NEC classification - Class and division definitions

1. Class I Locations are those in which flammable gases or vapors are, or may be, present in the air in quantities sufficient to produce explosive or ignitable mixtures.

2.  Class I, Division 1 Location

Where hazardous concentrations of flammable gases or vapors exist continuously, intermittently, or periodically under normal conditions, Where hazardous concentrations of such gases or vapors may exist frequently because of repair or maintenance operations or because of leakage, or Where breakdown or faulty operation of equipment or processes which might release hazardous concentrations of flammable gases or vapors, might also cause Simultaneous failure of electrical equipment.

3.  Class I, Division 2 Location

Where flammable volatile liquids or flammable gases are handled, processed or used, but in which the hazardous liquids, vapors, or gases will normally be confined within closed containers or closed systems from which they can escape only in case of accidental rupture or breakdown of such containers or systems, or in case of abnormal operation of equipment, Where hazardous concentrations of gases or vapors are normally prevented by Positive mechanical ventilation, but which might become hazardous through failure or abnormal operation of the ventilating equipment, or Which are adjacent to Class I, Division 1 locations, and to which hazardous concentrations of gases or vapors might occasionally be communicated unless such communication is prevented by adequate positive pressure ventilation from a source of clean air, and effective safeguards against ventilation failure are provided.

4.  Class II Locations are those which are hazardous due to the presence of Combustible dust.

5. Class III Locations are those which are hazardous due to the presence of easily ignitable fibers or flyings, but in which such fibers or flyings are not likely to be in suspension in air in quantities sufficient to produce ignitable mixtures.

Note : It cannot be assumed that equipment suitable for Class I locations is therefore suitable for Class II and III, or that equipment suitable for Class II locations is similarly automatically suitable for Class III locations