A Class I, Division 2 location is a location: (1) in which volatile flammable 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: or (2) in which 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 (3) that is adjacent to a Class I, Division 1 location, and to which hazardous concentration 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. (See: NEC & API RP 500B.) Group C: Atmospheres that do or may contain hydrogen sulfide (H2S), or other substances specified by NFPA 497M. Group D: Atmospheres that do or may contain natural gas, hydrocarbons or other substances specified by NFPA 497M. Unclassified Locations: An unclassified location is a location not classified as Division 1 or Division 2. (RT 14F)