Hey,

I am an electrical engineering student that is doing a study on arc flash and trying to figure out how to develop the protection boundaries. I have been using the NFPA 70E formulas and was confused by a couple of things. When using formula D.7.5 (Arc Flash Protection Boundary), what time (t) do I use? I read somebody else on here uses 2 seconds. Also, the incident energy at the distance of the protection boundary (E sub b) should be the different categories right (ex. Cat 0 should be 1.2 cal/cm^2 converted to 5.0208 J/cm^2 right?) Also, I was wondering if anybody could give me approximately where the boundaries should be for a general 480 V transformer, just to see if my numbers are close?

Thank you.

Hey yourself, welcome to the forum.



2 Seconds is a default time used when the OCPD won't clear the fault, it is based on the time for a person to react and move. You need to be careful using this depending on the configuration of the equipment and worker.

The time you should use is the time that the OCPD will take to clear the fault at the arcing current you are using. You need to look at more than one senario, arc flash software will run many different ones and give you the worse case. Many times the lower arcing currents result in higher Ei's due to the longer clearing times.



No, the definition of the arc flash boundary is the distance where the Ei is = 1.2 cal/cm2. When working inside the arc flash boundary the Ei is calulated at the assumed working distance and the proper PPE is selected based on the calculated Ei.



A "typical" 480V transformer to me is 3000kVA, to others it may be 100kVA, can you be more specific?

Glad to hear they are covering this stuff in EE school these days.

Thank you for the help. So, first I need to get the arc flash boundary using 1.2 cal/cm^2. After that, can I calculate the distances that each type of ppe is required? For example, between 3 and 2 feet away, category 2 clothing is required? Or is this the wrong way of going about it? Also, I'm using a 480V 1000kVA 5.75% impedance transformer.

Thank you.

Yep, also refered to as the flash ahzard boundary and the curable burn distance in older texts, did not become standard until the 2009 70E. 1.2 cal/cm2 will cause a 2nd degree burn of exposed skin in 0.1 seconds.



Not really "wrong" just will be too confusing. Assume a working distance, this is usually assumed to be 18" for most equipment (Based on length of outstreched arm to chest). Then calulate the Ei @ that distance.

Okay,

Thank you for the help.