When evaluating tasks, I think Roberts summarized it best by suggesting that there are some tasks where the inherent risk of an arc flash is low by the nature of the task. For instance resetting an overload relay. Then there are some tasks where the inherent hazard is low such as working on low voltage equipment or working inside the low voltage compartment of switchgear. It's only when we combine these two issues (risk and hazard) that a serious potential injury results.
wkeller wrote:
1) Is there a voltage level below which arc-flash or arc-blast is of little concern? (notwithstanding the need to still use insulating gloves and face protection due to minor arcing events)
First off arc blast has been shown to be much less of a concern than previously suggested via several tests published at the ESW. I've done some calculations that show similar results, as has CIGRE (in a round about way). Other than turning doors into projectiles by pressuring the enclosures, a "blast" (as in concussive force) risk has been proven by testing to be a nonissue. It might blow your ear drums out but it's not going to cause much else.
Second the voltage issue has long been a point of contention. Test results done at Bruce (Kinetrics) for Duke show that up to 130 VDC at up to 20 kA barely reaches the 1.2 cal/cm2 threshold. Simply put, the arc extinguishes. Somewhere in the neighborhood of 200 to 300 V there is a threshold for AC systems where the hazard becomes significant. There is an article in the articles section where I summarized most of the information and research out there on the subject.
The test standard is based on exposure to a second degree or more severe burn at the face/chest area. Hands, arms, and legs have always been considered nonfatal and basically sacrificial because severe burn injuries in those areas are rarely fatal. Thus a 1.2 cal/cm2 cutoff definitely would not require face/head protection at all. Gloves might be a good idea but once we're below that threshold even arc rated shirts aren't required either so the arms are just as exposed. 1.2 cal/cm2 is a threshold where PPE is not required. As in zero, zilch, none, for arc flash purposes. Shock protection is separate and a different hazard.
I really recommend NOT doing a calculation for arc flash below 250 VAC because it's not really all that valid. The table based approach should be used instead.
Quote:
2) Most arcflash studies that I'm aware of seems to stop at the local shut-off box that feeds power to a machine. Is there any reason why arc flash studies do not seem to include the in-feed point at the machine itself (including an arcflash label)?
Two reasons. First, NEC only includes distribution type equipment in the list of examples of where it needs to be done. It has the catch-all phrase where routine energized maintenance occurs but few people seriously think that one through. So if we go by the strict wording of NEC then the point of utilization does not need to be considered. In general the only violations I have typically run into are local disconnects where testing often occurs outside of the normal switchgear, starters, etc., and ignoring controls and other low voltage circuits which are generally not a problem. Again, consider 277 V lighting as a serious concern/exception or very old starters which used 480 V power directly through the controls instead of a control power transformer but that isn't all that common these days. So there are exceptions but it's not the normal situation. If it is, work practices can be changed to eliminate it or evaluations done on the exceptions. A lot of plants have eliminated 480 V controls and 277 V lighting as a result of arc flash studies and been all the better for it in the long run with much less damage when a fault occurs.
Second the hazard is usually very low once you get through the typical starter or distribution disconnect/breaker, especially as instantaneous tripping is generally the norm rather than the exception. With the exception of zone selective protection, having instantaneous turned on in a breaker at multiple levels is a problem because of severe miscoordination. Yes series ratings and dynamic impedance exist but don't rely on them. I just evaluated a customer's system last week that had a 2000 A main, a 600 A feeder breaker, and a 400 A (RK5) fused disconnect. The motor burned up and shorted out. The fuses did not trip but both breakers tripped. The instantaneous settings (both were turned on) were basically right on top of each other and set so low that the fuse curve was almost entirely above all of the settings until the motor overload kicked in...the fuses did nothing. Customer wanted to know why both breakers tripped which was pretty obvious once I looked at the TCC's. Someone doing the walk through though should be aware of when "exceptions" and concerns exist like a lack of instantaneous protection or lots of local disconnects used for troubleshooting and take it into consideration.
