Satisfying NESC 41

If I were to do an arc flash analysis of a decent sized R.E.C. distribution system (40 subs or so) would I be satisfying NESC 41 if I chose roughly 300 points throughout the system and assesed them, or would that not be enough? I'm trying to decide if it is necessary to all of the primary equipment (fuses, reclosers, VFI's) since I'll get different fault currents depending upon where that equipment is placed on the system.

Nesc

We are taking the approach that, since you could have personnel working on any part of an overhead system, checking selected points on the relay TCC from max to min circuit fault duty. If you are downstream from a fuse or recloser on 1 shot, you could use those curves since they should be faster than the substation breaker. You will probably notice that at levels of 150 to 200 % pickup, the energy can get quite high due to most inverse type curves requiring 5 to 10 seconds or more to clear at these levels.

We are considering adding maintenance switches to invoke alt. settings which would lower the time and inst pickups for those cases where the energy levels are high and the necessary micro-processor relays exist.

We are also considering using a max of 2 to 5 seconds for arc duration but are not comfortable using an arbitrary value yet.

Hope it helps.
Alan

Alan,

We established an 8 cal/cm2 min rating for our clothing systems (you could pick 4) and established equivalent "relay type curves" for our MWD. We then did a coordination study using this curve againt our line devices and found the "miscoordiantions" i.e. exception areas. The choices then become increase the working distance or increase the clothng. For rubber gloving, we had no choice but to increase the clothing. Works for transmission lines also.

Hope that helps,
Marcia Eblen, PG&E Principal Grounding Engineer

FR Clothing

Marcia,

So far I have been recommending a minimum of 8 cal clothing to our clients as well. On the high voltage (15 and 25 kV) distribution systems there are very few cases I see that even get to 5 or 6 cal levels. Most of the bus fault duties are in the 5 to 7 k range. Ground mounted equipment at 480 or 208 is a different animal and I am not at all comfortable with the minimum 4 cal in the NESC. Self contained polyphase meter bases I believe will be an issue as well.

Thanks,
Alan