How are hazardous locations classified for electrical installations?

Hazardous locations are classified by NEC Article 500 based on type of hazard and likelihood of presence. Class I involves flammable gases/vapors, Class II combustible dusts, Class III ignitable fibers. Division 1 means hazard is normally present; Division 2 means present only under abnormal conditions. Alternatively, Zone classification (0, 1, 2 for gases; 20, 21, 22 for dusts) indicates probability of hazardous atmosphere. Classification determines required equipment ratings and installation methods.