possible,...they would say, but we are only putting in one (couple of) VFD(s), panel(s).

Thanks again for the help.


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PostPosted: Mon May 31, 2010 1:44 pm 
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I see that time and time again. Unless someone has actually performed the arc flash calculation study, they have no idea how labor intensive it is. I hear it all the time in my arc flash studies class.

About your only remaining option would be to use the NFPA 70E tables which are permitted to be used in lieu of calculations.

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PostPosted: Mon May 31, 2010 4:07 pm 
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aregular wrote:
Thanks for the quick reply, brainfiller.

The scenarios that have been troubling us are when there is no study for the existing system to begin with and in cases where they can't provide us with the existing study?
...
Now in cases where there are no existing Arc Flash Studies, when we tell them that we need to go down to the site collect data of the existing system, model the system as accurately as possible,...they would say, but we are only putting in one (couple of) VFD(s), panel(s).


If they provide all the fault currents necessary to do calculate IE, then use these in the analysis of the the addition. Qualify your report to the effect that it is based on client furnished fault data and does not address the rest of the system. If the client cannot furnish the fault currents, then you have no choice except to gather the data to do a fault current study. If you do the fault current study and only calculate IE only for the additions, you still have to qualify your report that it does not address the existing system.

We have done studies where the client supplies fault current and interrupting times at specific locations. We merely calculate the IE and boundary at each point.


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PostPosted: Tue Jun 01, 2010 9:20 am 

Joined: Mon Feb 15, 2010 8:50 pm
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Thanks, jghrist

That is basically what we have been trying to do. I just wanted to know how others in the industry deal with this issue and also learn from their experiences.

Thanks again everybody for the help. It is a great resource you have here


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PostPosted: Thu Nov 03, 2011 8:14 am 

Joined: Mon Jul 20, 2009 11:26 am
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Location: Ontario, Canada
Rule of Thumb - Significant Motor Loading?

brainfiller wrote:
Good Question!

NFPA 70E 2009 Edition States:

130.3 Arc Flash Hazard Analysis ... The arc flash hazard analysis shall be updated when a major modification or renovation takes place. It shall be reviewed periodically, not to exceed five years, to account for changes in the electrical distribution system that could affect the results of the arc flash hazard analysis.

The intent is to catch any changes that could impact the results of the study. If there are system additions, they should be included in the study. Does this mean that you have to update the entire study? Probably not but it depends on when the original study was last performed and what types of additions are being made.

For example, if the change is only the addition of a panel or something similar and the original study is up to date (i.e. no other major system / utility changes) then I would just add the new data to the existing study.

However, if the new change includes significant motor load that could impact motor contribution, you will want to be sure that new motor loads do not affect the existing study. Also, changes that might require an increase in the size of upstream protective device settings would also be a good example where further review is necessary.

Hope this helps. Check back if you have any other questions.


Hi - Our company got an Arc Flash Study completed for our facility, however we left it as part of the project for the company completing the study to do all the work and just utilize their software package with the agreement going forward that we'll contract them to update the study when we require changes to be made.

Understanding the information I've quoted, I'm wondering if there should be a HP value (whether it be added or subtracted from our systems) which should "trigger" my company to want to get the study updated. I wasn't sure whether a single event where >50HP or a number of events where an accumulation of >50HP would be a good point or whether that is too sensative.

We are trying to come up with a system to make sure that items/modifications/additions/subtractions don't get lost through the cracks and I'm just hoping someone could tell me if there is a rule of thumb in industry which companies may be following.

I want to be dilligent in making sure our study is correct, however I don't want to get raked over the coals by the supplier company who completed our study by us going back to them at unreasonable intervals.

Many thanks in advance!

Best regards,
Dylan


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PostPosted: Thu Nov 03, 2011 5:37 pm 

Joined: Wed Jan 26, 2011 12:04 pm
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jghrist wrote:
If they provide all the fault currents necessary to do calculate IE, then use these in the analysis of the the addition. Qualify your report to the effect that it is based on client furnished fault data and does not address the rest of the system. If the client cannot furnish the fault currents, then you have no choice except to gather the data to do a fault current study. If you do the fault current study and only calculate IE only for the additions, you still have to qualify your report that it does not address the existing system.

We have done studies where the client supplies fault current and interrupting times at specific locations. We merely calculate the IE and boundary at each point.


Is it safe to say that the above should be done by qualified engineers or specialists? Let me be more specific: I know the fault currents on a new electric plant from drawings, but should PE's or persons with higher-level arc flash calculation knowledge do the actual calcs?

Although this might seem like an obvious "Yes" answer, I think the provision of the 70E tables themselves is acknowledging that not everyone can afford the study. Can people comfortable with the formulae in the 70E appendices dig in, find the clearance times, and do the calcs or is it better left to the experts?


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PostPosted: Thu Nov 03, 2011 6:01 pm 
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Joined: Mon Jun 02, 2008 11:58 am
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Location: Charlotte, NC
Quote A= $82,000
Quote B = $80,500
Quote C = $19,200

If your company gave the study to "C" I have little sympathy, however I have had to go clean up plenty of "C's" messes, as have a lot of other "A" and "B" guys out there.


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PostPosted: Fri Nov 04, 2011 8:39 am 

Joined: Tue Jul 24, 2007 5:00 pm
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Location: Chicago, IL
Zog wrote:
Quote A= $82,000
Quote B = $80,500
Quote C = $19,200

If your company gave the study to "C" I have little sympathy, however I have had to go clean up plenty of "C's" messes, as have a lot of other "A" and "B" guys out there.


I have a friend that has made a pretty good living at going in after the "C's" have completed(?) the study to clean up their mess. There are a few of these "C" companies that look impressive on the internet so they must be good. :eek:

The good news is the overwhelming majority of consultants do a very good job. It is just a few of the "low bid artists" that talk a good line and then greatly disappoint the client.


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