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Flow Meter Selection Guide

Hazardous Area Classifications Guide

The National Electrical Code (NEC) defines hazardous locations as those areas "where fire or explosion hazards may exist due to flammable gases or vapors, flammable liquids, combustible dust, or ignitable fibers or flyings."

Hazardous locations are classified in three ways by the NEC: Type, Condition, and Nature.

Hazardous Location Types - Classes

According to the NEC, there are three types of hazardous locations.

Class I: The first type of hazard is one which is created by the presence of flammable gases or vapors in the air, such as natural gas or gasoline vapor. When these materials are found in the atmosphere, a potential for explosion exists, which could be ignited if an electrical or other source of ignition is present.

Class II: The second type of hazard listed by the National Electrical Code are those areas made hazardous by the presence of combustible dust. These are referred to in the Code as "Class II Locations." Finely pulverized material, suspended in the atmosphere, can cause as powerful an explosion as one occurring at a petroleum refinery.

Class III hazardous locations, according to the NEC, are areas where there are easily-ignitable fibers or flyings present, due to the types of materials being handled, stored, or processed. The fibers and flyings are not likely to be suspended in the air, but can collect around machinery or on lighting fixtures and where heat, a spark or hot metal can ignite them.

Hazardous Location Conditions - Divisions

In addition to the types of hazardous locations (Class) the NEC also designates two types of conditions under which these hazards are present. These two conditions are described as, first, normal conditions, and, second, abnormal conditions.

Division 1 - In the normal condition, the hazard would be expected to be present in everyday production operations or during frequent repair and maintenance activity.

Division 2 - When the hazardous material is expected to be confined within closed containers or closed systems and will be present only through accidental rupture, breakage or unusual faulty operation, the situation could be called "abnormal."

Nature of Hazardous Substances - Groups

The gases and vapors of Class I locations are broken into four groups by the Code: A, B, C, and D. These materials are grouped according to the ignition temperature of the substance, its explosion pressure, and other flammable characteristics.

  • Group A - Acetylene
  • Group B - Hydrogen (or gases of equivalent hazard)
  • Group C - Ethylene (or gases of equivalent hazard)
  • Group D - Gasoline (or gases of equivalent hazard)

Groups E, F and D are reserved for Class II Dust locations. These groups are classified according to the ignition temperature and the conductivity of the hazardous substance. Conductivity is an important consideration in Class II locations, especially with metal dusts.

  • Group E - Metal Dusts such as aluminum, magnesium, etc.
  • Group F - Coal Dust, carbon black, charcoal dust, coal, coke dust & similar types of materials.
  • Group G - Grain Dust, flour, starch, cocoa and similar types of materials.

Meet the Author
Flow Specialist - Bill Michie Written By: Bill Michie  
Flow Applications Specialist
Cross Company
Instrumentation Group

Phone: (866) 905-9790 (M-F, 8am-5pm Eastern)
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