AndrewKGentile wrote:
From a mathematical stand point, I agree with you. From an engineering standpoint it is clear that this is wrong. The basis of this analysis is dependent on the arc fault being self sustaining, which is very unlikely. The equation calculates energy, even when it's not there.
Imagine doing the same analysis on a 1 kva transformer with a 120V secondary. If you protect the transformer with a large enough fuse, a fault on the secondary will never clear. That doesn't make a fault on the secondary of this tiny transformer an arc hazard, even though the analysis would indicate it is. It's a misapplication of the equation.
I agree 100% with this statement. If we assume stability, we get the ETAP result. Mathematically it's correct though it might be a little confusing because the result is based on energy which is power multiplied by TIME and in this case both power and time are changing in opposite directions. The good news is that we are pretty sure intuitively that the arc is NOT stable so this is really just an upper bound. The real arc flash hazard won't exceed this number, but it leaves a bad taste in the mouth, a lot more inspection, and it's obviously just the wrong approach.
But if we assume or try to estimate stability, we're in trouble for the simple reason that we don't honestly have a comprehensive standard. We have very few options to go by. Let's take them in turn. First, we have IEEE C2 (NESC). This is a utility standard and the front matter clearly documents that it is not really intended to apply to industrial settings as a general standard so we have to overcome and accept this. But it has a very simple table based approach that gets us to ATPV 4 without face protection except a hard hat. This is perfect if you are working in a metal casting shop, a refinery, natural gas plant, or similar operations where NFPA 2112 for flash fires rules and you are already wearing that level of PPE for other reasons but it is problematic for all other industries where FR PPE is not the norm for daily work wear. Some plants have simply accepted this and the electricians are required to wear it though so it's not an impractical standard. Personally I go pretty much everywhere. My uniforms include FR pants and I have a choice of a short sleeve non-FR shirt or a long sleeve FR shirt, and I carry a 40 cal suit on the truck in case that's not enough. I also carry a full change of clothes and I even have an old Hi Viz+FR lineman's shirt for those very rare situations where I need both. I think I've got both FR and regular hair nets too. It also comes in handy if I end up having to stay overnight or I get so dirty I need to change to drive home. This covers me with all my clients. All line crews around here work every day in the hot summer sun in the South in full 4+ ATPV uniforms. They hated it when it changed but they've learned to live with it. There is also a plant in Trinidad that I know the engineer at that has the same standard as every day workwear for all of their electricians and they've had no increases in heat related illnesses. The advantage of this approach is not only is IEEE C2 revised a lot more often (5 year cycle) but the results haven't been thrown into question by test results. The downside of course is there is always minimal PPE.
But if we want to get to NO PPE, our options are a little more limited. Right now there is a consensus safety standard, IEEE 1584-2002, that gives a specific criteria for no PPE which is the "<240 V, <=125 kVA" rule. Just as with other consensus standards such as NEC (NFPA 70) or NFPA 70E the normal result is that consensus safety standards are standards. They are however subject to review and change over time. On occasion an experimental result or some other new information will get published. This becomes public input which is used by the standards committee to be considered for review and revision of the standard that the committee maintains. The revised standard eventually gets published. Then the various organizations that base their results on the standard review and accept, reject, or modify, the new safety standard. Due to the extensive review process itself, consensus safety standards have the legal weight of just below regulations. So you could ignore all the machinations and simply hold fast to PUBLISHED, APPROVED, FINAL consensus safety standards. That's the normal way things are done.
Now as engineers of course we should review and accept, reject, or modify the underlying standards that we use as the basis for a decision. As an example if I'm working in the mining business in the U.S., the federal mine safety administration (MSHA) recognizes a part of the 1976 edition of NEC as part of their regulations. Mines fall generally outside state regulatory authority. This means that unlike most commercial/industrial jurisdictions, mines are really only required to meet the 1976 NEC standard. They can freely adopt or ignore NEC. Most more progressively oriented mines adopt a more current NEC standard as an engineering and safety standard though because they recognize that the 1976 standard and the mine regulations leave a LOT to be desired. This would be equivalent to following OSHA 1910 Subchapter S for general industry...it is certainly a regulatory requirement but far from a best practice today.
The danger of course is that if you develop your own standard, you are on your own to defend the underyling basis for that decision as well as whether or not you followed it. This is a far cry different from following a consensus safety standard in which case from a legal point of view the consensus safety standard has more legal weight than a bunch of engineers arguing over test results that were done in a lab and not necessarily verified or subject to an extensive review process. However there is a way to marry the two and keep yourself out of trouble. If you modify the standard to use a set of requirements that is more strict than the underlying standard, and you state that is in your standards review, you can pretty much freely implement any modified version of the underlying standard you want. This allows you to use experimental results directly, or use interim drafts of IEEE 1584, or simply pull something out of thin air because at the end of the day from a legal point of view, you are following IEEE 1584-2002 OR BETTER so IEEE 1584-2002 is the fall back support for this approach. As long as the path to proving the "OR BETTER" part of your criteria is easily followed, you honestly can't go wrong. So if you want to take this approach IEEE 1584 Second edition Draft 6 uses <240 VAC, <2 kA available fault current as the cutoff but for obvious reasons don't take my word for it. Download and read if for yourself.
So again you have 4 options:
1. Ignore arc extinguishment. Use IEEE 1584-2002 (empirical or Lee as appropriate) for single phase using the guidance from IEEE 1584-2002 to treat single phase as 3 phase. I'd be a little more nervous about using ETAP's single phase model without some kind of documentation that traces back to some kind of standard since it may not carry the same weight as a consensus safety standard.
2. Follow IEEE C2 (NESC) which is probably the strongest support among standards based approaches right now although it has a minimum PPE requirement. Whole industries and many plants have done this.
3. Follow IEEE 1584-2002 as it stands. Let the review process run it's course like it normally does. The equations themselves aren't all that great and a big improvement is coming on those too along with enclosure sizes. It's been over a decade since the last revision in an active research area so no wonder the standard shows it's age.
4. Modify IEEE 1584-2002 based on a more restrictive approach based on something you develop yourself. The engineering basis for your standard is not really in question since you are essentially following IEEE 1584-2002. You can almost say anything you want here.
5. Strike out on your own? This seems like serious trouble. Granted since IEEE 1584-2002 empirical results among others is just that...empirical, and not based on theory, the only arguments other than consensus review are goodness of fit to the data. So it is possible to use say the simplified arc flash equations published on an IEEE forum by a respected researcher, but we're treading on thin ice here.