Lower ka limit for arc flash calculation 1

[TerryOL]

I have been reading a number of articles recently dealing with arc flash. In one of the articles I came across a statement advising that below a certain value of fault current (ka) and a certain size of circuit breaker it is not worth conducting an analysis as the category will be zero. I think the values were 25ka and 100 amp, 433v 3 phase. Unfortunately I have not been able to locate this article again. I am interested in confirming these values as I am writing a standard operating procedure (SOP) dealing with arc flash and have made the statement that this analysis is required on all "high fault current circuits". I am therefore looking for a definition (lower cut-off point) for "high fault current".

Thank you for your help.

 

[jghrist]

Sometimes lower fault current will result in higher arc flash incident energy because the fault clearing time will be higher. If you aren't calculating the incident energy, you must use Table 130.7(C)(15)(a) of NFPA-70E (if you are in the USA). Make sure that you consider the fault current and clearing time parameters.

 

[TerryOL]

Thank you jghrist for your response. I expect the author of the article I referred to was using the operating parameters of a three phase "C" curve MCCB to come up with the ka rating of the supply. I will continue to review my reference material until I find the article again.

ps; I am in Australia, not the USA

Thanks again.

 

[RonShap]

Are you referring to the IEEE 1584 statement about equipment below 240 V need not be considered unless it involves at least one 125 kVA or larger low-impedance transformer in its immediate power supply.

This statement refers to the incident energy exposure possible under these conditions as observed during testing. The IEEE statement means that it is not necessary to calculate incident energy on equipment under 240V fed from a transformer less than 125 kVA, because the available fault current is not high enough to sustain an Arc-Flash and release significant energy. It is important to remember that this statement only refers to incident energy calculations, and does not release employers from the responsibility to assess all equipment operating at 50 volts and higher in the workplace for other electrical hazards, such
as shock and overload conditions which may lead to electrocutions, fires, or other hazards.