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As defined in 505.5(B)(2) of NFPA 70, a Class I, Zone 1 location shall be any location where one of the following conditions exists:
  1. An ignitible concentration of flammable gas or vapor is likely to exist under normal operating conditions.
  2. An ignitible concentration of flammable gas or vapor might exist frequently because of repair or maintenance operations or because of leakage.
  3. Breakdown or faulty operation of equipment or processes might release an ignitible concentration of flammable gas or vapor and might also cause simultaneous failure of electrical equipment in such a way as to directly cause the electrical equipment to become a source of ignition.
  4. An ignitible concentration of flammable gas or vapor might occasionally be transmitted from an adjacent Class I, Zone 0 location, unless such transmission is prevented by positive pressure ventilation from a source of clean air and effective safeguards against ventilation failure are provided.
[33:6.3.1.4]

Related Code Sections

(1) Special Occupancies, Inadequate Ventilation
In a Class I, Zone 1 location that is so classified due to inadequate ventilation, electrical equipment suitable for Class I, Zone 2 locations shall ...