Noah wrote:
We have been asked to provide arc flash warning label for a distribution system including some 50kVA, single phase, 7200V primary, 120/240V secondary transformers. Normally how do you do the study since IEEE 1584 doesn't provide single phase equation. Can I simply convert single phsae transformers to three phase to simulate the incident energies and divide result value by Duke's factor of 2.8? Or we have to use Duke's Power Heat flux calculator only?
1. Duke Heat Flux Calculator strongly under-estimates incident energy relative to known test data. It's a theoretical calculation but I'd advise against using it altogether.
2. If this is open air utility-based equipment (which it sounds like), I'd advise using the information from NESC which applies to utility equipment rather than IEEE 1584 and 70E which are intended for industrial equipment. NESC provides two tables. One table corresponds to calculations performed using Kinetrics (Ontario Hydro) ArcPro which is a theoretical model intended to simulate a vertical arc in air. The other table is a collection of equipment-specific values which are based on actual test data performed by EPRI, Duke, PG&E, and others. Utility distribution systems are almost inherently single phase arcing due to the large space between conductors, so their results are effectively single phase.
3. You cannot assume that single phase arcs are automatically and significantly lower energy than 3 phase arcs. Although it makes intuitive sense that single phase arcs should produce less energy than three phase arcs, this is not automatically the case in actual testing, and the leading source of test data seems to be EPRI.
4. IEEE 1584 testing had a difficult time sustaining an arc below 250 V. "It was difficult to sustain an arc at the lower voltages. An arc was sustained only once at 208 V in a 508 mm × 508 mm × 508 mm box. In all other tests with that box and the 305 mm × 368 mm × 191 mm box, the arc blew itself out as soon as the fuse wire vaporized. An arc was sustained several times at 215 V in a device box (100 mm × 100 mm × 50 mm size). It appeared from the arc flash photos from the 305 mm × 368 mm × 191 mm box that testing arcs usually jumped from the electrodes to the box wall and from another point on the box wall back to another electrode. The magnetic forces created by these arc currents forced them away from each other and into the box wall."
5. IEEE 1584 contains an "exception" (more of a comment): "Equipment below 240 V need not be considered unless it involves at least one 125 kVA or larger low impedance transformer in its immediate power supply." This does not apply in this case since the cutoff is below 240 V, which your equipment (120/240 V) does not match.
6. The Z462 standard gives a rating of 4 cal/cm^2 for the 120/240 V side assuming up to 25 kA short circuit and 2 cycle fault time. The latter is usually achieved with a 100 A or smaller MCCB but not necessarily otherwise and the breaker itself would still be over this rating.
7. Although this is for 3 phase quadraplex open air wiring, EPRI Report #1022002 (publicly available for free from
www.epri.org) contains a fairly realistic real world scenario (screwdriver jammed into the conductors). The highest measured incident energy is 3.5 cal/cm^2 at 25 kA. You haven't given the transformer impedance but it doesn't sound like you'll be able to reach that much fault current.
8. Gap length is very important for 120/240 V. IEEE paper #PCIC-2006-6 tested the effect of insulating barriers on arc flash. Without a barrier, "the authors conducted arcing fault tests at 208V and 250V with arcing gaps of 12.7mm, 32mm and 50.8mm. At 208Varcing could not be sustained at 10kA or less, even with the shortest (12.7mm) gap." With a barrier, "With a gap of 12.7mm, sustained arcing was obtained at 4.5kA, 10kA and 22kA. When the gap was increased to
32mm, the arcs were self-extinguishing at 4.5kA, but sustained arcs were obtained at 10kA and 22kA. For the 12.7mm gap, incident energies at 0.1s up to 2.7 cal/cm2 were measured, while for the 32mm gap this increased to 3.2 cal/cm2.” “For low X/R arcs were sustainable at 208V with a 12.7mm gap from 4.5kA upwards, but arcing could not be sustained with 32mm gap at any level of bolted-fault current.” Thus as I mentioned earlier, the PG&E results may not be valid considering demonstrated proof that 208 V arcing exists under the right conditions. It then becomes a matter of determining how much incident energy there is. Data from the PCIC paper results in 3 cal/cm^2 at up to 6 cycles. Without barriers the IEEE 1584 "exception" holds true. Once barriers are introduced however, the cutoff drops down to as low as 4.5 kA. With a 45 kVA transformer and a 3% impedance, this corresponds to the 1.2 cal/cm^2 threshold. So if your impedance is high enough, you might still qualify.
So...what I'm recommending is not a calculation but rather a careful analysis of the available test data out there and making conservative estimates using the test data as upper limits. Effectively this means that the EPRI/NESC-2012 upper limit of 4 cal/cm^2 PPE should be imposed for 120/240 V equipment as a general rule. Exceptions could then be made for instance in relatively open air conditions or where very wide arc gaps exist such that an arc cannot be sustained.This approach will have to suffice until such time as the mathematical modelling catches up with the experimental test work.
