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Definitions of Class, Division and Group Hazardous Locations in N.A. Market

From: www.njzlighting.com May 16, 2017
In North America, the classification system that is most widely utilized is defined by the NFPA Publication 70, NEC, and CEC. They define the type of hazardous substances that is, or may be, present in the air in quantities sufficient to produce explosive or ignitable mixtures. The NFPA establishes area classifications based on Classes, Divisions, and Groups which are factors combined to define the hazardous conditions of a specific area.

Class Definition

The NFPA Publication 70, NEC, and CEC define three categories of hazardous materials that have been designated as Class I, Class II, or Class III. The Classes define the type of explosive or ignitable substances which are present in the atmosphere such as:
• Class I locations are those in which flammable vapors and gases may be present.
• Class II locations are those in which combustible dust may be found.
• Class III locations are those which are hazardous because of the presence of easily ignitable fibers or flyings.

Division Definition

Each of the three Classes, discussed earlier, is further subdivided into two Divisions, Division 1 or Division 2.The Division defines the likelihood of the hazardous material being present in a flammable concentration.

Division Definitions
Division 1 In which ignitable concentrations of hazards exists under normal operation conditions and/or where hazard is caused by frequent maintenance or repair work or frequent equipment failure.
Division 2 In which ignitable concentrations of hazards are handled, processed or used, but which are normally in closed containers or closed systems from which they can only escape through accidental rupture or breakdown of such containers or systems.

Figure provides a graphical illustration to help with the understanding of the Division definition. The NEC and CEC, like all other such codes at the present time, do not attempt to quantify the definition of Division 1 and Division 2.

The installation and conduit requirements for Division 1 locations are more stringent than for Division 2 locations.

Figure. Division Definition

Group Definition

The explosive characteristics of the air mixtures of gases, vapors, or dusts vary with the specific material involved. Materials have been placed in groups based on their ignition temperatures and explosion pressures. Class I and Class II Divisions are further subdivided into Groups of hazardous materials. The Groups define substances by rating their flammable nature in relation to other known substances.

Combustible and flammable gases and vapors are divided into four Groups. The classification is based on maximum explosion pressures. Refer to Appendix for diagrams that show the relationship between Classes, Divisions and Groups.

The table below provides examples of which materials actually make up specific Groups.

Class Division Group Flammable Material Maximum Experimental Safe Gap (MESG) Minimum Igniting Current Ratio (MIC)
Class I Division 1 & 2 A Acetylene
Class I Division 1 & 2 B • Hydrogen
• Butadiene
• Ethylene Oxide
• Propylene Oxide
£ 0.4 mm £ 0.4
Class I Division 1 & 2 C • Ethylene
• Cyclopropane
• Ethyl Ether
> 0.45 mm
£ 0.75 mm
> 0.4
£ 0.8
Class I Division 1 & 2 D • Propane
• Acetone
• Ammonia
• Bezene
• Butane
• Ethanol
• Gasoline
• Methanol
• Natural Gas
³ 0.75 mm > 0.8


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