The refinery areas to which this publication applies are those in which flammable petroleum gases and volatile liquids are: a, processed; b, stored; and c, loaded, unloaded, or otherwise handled. In establishing the limits of classified areas, it is assumed that the vapors are heavier-than-air Group D hazardous materials as defined in Art. 500, NFPA Bulletin No. 70: National Electrical Code (see Appendix). If there are different conditions than average, or if the flammable vapors are lighter than air (NEC Groups A, B, or C), it may be necessary to make suitable modification of the practices out-lined herein. However, the classification of areas based on heavier-tha-air vapors should normally be conservative for lighter-than-air vapors. Refinery processing facilities consist of specialized equipment within which liquids or vapors are continuously processed at high rates and at elevated temperatures and pressure. Both chemical and physical changes occur in these materials, and during "abnormal" conditions the composition and properties of stocks may change drastically. These conditions, together with considerations of operating continuity, dictate standards of refinery design that are not warranted in other operations of the petroleum industry. For these reasons, it is considered that the recommendations of this publication do not apply outside of refineries. Factors such as corrosion, weather, maintenance, equipment standardization and inter-changeability, and possible process changes or expansion frequently dictate the use of special enclosures or installations for electrical systems. Such factors are outside the scope of this publication, which is concerned entirely with proper application of electrical equipment to avoid ignition of flammable vapors and gases.