# Arc Flash boundary mitigation



## JohnR (Apr 12, 2010)

I have a location that has a 400A main breaker, old, hasn't been reset in the last 20... has about a 3 or 4" area that the breaker is exposed. I am doing a arc flash, boundary study and fully expect the answer to come back at 40+cal for working on the equipment. 
Fed by a trans 50ft away, 750kcmil AL 120/208 

In the same room, luan sliding closet doors between, there is a receiving office. What can be done if needed, to mitigate the approach boundary to a level where the office can be used without having the workers have to wear AF clothes all day.


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## Big John (May 23, 2010)

I'd be pretty surprised if your study came back with a really high boundary for that. I haven't seen a 40 cal. incident energy on a 208 volt panel.

But if it is, you've got three choices: Reduce the available current, reduce the clearing time, or create a physical barrier.

How hard would it be to add a fused disconnect at the transformer?

-John


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## JohnR (Apr 12, 2010)

It is a UG fed service from the transformer at the front of the building by their entrance. IDK about adding a disco there. I am kind of drawn to the physical barrier, but don't know if full swing doors can be added to the GE equipment and keep the UL rating of it. 

I got the estimated cal rating from doing a online calc, I know that is just a idea of what it might be. I am NOT using that for the study, I have an EE who will be doing the correct one.
The guys insurance is making him step up and do some compliance. Signs etc.


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## Zog (Apr 15, 2009)

JohnR said:


> I have a location that has a 400A main breaker, old, hasn't been reset in the last 20... has about a 3 or 4" area that the breaker is exposed. I am doing a arc flash, boundary study and fully expect the answer to come back at 40+cal for working on the equipment.
> Fed by a trans 50ft away, 750kcmil AL 120/208
> 
> In the same room, luan sliding closet doors between, there is a receiving office. What can be done if needed, to mitigate the approach boundary to a level where the office can be used without having the workers have to wear AF clothes all day.


What is the size of the transformer feeding it? Any OCPD's between the transformer and the 400A main?


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## JohnR (Apr 12, 2010)

Still trying to get the POCO to give me that info. No other limiting devices either. I have been told by an inspector that the 500 k delta wye 208v are the worst as the impedance is ( I think he said) 100 k. 

If that intact is the case, I may advise the business owner to have the tranny swapped out to something closer to what he really needs. Like a 250. Will have to do a full load calc before that though, and get the results of the Analisys back.


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## Zog (Apr 15, 2009)

100k what? Transformer impedences are in %, you use the kVA rating and %Z to find your fault current at the transformer and then do a point to point to your panel, then do your arc flash calculation based on that, if there is no OCPD use a 2 second clearing time.


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