Criteria of Electrical Fittings for Hazardous Locations
Hazardous Area Classifications – an Overview
The intent of this document is to provide a broad overview of hazardous area classifications and the types of protection techniques involved. The information provided in this bulletin is for educational purposes and should not be used in place of any other source or governing documents
The National Electrical Code (NEC) defines hazardous locations ( HAZLOC) 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.” A substantial part of the NEC is devoted to the discussion of hazardous locations. Electrical equipment installed in such locations could provide an ignition source, due to electrical arcing, or high temperature. Standards and regulations exist to identify such locations, classify the hazards, and design equipment for safe use in such locations.
When electrical equipment is used in, around, or near an atmosphere that has flammable gases or vapors, flammable liquids, combustible dusts, ignitable fibers, or flyings, there is always a possibility or risk that a fire or explosion might occur. Those areas where the possibility or risk of fire or explosion might occur due to an explosive atmosphere and/or mixture is often called a hazardous (or classified) location/area.
Currently, there are two systems used to classify these hazardous areas; the Class/Division system and the
Zone system. The Class/Division system is used predominately in the United States and Canada, whereas the rest of the world generally uses the Zone
system. However, the United States and Canada are trending more towards the Zone System.
Class/Division System Hazardous locations per the Class/Division system are classified according to the Class, Division, and Group
1. Class—The Class defines the general nature (or properties) of the hazardous material in the surrounding atmosphere which may or may not be in sufficient quantities.
Class I—Locations in which flammable gases or vapors may or may not be in sufficient quantities to produce explosive or ignitable mixtures.
Class II—Locations in which combustible dusts (either in suspension, intermittently, or
periodically) may or may not be in sufficient quantities to produce explosive or ignitable mixtures.
Class III—Locations in which ignitable fibers may or may not be in sufficient quantities to produce explosive or ignitable mixtures.
2. Division—The Division defines the probability of the hazardous material being able to produce an explosive or ignitable mixture based upon its presence.
Division 1 indicates that the hazardous material has a high probability of producing an explosive or ignitable mixture due to it being present continuously, intermittently, or periodically or from the equipment itself under normal operating conditions.
Division 2 indicates that the hazardous material has a low probability of producing an explosive or ignitable mixture and is present only during abnormal conditions for a short period of time.
3. Group—The Group defines the type of hazardous material in the surrounding atmosphere.
Groups A, B, C, and D are for gases* (Class I only) while groups E, F, and G are for dusts and flyings (Class II or III).
Group A—Atmospheres containing acetylene.
Group B—Atmospheres containing a flammable gas, flammable liquid-produced vapor, or combustible liquid-produced vapor whose MESG is less than 0.45 mm or MIC ratio is less than 0.40.
- Typical gases include hydrogen, butadiene, ethylene oxide, propylene oxide, and acrolein.
Group C—Atmospheres containing a flammable gas, flammable liquid-produced vapor, or combustible liquid-produced vapor whose MESG is
greater than 0.45 mm but less than or equal to 0.75 mm or MIC ratio is greater than 0.40 but less than or equal to 0.80. Typical gases include ethyl ether, ethylene, acetaldehyde, and cyclopropane.
Group D—Atmospheres containing a flammable gas, flammable liquid-produced vapor, or combustible liquid-produced vapor whose MESG is
greater than 0.75 mm or MIC ration is greater than 0.80. Typical gases include acetone, ammonia, benzene, butane, ethanol, gasoline, methane,
natural gas, naphtha, and propane.
Group E—Atmospheres containing combustible metal dusts such as aluminum, magnesium, and their commercial alloys.
Group F—Atmospheres containing combustible carbonaceous dusts with 8% or more trapped volatiles such as carbon black, coal, or coke dust.
Group G—Atmospheres containing combustible dusts not included in Group E or Group F. Typical dusts include flour, starch, grain, wood, plastic, and chemicals.