Definitions of Hazardous (Classified) Locations Equipment

For complete details, reference the National Electrical Code (NEC^®), NFPA 70 or the appropriate NFPA or UL Standard.

Class I, Division 1

A Class I, Division 1 location is a location where ignitable concentrations of flammable gases, vapors or liquids:

can exist under normal operating conditions;
may exist frequently because of repair or maintenance operations or because of leakage; or
may exist because of equipment breakdown that simultaneously causes the equipment to become a source of ignition

Equipment intended for use in a Class I , Division 1 area is usually of the explosionproof, intrinsically safe, or purged/pressurized type.

Explosionproof apparatus: "Apparatus enclosed in a case that is capable of withstanding an explosion of a specified gas or vapor that may occur within it and of preventing the ignition of a specified gas or vapor surrounding the enclosure by sparks, flashes, or explosion of the gas or vapor within and that operates at such an external temperature that a surrounding flammable atmosphere will not be ignited thereby." (See NFPA 70)

Intrinsically safe apparatus: "Apparatus in which all the circuits are intrinsically safe." (See UL 913)

Intrinsically safe circuit: "A circuit in which any spark or thermal effect is incapable of causing ignition of a mixture of flammable or combustible material in air under prescribed test conditions." (See UL 913)

Purging: "The process of supplying an enclosure with a protective gas at a sufficient flow and positive pressure to reduce the concentration of any flammable gas or vapor initially present to an acceptable level." (See NFPA 496)

Pressurization:"The process of supplying an enclosure with a protective gas with or without continuous flow at sufficient pressure to prevent the entrance of a flammable gas or vapor, a combustible dust, or an ignitable fiber." (See NFPA 496)

Class I, Division 2

where volatile flammable liquids or flammable gases or vapors exist, but are normally confined within closed containers;
where ignitable concentrations of gases, vapors or liquids are normally prevented by positive mechanical ventilation; or
adjacent to a Class I, Division 1 location, where ignitable concentrations might be occasionally communicated.

Equipment intended for use in a Class I, Division 2 area is usually of the nonincendive, non-sparking, purged/pressurized, hermetically sealed, or sealed device type.

Nonincendive circuit: "A circuit in which any arc or thermal effect produced under intended operating conditions of the equipment is not capable, under the test conditions specified, of igniting the specified flammable gas- or vapor- air mixture."(See UL 1604)

Nonincendive component: "A component having contacts for making or breaking an incendive circuit and the contacting mechanism shall be constructed so that the component is incapable of igniting the specified flammable gas- or vapor-air mixture. The housing of a nonincendive component is not intended to:

Nonincendive field circuit: "A circuit that enters or leaves the equipment enclosure and that under intended operating conditions is not capable, under the test conditions specified, of igniting the specified flammable gas- or vapor-air mixture or combustible dust." (See UL 1604)

Non-sparking apparatus: "Apparatus that has no normally arcing parts or thermal effects capable of ignition. Examples of normally arcing parts are relays, circuit breakers, servo-potentiometers adjustable resistors, switches, non-latching type connectors and motor brushes. (See UL 1604)

Hermetically sealed component: "A component that is sealed against entrance of an external atmosphere and in which the seal is made by fusion, such as soldering, brazing, welding, or the fusion of glass to metal." (See UL 1604)

Sealed device: "A device that is constructed so that it cannot be opened, has no external operating mechanisms, and is sealed to restrict entry of an external atmosphere without relying on gaskets. The device may contain arcing parts or internal hot surfaces." (See UL 1604)

A Class I, Zone 0 location is a location where ignitable concentrations of flammable gases, vapors or liquids:

Equipment intended for use in a Class I, Zone 0 area is usually of the intrinsically safe, "ia," type.

A Class I, Zone 1 location is a location where ignitable concentrations of flammable gases, vapors or liquids:

are likely to exist under normal operating conditions;
may exist frequently because of repair or maintenance operations or leakage; or
may exist because of equipment breakdown that simultaneously causes the equipment to become a source of ignition; or
are adjacent to a Class I, Zone 0 location from which ignitable concentrations could be communicated.

Equipment intended for use in a Class I, Zone 1 area is usually of the flameproof, purged/pressurized, oil immersed, increased safety, encapsulated or powder filled type.

Flameproof: "The enclosure of the equipment will withstand an internal explosion, and prevent passage of flame to the surrounding atmosphere. Care must be taken to maintain the length and clearance (gap) of flameproof joints in service." (See UL 2279)

Oil immersion: "Arcing contacts are immersed in a protective liquid." (See UL 2279)

Increased safety: "The equipment contains no normally arcing parts, and additional measures (such as larger spacings among wiring connections) are taken to prevent the possibility of high temperatures or sparks. A minimum IP rating of IP 54 is required." (See UL 2279)

Encapsulation: "Arcing contacts are completely surrounded by an encapsulating material." (See UL 2279)

Powder filling: "Arcing contacts are surrounded by a filling material (glass or quartz powder)." (See UL 2279)

where ignitable concentrations of flammable gases, vapors or liquids are not likely to occur in normal operation or, if they do occur, will exist only for a short period;
where volatile flammable liquids, or flammable gases or vapors exist, but are normally confined within closed containers
where ignitable concentrations of gases, vapors, or liquids are normally prevented by positive mechanical ventilation;
adjacent to a Class I, Zone 1 location from which ignitable concentrations could be communicated.

Equipment that is intended for use in a Class I, Zone 2 area is usually of the nonincendive, non-sparking, restricted breathing, hermetically sealed or sealed device type.

Restricted breathing: "The enclosure relies on tight seals and gaskets to prevent diffusion of the explosive atmosphere into the equipment enclosure. Provision for checking that the restricted breathing properties of the enclosure are maintained is provided." (See UL 2279)

Sealed device: "A device that is constructed so that it cannot be opened and is sealed to restrict entry of an external atmosphere. The device may contain arcing parts or internal hot surfaces." (See UL 2279)

Class II, Division 1

ignitable concentrations of combustible dust can exist in the air under normal operating conditions;
ignitable concentrations of combustible dust may exist because of equipment breakdown that simultaneously causes the equipment to become a source of ignition; or
electrically conductive combustible dusts may be present in hazardous quantities.

Equipment intended for use in a Class II, Division 1 area is usually of the dust-ignition-proof, intrinsically safe, or pressurized type.

Dust-ignition-proof : "Enclosed in a manner that will exclude dusts and, where installed and protected in accordance with the NEC, will not permit arcs, sparks or heat otherwise generated or liberated inside of the enclosure to cause ignition of exterior accumulations or atmospheric suspensions of a specified dust on or in the vicinity of the enclosure." (See NFPA 70)

combustible dust is not normally in the air in ignitable concentrations;
dust accumulations are normally insufficient to interfere with normal operation of electrical equipment;
dust may be in suspension in the air as the result of infrequent malfunctioning of equipment; or
dust accumulation may be sufficient to interfere with safe dissipation of heat or may be ignitable by abnormal operation.

Equipment intended for use in a Class II, Division 2 area is usually of the dusttight, nonincendive, non-sparking, or pressurized types.

Dusttight: "Constructed so that dust will not enter the enclosing case under specified test conditions. An example of such conditions would be a UL Type 12 enclosure." (See NFPA 70)

A Class III, Division 1 location is a location where easily ignitable fibers or materials producing combustible flyings are handled, manufactured or used.

Equipment intended for use in a Class III, Division 1 area is usually of the dusttight or intrinsically safe type (both previously defined).

A Class III, Division 2 location is a location where easily ignitable fibers are stored or handled.

Equipment intended for use in a Class III, Division 1 area is usually of the dusttight or intrinsically safe type (both previously defined).

Definitions of commonly used hazardous locations terminology

Class I, Division 1

Class I, Division 2

Class II, Division 1