NFPA 497-2021 Recommended Practice for the Classification of Flammable Liquids, Gases, or Vapors and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas

NFPA 497-2021 Recommended Practice for the Classification of Flammable Liquids, Gases, or Vapors and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas.
3.3.11 Minimum Ignition F.nergy (MIE). The minimum energy required from a capacitive spark discharge to ignite the most easih ignitible mixture ala gas or vapor.
3.3.12 UncIanified Locaiions Locauons determined to be neither Class 1, Division 1; Class 1, Division 2; Zone 0; Zone 1; 7mw 2; Class TI, Division 1; Class 11, Division 2; Class 111, Division 1; Class III, Division 2; Zone 20; Zone 21; Zone 22; nor any combination thereof. [70:100]
C]apter 4 Classification of Combustible Materials
4.1 Natha,d Becb*izl Cede Criteria.
4.1.1 Articles 500 and 505 of the NEC classify a location in which a combustible material is or may be present in the atmosphere in sufficient concentrations to produce an ignitible mixture.
4.l.2’ In a Class I hazardous (classified) location, the combustible material present is a flammable gas. flammable liquid— produced vapor, or combustible liquid—produced vapor mixed with air that could burn or explode.
4.2 Behavior of Class I (Combustible Matcrial) Gases, Vapors, and Liquids.
4.2.1 lighter-than-Air (apor Density Less than 1.0) Gases. These gases tend to dissipate rapidly iii the atmosphere. They will not affect as great an area as will heavier-than-air gases or vapors. Except in enclosed spaces, such gases seldom accumulate to form an ignitible mixture near grade level, where most electrical installations are located. A lighter-than-air gas that has been cooled sufficiently could behave like a heavier-than- air gas until it absorbs heat from the surrounding atmosphere.
4.2.2 Heavier-than-Air (Vapor Density Greater than 1.0) Gases. These gases tend to fall to grade level when released. 11w gas could remain for a significant period of time, unless dispersed 1w natural or forced ventilation. A heavier-than-air gas that has been heated sufliciendy to decrease its density could behave like a lighter.than-air gas until cooled I’ the surrounding atmosphere.
4.2.3 Applicable to All Densities. As the gas difhz.ses into the surrounding air, the density of the mixture approaches that of air.
4.2.4 Compressed Liquefied Gases. These gases are stored above their normal boiling point but art- kept in the liquid state by pressure. When released, the liquid immediately expands and vaporizes, creating large volumes of cold gas. The cold gas behaves like a heavier-than-air gas.NFPA 497 pdf download.