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Hazardous Locations: Classes, Divisions, and Groups

General-purpose electrical equipment can cause explosions in certain atmospheres. Equipment used in areas where explosive concentrations of dusts or vapors may exist must be equipped with special wiring and other electrical equipment for safety purposes. Hazardous (classified) locations such as these might exist in areas such as aircraft hangars, gasoline stations, paint-finishing locations, or agricultural areas such as grain bins. In order to make sure the correct equipment is used, these environments have been broken down into classes, divisions, and groups to more specifically identify the hazards.

Class I

Locations or areas where flammable gases or vapors are/could become present in concentrations suitable to produce explosive and/or ignitable mixtures. Class I locations are further divided into 2 divisions:

Class I, Division 1: There are three different situations that could exist to classify an area as a Class I, Division 1 location.

1. When the atmosphere of an area or location is expected to contain explosive mixtures of gases, vapors, or liquids during normal working operations. (This is the most common Class I, Div. 1)
2. An area where ignitable concentrations frequently exist because of repair or maintenance operations.
3. The release of ignitable concentrations of gases or vapors due to equipment breakdown, while at the same time causing electrical equipment failure.

Class I, Division 2: One of the following three situations must exist in order for an area to be considered a Class I, Division 2 location.

1. An area where flammable liquids and gases are handled, but not expected to be in explosive concentrations. However, the possibility for these concentrations to exist might occur if there was an accidental rupture or other unexpected incident.
2. An area where ignitable gases or vapors are normally prevented from accumulating by positive mechanical ventilation, yet could exist in ignitable quantities if there was a failure in the ventilation systems.
3. Areas adjacent to Class I, Division 1 locations where it is possible for ignitable concentrations of gas/vapors to come into this area because there isn't proper ventilation.

Class II Class II locations are areas where combustible dust, rather than gases or liquids, may be present in varying hazardous concentrations.

Class II, Division 1: The following situations could exist, making an area become a Class II, Division 1 locations:

1. Where combustible dust is present in the air under normal operating conditions in such a quantity as to produce explosive or ignitable mixtures. This could be on a continuous, intermittent, or periodic basis.
2. Where an ignitable and or explosive mixture could be produced if a mechanical failure or abnormal machinery operation occurs.
3. Where electrically conductive dusts in hazardous concentrations are present.

Class II, Division 2: Class II, Division 2 locations exist in response to one of the following conditions:

1. Where combustible dust is present but not normally in the air in concentrations high enough to be explosive or ignitable.
2. If dust becomes suspended in the air due to equipment malfunctions and if dust accumulation may become ignitable by abnormal operation or failure of electronic equipment.

Class III

Class III locations contain easily ignitable fibers or flyings, but the concentration of these fibers or flyings are not suspended in the air in such quantities that would produce ignitable mixtures.

Class III, Division 1: These locations are areas where easily ignitable fibers or items that produce ignitable flyings are handled, manufactured, or used in some kind of a process.

Class III, Division 2: These locations are areas where easily ignitable fibers are stored or handled.

Groups

Class I and Class II locations are further divided into groups: Groups A-G for gases, vapors, and liquids. Groups A-D relate to Class I environments. Groups E-G describe different dusts that may be encountered in a Class II environment. The actual determination of what group a chemical or dust would fall into is determined by the specific properties of that chemical or dust.

Requirements for Equipment used in Hazardous Locations:

The specific hazardous atmosphere must be considered when deciding what type of equipment to use in those areas. The equipment must be designed and tested to ensure it will perform properly and not cause additional hazards in that environment. According to 29 CFR 1910.307(b)(2)(ii), "Equipment shall be marked to show the class, group and operating temperature or temperature range, based on operating in a 40°C ambient temperature for which it is approved." The National Electric Code (NEC), NFPA 70, has set guidelines for the design of equipment installed in hazardous locations.

Equipment for Class I Locations
The equipment used in Class I locations are housed in enclosures designed to contain any explosion that might occur if hazardous vapors were to enter the enclosure and ignite. These closures are also designed to cool and vent the products of this explosion as to prevent the surrounding environment from exploding. The lighting fixtures used in Class I locations must be able to contain an explosion as well as maintain a sur-face temperature lower than the ignition temperature of the surrounding hazardous atmosphere.

There are some exceptions to the marking requirement set up by OSHA under 29 CFR 1910.307(b)(2)(ii)(B). According to OSHA, "Fixed lighting fixtures marked for use in Class I, Div. 2 locations only, need not be marked to indicate the group." Also, fixed general-purpose equipment in Class I locations, other than lighting fixtures, do not need to be marked with the class, group, division, or operating temperature, but must still be acceptable for the specific class, division, and group for which it is in.

Equipment for Class II Locations
Class II locations make use of equipment designed to seal out dust. The enclosures are not intended to contain an internal explosion, but rather to eliminate the source of ignition so no explosion can occur within the enclosure. These enclosures are also tested to make sure they do not overheat when totally covered with dust, lint, or flyings.

Again, OSHA has an exception for the marking requirements for equipment used in Class II locations. According to 29 CFR 1910.307 (b)(2)(ii)(D) permanently fixed dust-tight equipment, other than lighting fixtures, which is acceptable for use in Class II, Division 2 locations does not need to be marked with the class, group, division, or operating temperature.

Equipment for Class III Locations
Equipment used in Class III locations needs to be designed to prevent fibers and flyings from entering the housing. It also needs to be constructed in such a way as to prevent the escape of sparks or burning materials. It must also operate below the point of combustion. The same exception for the Class II locations holds true for the Class III locations-fixed, dust-tight equipment, other than lighting fixtures, does not need to be marked with the class, group, division, or operating temperature, as long as it is acceptable for Class III locations.

International Comparison

The International Electrotechnical Committee (IEC) is an international organization that strives to create standards to help promote international trade. They use a series of zones rather than classes and divisions to classify hazardous areas. The table below compares the NEC classifications to the equivalent IEC classifications:

Definitions:

Explosion Proof: a term frequently encountered when looking at products and wiring for hazardous areas. This term is usually seen when describing Class I Division 1 equipment. There are basically two requirements for components to be considered explosion proof for Class I Division 1 atmospheres. First, the device must be able to withstand an internal explosion if it should occur and secondly, it must work to prevent the spread of the internal explosion to the surrounding saturated atmosphere. This is typically accomplished by joints and closures built into the actual device. The devices themselves may be damaged, but they are designed so as not to allow the explosion or other possible sources of ignition to reach the hazardous atmosphere.

Intrinsically Safe: another term that is often seen when looking for products to use in hazardous locations. According to the Fire Protection Handbook, intrinsically safe is defined as "...equipment and wiring incapable of releasing sufficient electrical energy under normal or abnormal conditions to cause ignition of a specific hazardous atmospheric mixture." Or simply stated, equipment that is intrinsically safe is incapable of igniting the atmosphere surrounding it.

Commonly Asked Questions

Sources for More Information:

29 CFR 1910.307 -- Hazardous (classified) Locations.
UL 1203 -- Explosion-Proof and Dust-Ignition-Proof Electrical Equipment.
Fire Protection Handbook 18th edition -- National Fire Protection Association.
29 CFR 1910.399 -- Definitions Applicable to this subpart.
NFPA 70

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Please Note: The information contained in this publication is intended for general information purposes only. This publication is not a substitute for review of the applicable government regulations and standards, and should not be construed as legal advice or opinion. Readers with specific questions should refer to the cited regulation or consult with an attorney.