The National Electrical Code (NEC) defines hazardous locations as areas where fire or explosion conditions may exist due to the presence of flammable gases or vapors, flammable liquids, combustible dusts, or easily ignitable fibers or flyings in sufficient quantities or concentrations to present a potential explosion hazard.
A Class/Division type of electrical codes and standards system is used in North America as the basis for classification of hazardous locations. Hazardous locations are divided into three Classes and two Divisions because hazards and methods of protecting electrical equipment against these hazards differ for different materials. The Classes are separated based on the type of hazard and the explosive characteristics of the material. The Divisions are based on the occurrence or risk of fire or explosion that the material presents. The Classes or type of material is further separated into Divisions based on the risk of fire or explosion that the material poses.
The Class is defined by the general nature (or properties) of the hazardous material in the surrounding atmosphere.
- Class I: Hazardous because flammable gases or vapors are present (or may be present) in quantities sufficient to produce explosive or ignitable mixtures
- Class II: Hazardous because combustible or conductive dusts are present (or may be present) in quantities sufficient to produce explosive or ignitable mixtures
- Class III: Hazardous because ignitable fibers or flyings are present (or may be present) in quantities sufficient to produce explosive or ignitable mixtures
The Division is defined by the probability of the hazardous material being present in an ignitable concentration in the surrounding atmosphere.
- Division 1: The substance referred to by class 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: The substance referred to by class has a low probability of producing an explosive or ignitable mixture and is present only during abnormal conditions for a short period of time - such as a container failure or system breakdown
The Group is defined by the type of the hazardous material in the surrounding atmosphere.
- 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
- Group C: Atmospheres containing a flammable gas, a flammable liquid produced vapor or a combustible liquid-produced vapor whose MESG is greater than 0.75 mm or MIC ratio is greater than 0.40 and less than 0.80
- Group D: Atmosphere containing flammable gas, flammable liquid produced vapor, or combustible liquid produced vapor mixed with air that may burn or explode, having either a MESG value greater than 0.75 mm or a MIC ratio greater than 0.80
- Group E: Atmospheres containing combustible metal dusts, including aluminum, magnesium, bronze, chromium, titanium, zinc and their commercial alloys or other combustible dusts whose particle size, abrasiveness and conductivity present similar hazards in connection with electrical equipment
- Group F: Atmospheres containing carbonaceous dusts, carbon black, coal black, charcoal, coal or coke dusts that have more than 8% total entrapped volatiles or dusts that have been sensitized by other materials so that they present an explosion hazard
- Group G: Atmosphere containing combustible dust not included in Groups E and F
- Maximum Experimental Safe Gap (MESG): The maximum clearance between two parallel metal surfaces that has been found under specified test conditions to prevent an explosion in a test chamber from being propagated to a secondary chamber containing the same gas or vapor at the same concentration
- Minimum Igniting Current (MIC) Ratio: The ratio of the minimum current required from an inductive spark discharge to ignite the most easily ignitable mixture of a gas or vapor, divided by the minimum current required from an inductive spark discharge to ignite methane under the same test conditions