Utility and short circuit current for AFH

[tem1234]

Hi,

At this moment, we ask for short-circuit current often to utility for arc flash study.

Utility have study this problem for long time and went with a solution, but a very conservative solution.

For MV, they say the short circuit current can vary from 0.5 to 12 kA. When you ask it for coordination, they will give you the real simulated short circuit current, maybe 4 kA but they say to not take this for arc flash.

For LV, they will take the 0.5 to 12 kA MV source, with all the range of transformer impedance they bought in the last xxx years. So by example, the short circuit current will be between 10 and 50 kA. They don't want to tell us what the real impedance of the transformer because they say that if they change it in emergency, there's ne warranty that the new one will have the same impedance.

i understand that their short circuit can vary, but it is quite difficult to lower the arc flash categorie with such a wide range of short-circuit current. The problem is that it's more a lawyer thing instead of engineering. They can give you a much better approximate, but their lawyer don't want to take any responsability.

So, i just want to know how people in other place deal with it, and what informations other utility give.

Thanks

 

[dpc]

You can do calculations for a range of available fault currents and take the worst case.

A range of 500 to 12,000 A seems a little extreme, I have to agree - unless there is something unusual about this system.



David Castor

http://www.cvoes.com

 

[rbulsara]

You do the best you can with the information you have. I generally have had good success with utility companies providing me the data of the actual transformer serving the facility; even when they insisting on a wide range of available SCC, and rightly so.

I base the AF calcs on the actual data to the extent possible as it represents a more probable scenario.

The SCC calcs on the other hand should be based on the highest probable SCC fault current, including the highest fault level advised by the utility co.

Just remember that the whole arc flash analysis as we know today is still approximate and should only serve as a guide for only one of several safety precautions to be taken during a live work.

Rafiq Bulsara

http://www.srengineersct.com

 

[tem1234]

Hi guys,

That's what we usually do. Take the worst case on a range of possible short circuit current.

But with such a large band, it's sure that you will have a very bad scenario specially with low fault current, so the risk categorie are very high.

I don't really like to give a report with a lot of cat 4 or worst. When this is really the case, it's not a problem, but not when it's because you're over conservative...

Thanks