SRW wrote:
If the results of an arc flash hazard analysis were that the incident energy was 7.5 cal but the available bolted fault current exceeded the values, at the system voltage, in Table H.4 would it still be acceptable to use 8 cal PPE?
1. If the available bolted fault exceeds the tabular result, then you can't use the tabular results because you violated the assumptions.
2. If you did the analysis, then tabular results don't matter anyways. You use the actual calculated result.
3. The PPE tables even with the "4 level' system (0-4) are not intended to apply to actual arc flash hazard calculations anyways. Those are intended for use with the other tables.
With that out of the way, we can get down to discussing PPE if you have actually calculated your incident energy:
1. In practice, most people read those tables and apply their calculated incident energies to the tabular results just because it is a convenient way to determine required PPE. As long as the PPE specified exceeds the incident energy calculated, then it is sufficient. Note that some manufacturers actually specify a higher cal/cm^2 value for their PPE and give both ratings (ATPV and tabular PPE rating). Since you used the calculation method, you can use either value. The actual ATPV value will be higher.
2. Since you have calculated values, there is nothing stopping you from implementing as many or as few "levels" as you desire. Two examples:
A. Many companies have a "2" level system; 1.2 cal/cm^2 and 40 cal/cm^2. They don't bother with the ones in between, including the one I currently work for. There are practical problems with doing this for electricians though so you may want to add a third level.
B. Along the same lines, many go for 1.2, 8, and 40 cals/cm^2 as suggested in the appendix, although often the "8" level becomes more like 10-12 depending on the PPE manufacturer. The reason for picking "8" instead of "4" is because 12 oz cotton treated with the ammonia "FR" process almost automatically yields around an 8-12 cal/cm^2 ATPV. The only fundamental PPE difference between 4 and 8 cal/cm^2 is the inclusion of the balaclava or some sort of hood which was a recent (2009) addition.
C. I (and others) have also implemented another variation on the 2 level system in the past: 4, and 40. Especially in a glass, welding, or iron/steel industry shop, FR clothing is usually mandatory to begin with. Except for the balaclava or hood, the minimum standard work wear is already 4 cal/cm^2, so 1.2 cal/cm^2 is not even a consideration. It is also very easy to make a 3 level system (4/8/40) with minor additional PPE requirements.
There is also nothing stopping you from "mixing it up" in terms of using these simplified approaches with the tabular approach. Regardless of how you determine the "levels" to be used, the fundamental rule is that the PPE to be worn must equal or exceed the available incident energy level if PPE is required. The determination of whether or not PPE is required depends on your risk assessment, and that in turn depends on the task to be done, as made abundantly clear by all the references to looking at electrical hazards on a task-by-task basis in the 2012 edition of 70E.
