Hi Everyone,

I'm using SKM to do the arc flash hazard analysis for a 208V Panelboard that is being fed by a small 75kVA transformer. I setup the arc flash option in SKM so that it will report a 1.2 cal/cm2 at this panelboard. Now I noticed that SKM uses an arc flash boundary of 18 inches for this default result. SKM also uses a clearing time of 2 seconds for this default result.

I was reading the arc flash protection boundary in NFPA-70E (2009) and what I understand from NFPA-70E is that we need to use a clearing time of 2 cycles (0.033 seconds), and an arc flash boundary of 4 feet for cases where detailed arc flash analysis are not performed.

My question is, do I need to change the clearing time and the arc flash boundary in SKM to be consistence with NFPA-70E? Thanks!

Viet

The 2 second default is from IEEE 1584. That is the lowest duration cut off point listed in any standard. If you have a long clearing time, it is reasonable to cut of the duration at 2 seconds if there is room for escape - careful judgement must be used.

The 4 foot boundary default is subject to limitations of short circuit and clearing time not exceeding 1667 amp-seconds. It is not a completely random distance and the limitations must be considered.

I believe I understand why you are thinking about using 1.2 cal as a default, (208V circuit <125kVA transformer) However I am not sure I would do this. To me, listing an incident energy implies there was an analysis / calculations performed.