NCCER Introduction to the National Electrical Code (26105-20) Practice Test

The concept being tested is how hazardous locations are classified by material type and whether ignition is possible in air. Class II covers ignitable dusts, which can form ignitable mixtures in air when present in the right amounts. Class III, on the other hand, covers ignitable fibers or flyings, where these fibers may be present but are not normally in the air in quantities large enough to form an ignitable mixture. The description—ignitable fibers or dust in the air, not in quantities to form ignitable mixtures—points to the fiber aspect, and the emphasis on not forming a combustible air mixture means it fits Class III best. If dust were present in sufficient quantity to form an ignitable mixture, that would be Class II.