Hazardous Locations: The ATEX Classification in the U.S. and Canada

The standards applied in North America for classifying areas where potentially explosive substances may be present (referenced in particular by NFPA 2, NFPA 70, NFPA 497, NFPA 499, API 500, NFPA 505) differ from those applied in Europe, but their ultimate aim is to identify and classify work areas according to the presence of potentially explosive substances.

North American regulations perform a classification of environments based on the type of substances or materials to which they are exposed.

There are 3 class locations and they are as follows:

• Class I locations: Gases or Vapors from flammable liquids (Class I areas are those where flammable gases, vapors produced from flammable liquids, or vapors produced from combustible liquids are or may be present in the air in quantities sufficient to produce explosive or flammable mixtures [500.5]).

• Class II locations: Dust (Class II locations are those hazardous due to the presence of combustible dust [500.5]).

• Class III locations: Flammable fibers and chips (presence of readily flammable fibers or where materials that produce combustible volatiles are handled, produced, or used, but where such fibers/volatiles are unlikely to be in suspension in the air in sufficient quantities to produce flammable mixtures[500.5]).

The regulations decide to perform a classification of zones, or rather hazardous zones, into two classes, which are based on the frequency of the presence of the hazard source.

The class divisions are as follows:

• Division I: Constant or occasional presence, during normal plant operation, of hazardous substance. Equivalent to zone 0/20 or 1/21
• Division II: Presence of the hazardous substance only when there is a fault (provided the fault is not frequent, in which case it falls under Division I). Equivalent to zone 2/22

The legislation also decides to make a classification of hazardous materials and substances according to their degree of hazardousness, using the MESG and MIC ratio as benchmarks by making a 7-letter division.

The first 4 belong to gases, while the remaining 3 belong to dust.

• Group A: Acetylene.
• Group B: Flammable gases, vapors produced from flammable liquids or vapors produced from combustible liquids mixed with air that may burn or explode, having a maximum experimental safety distance (MESG) value less than or equal to 0.45 mm or a minimum ignition current ratio (MIC ratio) less than or equal to 0.40.
• Group C: ditto …(MESG value) greater than 0.45 mm and less than or equal to 0.75 mm, or a minimum ignition current ratio (MIC ratio) greater than 0.40 and less than or equal to 0.80.
• Group D: ditto … (MESG) value greater than 0.75 mm or a minimum ignition current ratio (MIC ratio) greater than 0.80.
• Group E: (Metal dusts) Atmospheres containing combustible metal dusts, including aluminum, magnesium and their commercial alloys, or other combustible dusts whose particle size, abrasiveness and conductivity present similar hazards in the use of electrical equipment.
• Group F. Atmospheres containing combustible coal dusts that have more than 8 percent total trapped volatiles or that have been sensitized by other materials so as to present an explosion hazard. Coal dust, carbon black, coal, and coke are examples of carbon dust.
• Group G. Atmospheres containing combustible dusts not included in Group E or Group F, including flour, grain, wood, plastics, and chemicals.