The list of potential arc-flash hazards in 110.16 isn't meant to be a checklist you can use to see where arc-flash hazard warnings aren't required. The listing of equipment emphasizes, rather than limits, applicability. An arc-flash warning must be posted for any electrical equipment if people might access it, even for a visual examination, while it's energized or if it's not in a dwelling unit.

The NEC doesn't detail the arc-flash safety requirements, and neither does OSHA. These are in NFPA-70E, as noted in Informational Note 1 in 110.16. The NEC is concerned only with the posting of warning signs. This requirement may go by the wayside someday, because it's now redundant with other measures required by NFPA-70E.

The NEC requirement is a relic from the days when NFPA-70E was a weak document and we made incorrect assumptions about arc flash and energy levels. We now know that arc flash can be serious anywhere in an industrial or commercial environment, so the signs are warranted wherever you have electrical equipment.

In nearly all situations, spending time trying to figure out if people might examine, maintain, or adjust the equipment while it's energized is a waste of time. Assume they will, and post the warnings. But rely on a solid NFPA-70E training and implementation program, not signs, to protect people.