Fault calculations, line or load side of the breaker?

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msasser@craftelectricinc.

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In an existing facility the main cubical is rated category dangerous. The engineer who performed the arc flash analysis calculated from the line side of the main breaker. An engineer we have been using also calculates from the line side. A new engineer I am using says it is industry standard that it should be calculated on the load side of the main, thus making it a category 2 (as the rest of the cubicals). He said if calculating from the line side, almost every installation would be category dangerous, even some homes.
It would seem to me the line side is correct. Which is it?
Thanks,
Mike
 
msasser@craftelectricinc. said:
In an existing facility the main cubical is rated category dangerous. The engineer who performed the arc flash analysis calculated from the line side of the main breaker. An engineer we have been using also calculates from the line side. A new engineer I am using says it is industry standard that it should be calculated on the load side of the main, thus making it a category 2 (as the rest of the cubicals). He said if calculating from the line side, almost every installation would be category dangerous, even some homes.
It would seem to me the line side is correct. Which is it?
Thanks,
Mike
Click to expand...
Is the breaker in question rated as current limiting? If not I fail to see where the available fault current would be any different at the line or load side. With a CL breaker it would only limit fault current should it trip limiting the let through current.
 
When working on the main cubicle, are you exposed to the line side if there were a failure o the line side? If so, that calculation should be on the line side, not utilizing any protection offered by the main breaker.
 
Your 'new engineer' is wrong.

In the past 5 years or so, by far most of the studies I have seen make the calculations on the line side.
 
jim dungar said:
Your 'new engineer' is wrong.

In the past 5 years or so, by far most of the studies I have seen make the calculations on the line side.
Click to expand...
Maybe he is just a "company man" who is looking for a way to be able to get the work done without having to shut off the main.
And yes, Mike, as a matter of fact quite a lot of installations should properly be rated dangerous but have never had an arc flash analysis done.
 
templdl said:
Is the breaker in question rated as current limiting? If not I fail to see where the available fault current would be any different at the line or load side. With a CL breaker it would only limit fault current should it trip limiting the let through current.
Click to expand...

Yes it is. Everything in the lineup downstream is Cat 2.
thanks
 
I spoke with another engineer who is in agreement that the calculation as he put it should be taken on the load side because calculations are taken at the bus not conductors. I've looked at Annex D in 70 E and don't see anything spelling it out, only Example D4, Table D2, (1) heading says 'Bus Nominal Voltage Levels'. I'm still confused.
 
GoldDigger said:
Maybe he is just a "company man" who is looking for a way to be able to get the work done without having to shut off the main.
Click to expand...

The issue of working on energized equipment versus turning off the main has all most nothing to do with the level of PPE required by a possible arcing event.
NFPA70E and OSHA, severely restrict 'live work'.


As I said, it is not 'the industry standard' to assume that a protective device always protects itself or the equipment it is mounted in.
You would need to provide more information as to the construction of your specific gear before a judgement could be made against it.
 
It's what Ron said in Post #3. If the main breaker's line-side bus/terminal/cable is exposed, and the adjacent section has holes that expose it to the main breaker section, then you have to recognize that the arc will not be contained in the main section.

With some understanding of the arc-flash calculations you can rationalize a reduced rating for adjacent sections by modifying the working distance from the line-side of the main breaker.

But generally the PPE rating of the line side gets affixed to the entire cabinet.
 
Has your 'new engineer' taken into account the possible failure of the breaker in the cabinet in question?
If there were no failures there would be no arc flash, but you need to account for all possible failures.
Can you get information from manufacturer about exposure inside the cabinet?
 
I am not sure I know what you mean by saying the calculation is performed on the line side or the load side of the main breaker. Is it a matter of whether the calculation takes credit for the main breaker opening, as the means of terminating the event? Does ?calculating on the line side? mean that the initial short circuit that creates the arc flash is presumed to take place upstream of the main breaker, so that something further upstream will have to clear the fault? If that is what is meant, then I agree with the notion of performing the calculation on the line side.

My reason comes from a colleague who owns a company that does power systems analysis as their only service. He told me that they don?t take credit for a main breaker within a panel as being the thing that opens and terminates the event. He said that the presence of the arc within the panel could cause a short circuit across the main breaker?s line side lugs, even if the arc originated somewhere downstream of the main breaker. Therefore, even if the main breaker were to trip, it would not terminate the event.
 
fmtjfw said:
Has your 'new engineer' taken into account the possible failure of the breaker in the cabinet in question?
If there were no failures there would be no arc flash, but you need to account for all possible failures.
Can you get information from manufacturer about exposure inside the cabinet?
Click to expand...

The gear is 7 years old and GE has the drawings on it. They say only in the last few years have they started offering compartmentalizing the line side of the mains.
 
jim dungar said:
The issue of working on energized equipment versus turning off the main has all most nothing to do with the level of PPE required by a possible arcing event.
NFPA70E and OSHA, severely restrict 'live work'.
Click to expand...

The whole reason for the inquiry besides the line vs the load side calculation is the customer knows his employees or anyone for that matter cannot safely operate the breaker.
We were looking at changing out that section and reviewing the arc flash study is when the question arose.

I'm going to stick with the line side calculation.
Thanks everyone.
Mike
 
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