Hazardous locations are areas where fire or explosion hazards exist due to the presence of flammable gases or vapors, flammable liquids, combustible dusts, or ignitable fibers or flyings.

The determination that areas are classified as hazardous locations is based on the following:

The possible presence of an explosive atmosphere such as flammable gases, vapors, or liquids (Class I), combustible dusts (Class II) or ignitable fibers & flyings (Class III).

Identifies the conditions under which the explosive atmosphere is present (is the hazard normally present continuously or for long periods, under normal operating conditions, or under abnormal operating conditions?)

The ignition related properties of the explosive atmosphere that is present (auto ignition temperature, explosive pressure, minimum ignition current, and maximum experimental safe gap.

A location in which explosive gas atmospheres are present continuously or for long periods of time.

A typical example would include a vapor space in a vented tank.

A location in which explosive gas atmospheres are likely to exist in normal operation or may exist frequently because of repairs, maintenance operations, and leakage or where equipment breakdowns could release gases or vapors and also cause simultaneous failure of electrical equipment in a mode to cause the electrical equipment to become a source of ignition.

A typical example would include a container filling area in a refinery.

A location in which explosive gas atmospheres are not likely to occur in normal operation and, if they do occur, will exist for a short time only; or where volatile flammable liquids, flammable gas, or flammable vapors are handled, processed, or used, but are normally confined within closed containers or systems from which they can escape only as a result of accidental rupture or breakdown of the containers or system, or as a result of abnormal operation of the equipment with which the liquids or gases are handled, processed, or used; or where ignitable concentrations of flammable gases or vapors are normally prevented by adequate ventilation, but which may occur as a result of failure or abnormal operation of the ventilation system.

A typical example would include a container storage area.

Atmospheres containing explosive gas in underground coal mines. Electrical apparatus that is intended for use in underground mines.

Atmospheres containing explosive gas in non-mining applications and is subdivided according to the nature of the gas atmosphere as follows: Group IIC - Atmospheres containing acetylene, hydrogen (H2), or gases of equivalent hazard.

Atmospheres containing acetaldehyde, ethylene, or gases or vapors of equivalent hazard. Group IIA - Atmospheres containing acetone, ammonia, ethyl alcohol, gasoline, methane, propane, or gases or vapors of equivalent hazard. Comparing the two systems

Atmospheres containing acetone, ammonia, ethyl alcohol, gasoline, methane, propane, or gases or vapors of equivalent hazard.

Note: There is potential for confusion between the NEC/CE and IEC gas classification systems since the Group letters are reversed and even combined. Care should also be taken to avoid confusing Group II and Class II, since both use Roman numerals. An unintended result of specifying the IEC gas groups, which combine the traditional Groups A and B into Group IIC, is that equipment approved for hydrogen (H2) would also have to be approved for acetylene. Since very little equipment is designed for acetylene, the wording as originally adopted severely limits the availability of equipment for hydrogen applications. As a result, NEC Section 505-7(d) now allows for equipment to be listed for a specific gas or vapor, specific mixtures of gases or vapors, or any specific combination of gases or vapors. One common example is equipment marked for "IIB + H2".

At present, the NEC or CE Code does not recognize any CENELEC or IEC dust classifications.