gobraves wrote:
Hey,
Hey yourself, welcome to the forum.
gobraves wrote:
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.
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.
gobraves wrote:
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?)
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.
gobraves wrote:
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?.
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.