A Guide to Class, Divisions, Groups, and Zones

Hazardous locations are listed in both the National Electrical Code (NEC) for US standards and the Canadian Electrical Code (CEC) for Canadian standards. Within the NEC, hazardous locations are broken down into Class, Divisions, Groups and Zones. The CEC uses basically the same system, but the 23rd edition of the CEC that came out in 1998 adopted the International Electrotechnical Commission’s (IEC) “Three Zone Area” classification system for Class I hazardous locations that refer to gases, vapors and mists. A detailed breakdown and comparison are covered below. Though, the division of Class I hazardous locations will still be used in the next few editions.

Regardless of the classification nomenclature, the reason for the breakdown is the same; it allows operators to better understand if the area they are working in is considered dangerous so that proper, explosion proof equipment can be used for added safety. Before diving into the breakdown of hazardous location classifications, a brief overview of what a hazardous location will be helpful.

Both the NEC and CEC use the word ‘hazardous’ as a way to define how a specific location is classified due to either the flammability properties in that area or the specific use of that area. Simply put, a hazardous location is an area where the possibility of an explosion or fire exists, which can be created by the presence of flammable vapors or gases, combustible dust, or easily ignitable fibers or flyings.

Some examples of hazardous locations include places where gasoline or petroleum is refined, stored or dispensed, factories where starches, candy, medicines, fireworks and plastics are made, mills where flour, grains or feed are processed, plants or mills where shoes, textiles, leathers, wood or cotton are manufactured and more.

This article will explore the different hazardous location classifications in both the NEC and the CEC to give readers a better understanding of what each tier in the system refers to. This will enable people who work in potentially hazardous locations to get a better understanding of how to tell if their environment is considered a hazardous location and the proper precautions to take, such as using explosion proof lighting and equipment.

NEC & CEC Classes

Classes define the general nature of the hazardous material that may be in the surrounding atmosphere. They are the first tier in the NEC and CEC classification systems. The NEC and CEC put flammability properties into classes, such as Class I, II and III. Each class represents different types of flammable materials or elements.

Class I refers to flammable gases or vapors that are or may be present in the atmosphere in significant enough quantities to produce a fire or explosion if ignited. Class I is further broken down into two Divisions and three Zones in the CEC and six Zones in the NEC.

Class II refers to combustible or conductive dust particles that are or may be present in the atmosphere in significant enough quantities to produce a fire or explosion if ignited. Class II is further broken down into two Divisions.

Class III refers to easily ignitable fibers or flyings that are or may be present in the surrounding area in quantities significant enough to produce a fire or explosion if ignited. Class III is further broken down into two Divisions.

NEC and CEC Divisions

Divisions define the probability of a hazardous material or element being present in a surrounding atmosphere that has the ability to be ignited. They are the second tier in the NEC classification system and fall under Classes. Divisions are broken into two levels that address all hazardous materials.

A Division 1 classification means that the substance in the Class it is paired with has a high probability of producing an ignitable or explosive mixture due to it being present continuously, intermittently or periodically. The substance may also come from the equipment in the area under normal circumstances.

A Division 2 classification means that the substance in the Class it is paired with has a low probability of producing an ignitable or explosive mixture due to it being present only during abnormal circumstances or for a short period of time, such as a system breakdown or container failure.

In terms of the CEC system, Division 1 relates only to Class I and Division 2 relates only to Class II and can still be used for the maintenance and repair of existing facilities. However, any new construction must reference the newer International Electrotechnical Commission (IEC) Zone Classification. Simply put, Divisions are used for the pre-1998 version of the CEC and Zones are used for the IEC Classification of the 1998 CEC.

NEC & CEC Groups

Groups define the type of hazardous material in the area. Groups are broken out into seven different segments – A through G. The NEC lists and defines hazardous gases, vapors and dusts by Groups characterized by their combustible or ignitable properties. Two systems of Groups exist for the CEC, including the pre-1998 Gas Groups consisting only of Groups A, B, C and D. The IEC System consists of Groups IIA, IIB and IIC. Both Group classifications for Canada are accepted by the CEC.

[NEC & CEC] Group A refers to an area that contains acetylene. If an area has this element present or there is a possibility it could be in the atmosphere, the area is considered hazardous.

[NEC & CEC] Group B refers to an area that contains a flammable gas, liquid-produced vapor or combustible liquid-produced vapor mixed with air that has the potential to catch fire and burn or explode. The elements in this Group have an MESG (Maximum Experimental Safe Gap) value equal to or less than 0.45 mm or an MIC (Minimum Igniting Current) ratio equal to or less than 0.40. Some examples include hydrogen, butadiene, ethylene oxide, propylene oxide and acrolein.

[NEC & CEC] Group C refers to an area containing a flammable gas, liquid-produced vapor or combustible liquid-produced vapor mixed with air that has the potential to catch fire and burn or explode. The elements in this Group have an MESG higher than 0.75mm or an MIC ratio between 0.40 and 0.80. Some examples include carbon monoxide, ether, hydrogen sulfide, morphline, cyclopropane, ethyl, isoprene, acetaldhyde and ethylene.

[NEC & CEC] Group D refers to an area containing a flammable gas, liquid-produced vapor or combustible liquid-produced vapor mixed with air that has the potential to catch fire and burn or explode. The elements in this Group have an MESG value higher than 0.75 mm or an MIC ration higher than 0.80. Some examples include gasoline, acetone, ammonia, benzene, butane, ethanol, hexane, methanol, methane, vinyl chloride, natural gas, naphtha and propane.

[NEC] Group E refers to an area containing combustible metal dusts such as aluminum, magnesium, bronze, chromium, titanium, zinc and other commercial alloys.

[NEC] Group F refers to an area containing dusts with carbon or carbon compounds, carbon black, coal black, charcoal, coal or coke dusts.

[NEC] Group G refers to an area that contains combustible dusts that are not included in Groups E and F, such as flour, grain, sugar, wood, starch, chemicals and plastics.

As you can see by the breakdown, Groups A, B, C and D are for Class I (gases) only. Groups E, F and G are for Classes II and III (dusts and fibers or flyings) only.

[CEC] IIA refers to elements listed in Group D, including acetaldehyde, acetone, alcohol, ammonia, benzine, benzol, butane, cyclopropane, dichloride, ethylene, gasoline, hexane, isoprene, lacquer solvent vapors, naptha, natural gas, propane, propylene, styrene, vinyl acetate, vinyl chloride, xylenes or other gases or vapor that may be equally hazardous. If any of these elements are present or could possibly be in the area, the environment is considered hazardous.

[CEC] IIB refers to elements listed in Group C, including acrylonitrile, butadiene, diethyl ether, ethylene, ethylene oxide, hydrogen sulfide, propylene oxide, unsymmetrical dimethyl hydrazine (UDMH) or other gases or vapor that may be equally hazardous. If any of these elements are present or could possibly be in the area, the environment is considered hazardous.

[CEC] IIC refers to all gas types under Groups A and B, including acetylene, carbon disulphide, hydrogen, butadiene, propylene oxide and manufactured gases containing more than 30% hydrogen by volume. If any of these elements are present in an area or there is a possibility that they may be present, then the area is considered hazardous.

NEC & CEC Zones

Zones in the NEC and CEC classification systems define the properties of the hazardous material if it is a gas or a dust, and the probability of a hazardous material being present in the area in a form concentrated enough to ignite and start a fire. There are three levels of hazard for each dust and gas in the NEC Zone System (Zones 1, 2, 3, 20, 21 and 22), whereas the CEC Zone System only has three (Zones 0, 1 and 2) and the Division system has only two levels that encompass all hazardous materials.

Since the changes to the electrical code, there has been some confusion about Divisions and Zones. Typically, industries in the US have classified hazardous locations as either Division 1 or 2 based on the NEC or CEC. Since the changes to the code, hazardous locations with gases or vapors (Class I) present can be reclassified as Zone 0, 1 or 2.

When comparing Divisions and Zones, Division 2 is equivalent to Zone 2, but Division 1 is equivalent to either Zone 0 or 1. The key difference between Divisions and Zones relate to terminations. Under the IEC Zone System wire terminations that are rated as increased safety are not generally considered sources that can ignite and cause a fire or an explosion. These terminations are tested for their ability or inability to heat up or vibrate loose. However, under the Division System, wire terminations are considered sources that can ignite and potentially cause fires or explosions as they are assumed to vibrate loose, short out and create an arc. Below is a breakdown of the Zone System.

[NEC & CEC] Zones 0, 1 and 2 are reserved for gases, vapors and mists.

[NEC & CEC] Zone 0 refers to flammable gases or vapors that are present continuously or for long periods of time in ignitable concentrations.

[NEC & CEC] Zone 1 refers to flammable gases or vapors that are present in ignitable concentrations that are likely to occur under normal operating conditions.

[NEC & CEC] Zone 2 refers to flammable gases or vapors that are present in ignitable concentrations that are not likely to occur under normal operating conditions and are present for only a short period of time.

[NEC] Zones 20, 21 and 22 refer to dusts, fibers or flyings.

[NEC] Zone 20 refers to flammable dusts, fibers or flyings that are present continuously or for long periods of time in ignitable concentrations.

[NEC] Zone 21 refers to flammable dusts, fibers or flyings that are present in ignitable concentrations that are likely to occur under normal operating conditions.

[NEC] Zone 22 refers to flammable dusts, fibers or flyings that are present in ignitable concentrations that are not likely to occur under normal operating conditions and are present for only a short period of time.