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 Post subject: Calculator vs Software for Arc Flash studies
PostPosted: Sat Jan 16, 2021 3:33 am 

Joined: Fri Jan 15, 2021 3:44 pm
Posts: 5
Hello

Can anyone confirm that calculators/spreadsheets (like ) are not appropriate for arc flash studies (even for small/medium plants) , due to the fact that they don't take into account the contribution of other electrical elements to the overall fault current at a specific location.

Best regards


Last edited by wbd on Sat Jan 16, 2021 5:57 am, edited 1 time in total.
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 Post subject: Re: Calculator vs Software for Arc Flash studies
PostPosted: Mon Jan 18, 2021 7:37 am 
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hmida wrote:
Hello

Can anyone confirm that calculators/spreadsheets (like ) are not appropriate for arc flash studies (even for small/medium plants) , due to the fact that they don't take into account the contribution of other electrical elements to the overall fault current at a specific location.

Best regards


If the spreadsheet calculator is based on the IEEE 1584 equations, it should be correct. (assuming there were no errors in implementation) The issue that you point out is a separate matter. The IEEE 1584 standards does NOT address short circuit modeling which should be based on other standards such as IEEE 551.

The IEEE 1584 equations require an input variable of bolted short circuit current. How that value is obtained is outside the scope of the IEEE 1584 arc flash equations. Many commercial software companies integrate short circuit, coordination and arc flash all into one complete package with each being based in their respective individual standard.


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 Post subject: Re: Calculator vs Software for Arc Flash studies
PostPosted: Mon Jan 18, 2021 7:56 am 

Joined: Fri Jan 15, 2021 3:44 pm
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Thank you Jim.
So the problem is related to the value of Bolted Short Circuit current which should be obtained from other softwares ? The main difficulties lie in the calculation of positive and zero sequence impedances at specific point in circuit, right ?
So we can't use only calculators/spreadsheets for an arc flash study without having another tool for determining bolted SC currents. Therefore it's mandatory to use dedicated software for arc flash study.


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 Post subject: Re: Calculator vs Software for Arc Flash studies
PostPosted: Mon Jan 18, 2021 8:17 am 
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hmida wrote:
Thank you Jim.
So the problem is related to the value of Bolted Short Circuit current which should be obtained from other softwares ? The main difficulties lie in the calculation of positive and zero sequence impedances at specific point in circuit, right ?
So we can't use only calculators/spreadsheets for an arc flash study without having another tool for determining bolted SC currents. Therefore it's mandatory to use dedicated software for arc flash study.


Only positive sequence impedance is needed since the input value for IEEE 1584 is a three-phase bolted fault current.
The fault current calculations can be performed via another method if you are using a spreadsheet - that is what I do on occasion.

The commercial software integrates the various studies but it is not a requirement to do so as long as you obtain the correct fault current from another source.

Step 1 - calculated bolted fault current at location - can be a separate exercise.
Step 2 - use IEEE 1584 to calculate the equivalent arcing fault current
Step 3 - use the arcing fault current and the time current characteristic of the upstream device to determine the arc duration (device that should trip due to an arc flash and be unaffected by the arc event)
Step 4 - use the arc duration and fault current in the spreadsheet to determine the incident energy and arc flash boundary. Also need gap distance, working distance and a few other parameters defined in IEEE 1584.

Hope this helps!


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 Post subject: Re: Calculator vs Software for Arc Flash studies
PostPosted: Mon Jan 18, 2021 9:12 am 

Joined: Fri Jan 15, 2021 3:44 pm
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Well explained Jim, Thank you. So it's possible to conduct Arc Flash risk assessment whithout using commercial software, using only spreadsheet/calculator + software for Bolted fault current. However, I think it's more complicated and time consuming task. Based on your experience, do you know if there's a time ratio between the spreadsheet method and the commercial software method, for a small/ medium plant ?

I think that the step 3 requires also another software to exctract the arcing time when having several trip units in upstream, right ?

Thank you again for these details.


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 Post subject: Re: Calculator vs Software for Arc Flash studies
PostPosted: Mon Jan 18, 2021 10:20 am 
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Great question. It's desirable to keep costs down so people getting into studies try to avoid the purchase of software which can be expensive. ....until you try to perform the study manually with spreadsheets etc. and watch the billable hours add up quickly.

I don't have a break point on size of system but have looked at it if you only need calculations at a couple of buses, spreadsheets are OK but more than that and it really gets cumbersome. PLUS, now that you perform short circuit calculations, you also need to obtain conductor impedance and other data. Lots of room for mistakes.

Software usually has pretty extensive libraries of time current curves, conductor impedance and other info that you may need. I'm not making a pitch for software as I have no ties but you probably get the idea.

I teach a couple of classes on these subjects, and have had people say they will perform the study manually using the worksheets that I provide in class and making their own spreadsheets. For small systems that works. But then some may try that approach on larger systems and they usually end up buying software after they realize how time consuming it is.

I have many of the S/W companies listed on this week's survey question (upper right of forum home page) and you can do a search and many will have demo versions.

Good Luck!


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 Post subject: Re: Calculator vs Software for Arc Flash studies
PostPosted: Mon Jan 18, 2021 1:37 pm 

Joined: Fri Jan 15, 2021 3:44 pm
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Thank you Jim for these clarifications. I really appreciate these practical advices. Indeed, there's a great risk of introducing errors when using spreadsheets on large systems, which can have serious consequences for worker safety. I will try to identify the best alternative for my case after evaluating some demo.

Thank you again. Best regards


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 Post subject: Re: Calculator vs Software for Arc Flash studies
PostPosted: Tue Jan 19, 2021 1:09 pm 
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I keep seeing a reference to bolted fault current but just a reminder is that you need to start with the available fault current obtained from the utility company not infinite bus fault current.

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Barry Donovan, P.E.
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 Post subject: Re: Calculator vs Software for Arc Flash studies
PostPosted: Tue Jan 19, 2021 5:16 pm 

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Thank you. the utility should provide the available short circuit fault current and the associated X/R ratio at utility services, right ?


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 Post subject: Re: Calculator vs Software for Arc Flash studies
PostPosted: Wed Jan 20, 2021 7:35 am 
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hmida wrote:
Thank you. the utility should provide the available short circuit fault current and the associated X/R ratio at utility services, right ?


Yes but a lot of times you will receive an initial answer from a customer service rep who is looking at a table of transformer infinite bus short circuit values and that is what is provided. Easy enough to check,

However, I always model from the utility primary protective device to the transformer, so I ask for:
1. 3 phase available fault current and X/R at the primary fuse
2. SLG available fault current and X/R at the primary fuse
3. Primary fuse data
4. Riser cable data (if padmount txf)
5. Transformer data (kVA, primary connection, %Z, secondary connection)

These type of requests usually end up in engineering.

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Barry Donovan, P.E.
www.workplacesafetysolutions.com


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 Post subject: Re: Calculator vs Software for Arc Flash studies
PostPosted: Thu Jan 21, 2021 10:45 am 
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To finish up Barry's response...the request can be painful and take an inordinate amount of time. Further, many times utilities don't want to even entertain available fault current discussions. I understand the reasoning, but there has to be some type of "meet in the middle" approach to limit liability.

Mike


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