... all allow the use of a face shield with an arc rating less than the incident energy. Can anyone explain why? Thanks.

Perhaps this language from the Federal Register may partially answer the question,

"As explained previously in this section of the preamble, much overhead line work poses hazards involving exposure to single-phase arcs in open air. In such exposures, there is little or no reflected or convective heat energy. In addition, as also noted earlier, OSHA concluded that a reasonable distance from the employee to the arc for these exposures is 380 millimeters (15 inches), measured from the crotch of the employee’s hand to the chest.364 (See Table 14, earlier in this section of the preamble.) OSHA estimates that the employee’s face will likely be at least 455 millimeters (18 inches) from the arc. Because the heat energy from a single-phase arc in air drops in inverse proportion to the square of the distance, the roughly 20-percent increase in distance (from 380 to 455 millimeters) results in a drop in incident energy of nearly 30 percent (Ex. 0430). Therefore, because the incident energy at the employee’s head will be more than 30 percent lower than the estimated incident energy, which OSHA based on the exposure at the employee’s chest, OSHA concludes that the thresholds for requiring head and face protection for exposures involving a single-phase arc in air can be higher than the threshold for requiring head and face protection for three-phase exposures."

Thanks. Good information.

Follow up questions:

A note under the Minimum head and face protection table, "Arc rated hood or faceshield with balaclava" column in Appendix E states: "Note that § 1910.269(l)(8)(v) permits this type of head and face protection at any incident energy level."

1) Any IE? Without specifying an arc rating for the faceshield and balaclava? I can use an 8 cal rated faceshield and balaclava where the exposure is several time higher?
2) I've scoured 1910.269(l)(8)(v), and just can't find anything about any IE. Where is this?