125 kva exception for arc flash calculations

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mlutrey

Member
can some one shed some light on this item. I am using SKM software to calculate arc flash and I have the option to use a 125 kVa exception that lowers the catagory ratings of my panels fed by transformers below 125 kva. The only thing I can find is "The 125 kVA exception


Often referred to as the “125 kVA Exception,” the 2002 edition of IEEE 1584 contains language that permits a study to exclude calculations on circuits with voltages less than 240V and fed by transformers 125 kilovolt-amperes (kVA) and smaller. This exception was based on a few tests that indicated, that if an arc flash occurs at lower voltages and also with a lower magnitude of short-circuit current, it would be difficult to sustain the arc. Therefore, it would result in a lower level of incident energy.


Since 2002, significant testing has indicated, under certain conditions, it is possible to sustain an arc flash at much lower levels of short-circuit current. Although this exception is still under evaluation and review, it is likely that the cutoff will be greatly reduced from the existing 125 kVA transformer size.
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Is it safe to use this concept???
 

wbdvt

Senior Member
Location
Rutland, VT, USA
Occupation
Electrical Engineer, PE
Since it is still in the current industry standard, many people are still using it until the new IEEE 1584 is released, probably in 2 years. I have heard that it will be current based rather that kVA based.

Some companies will use an exemption of lower than 125kVA. The problem becomes, with the present formulas, when one has a high incident energy level on a 208V panel.
 

mlutrey

Member
Since it is still in the current industry standard, many people are still using it until the new IEEE 1584 is released, probably in 2 years. I have heard that it will be current based rather that kVA based.

Some companies will use an exemption of lower than 125kVA. The problem becomes, with the present formulas, when one has a high incident energy level on a 208V panel.

Thanks for your response.
So it is a decision on my part to either be overly conservative and go with the higher Cat ratings given when not using the 125 kva exception or use the exception and get lower Cat ratings on my panels thus making the electricians happy but possibly not safe and then wait 2 years for athe new IEEE. standard.
again thanks.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
So it is a decision on my part
NFPA70E has always required you to perform a risk analysis. In 2015 they became very blunt about it and inserted specific language, such as 110.1(G). Also there is the new definitions of Risk and Risk Assessment in which case you are supposed to evaluate the "likelihood" of an injury.

As a possible topic of discussion: if this 125kVA exception was totally 'irresponsible' wouldn't there be some readily available supporting documentation, such as number of injuries?

In my personal opinion, an electrician is more likely to get injured walking through 'fork-lift' traffic than they are from a 120V to ground arc flash using this exception. PPE is required for all risks faced by our people.
 

mlutrey

Member
NFPA70E has always required you to perform a risk analysis. In 2015 they became very blunt about it and inserted specific language, such as 110.1(G). Also there is the new definitions of Risk and Risk Assessment in which case you are supposed to evaluate the "likelihood" of an injury.

As a possible topic of discussion: if this 125kVA exception was totally 'irresponsible' wouldn't there be some readily available supporting documentation, such as number of injuries?

In my personal opinion, an electrician is more likely to get injured walking through 'fork-lift' traffic than they are from a 120V to ground arc flash using this exception. PPE is required for all risks faced by our people.

Thank you Jim, I should have said that it will be my decision based on my risk assessment. Again thanks
 

ron

Senior Member
Thank you Jim, I should have said that it will be my decision based on my risk assessment. Again thanks
It is somewhat similar to establishing a time limit for the incident energy calculation, such as 2 seconds. Although this is in the Annex of 1584, some use it and others don't because they think about "that time" the breaker didn't trip free, or a blast occurs and the equipment is positioned such that the worker would hit their head on the wall immediately behind them and would not be able to get away as inferred in the Annex.

It is the judgement of the person doing the arc flash hazard analysis.
 
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