Hazardous Areas Classification - North America

North American hazardous locations classification in classes, divisions and groups

Electrical devices used in hazardous areas need to be certified for use according the requirements specified for the area.

In North America certification is provided by

  • Factory Mutual - An approval agency primarily concerned with insurance underwriting.
  • Underwriters Laboratories - An independent, not-for-profit product safety testing and certification organization.
  • Canadian Standards Association - A not-for-profit membership-based association serving business, industry, government and consumers in Canada developing standards addressing public safety and health.

The types of protection required depends on the risk involved in the area.

In general hazardous locations in North America are separated by classes, divisions, and groups to define the level of safety required for equipment installed in these locations.

Classes

The classes defines the general nature of hazardous material in the surrounding atmosphere.

ClassHazardous Material in Surrounding Atmosphere
Class I Hazardous because flammable gases or vapors are present in the air in quantities sufficient to produce explosive or ignitable mixtures.
Class II Hazardous because combustible or conductive dusts are present.
Class III Hazardous because ignitable fibers or flying's are present, but not likely to be in suspension in sufficient quantities to produce ignitable mixtures. Typical wood chips, cotton, flax and nylon. Group classifications are not applied to this class.

Divisions

The division defines the probability of hazardous material being present in an ignitable concentration in the surrounding atmosphere.

DivisionPresence of Hazardous Material
Division 1 The substance referred to by class is present during normal conditions.
Division 2 The substance referred to by class is present only in abnormal conditions, such as a container failure or system breakdown.

Groups

The group defines the hazardous material in the surrounding atmosphere.

GroupHazardous Material in Surrounding Atmosphere
Group A Acetylene
Group B Hydrogen, fuel and combustible process gases containing more than 30% hydrogen by volume or gases of equivalent hazard such as butadiene, ethylene, oxide, propylene oxide and acrolein.
Group C Carbon monoxide, ether, hydrogen sulfide, morphline, cyclopropane, ethyl and ethylene or gases of equivalent hazard.
Group D Gasoline, acetone, ammonia, benzene, butane, cyclopropane, ethanol, hexane, methanol, methane, vinyl chloride, natural gas, naphtha, propane or gases of equivalent hazard.
Group E Combustible metal dusts, including aluminum, magnesium and their commercial alloys or other combustible dusts whose particle size, abrasiveness and conductivity present similar hazards in connection with electrical equipment.
Group F Carbonaceous dusts, carbon black, coal black, charcoal, coal or coke dusts that have more than 8% total entrapped volatiles or dusts that have been sesitized by other material so they present an explosion hazard.
Group G Flour dust, grain dust, flour, starch, sugar, wood, plastic and chemicals.

The specific hazardous materials within each group and their automatic ignition temperatures can be found in Article 500 of the National Electrical Code and in NFPA 497.

Group A, B, C and D apply to class I locations. Group E, F and G apply to class II locations.

Temperature Code

A mixture of hazardous gases and air may ignite in contact with a hot surface. The condition for ignition depends on several factors as surface area, temperature and concentration of gas.

Equipment approved receives a temperature code indicating the maximum surface temperature of the equipment.

Temperature CodeMaximum Surface Temperature
oFoC
T1 842 450
T2 572 300
T2A 536 280
T2B 500 260
T2C 446 230
T2D 419 215
T3 392 200
T3A 356 180
T3B 329 165
T3C 320 160
T4 275 135
T4A 248 120
T5 212 100
T6 185 85

Equipment that not exceed a maximum surface temperature of 212 oF (104 oF ambient temperature) is not required to be marked with a temperature code (NEC).

Recommended reading for this topic:

  • National Electrical Code, NFPA 70, Chapter 5, Article 500
  • 29 CFR 1910 Subpart S, Electrical 1910.307
  • NFPA 497, "Classification of Gases, Vapors, and Dusts for Electrical Equipment in Hazardous Classified Locations"
  • NFPA Handbook, "Electrical Installations in Hazardous Locations, " by P. J. Schram and M. W. Earley
  • NFPA 70E, Chapter 5, "Hazardous (Classified) Locations"
  • NFPA (Fire) HAZ-10, "Fire Protection Guide to Hazardous Materials"
  • ANSI/UL 913, "Intrinsically Safe Apparatus"
  • NFPA 496, "Purged and Pressurized Enclosure for Electrical Equipment in Hazardous Locations."

Related Topics

  • Process Control - Instrumentation and process control systems - engineering and documentation
  • Miscellaneous - Miscellaneous engineering related topics like Beaufort Wind Scale, CE-marking, drawing standards and more
  • Risk, Reliability and Safety - Risk, Reliability and Safety in Process Control Systems

Related Documents

Tag Search

  • en: hazardous areas classes divisions groups
  • es: √°reas peligrosas clases de grupos de divisiones
  • de: explosionsgef√§hrdeten Bereichen Klassen Divisionen Gruppen

Search the Engineering ToolBox

- "Search is the most efficient way to navigate the Engineering ToolBox!"

Engineering ToolBox - SketchUp Extension - Online 3D modeling!

3D Engineering ToolBox Extension to SketchUp - add parametric components to your SketchUp model

Add standard and customized parametric components - like flange beams, lumbers, piping, stairs and more - to your SketchUp model with the Engineering ToolBox - SketchUp Extension/Plugin - enabled for use with the amazing, fun and free SketchUp Make and SketchUp Pro. Add the Engineering ToolBox extension to your SketchUp from the Sketchup Extension Warehouse!

Translate the Engineering ToolBox!
About the Engineering ToolBox!

close