Classified hazardous areas Class I II III
Classified Areas Class I II III
Section titled “Classified Areas Class I II III”In the United States and Canada, hazardous areas are classified using a system of 3 classes and 2 divisions according to NEC Article 500, where Class I covers flammable gases and vapors, Class II covers combustible dusts, and Class III covers ignitable fibers or particles. This classification determines the protection techniques required for electrical installations in environments with risk of explosion or fire, evaluating the nature, probability, and type of hazardous material present.
The Class/Division System
Section titled “The Class/Division System”The Class/Division/Group system, based on Article 500 of the National Electrical Code (NEC), classifies hazardous areas according to three main parameters: the general nature of the hazardous material, the probability of its presence, and the specific type of substance. In North America, this system coexists with the Zone system of NEC Articles 505 and 506, created to harmonize with the international IEC standard.
Definitions by Class
Section titled “Definitions by Class”The Class defines the general nature of the hazardous material present in the surrounding atmosphere. Each class corresponds to a distinct physical state of the combustible, determining the type of electrical protection required.
| Class | Nature of hazardous material |
|---|---|
| Class I | Hazard due to presence (or possible presence) of flammable gases or vapors in sufficient quantities to produce explosive or ignitable mixtures. |
| Class II | Hazard due to presence (or possible presence) of combustible or conductive dusts in sufficient quantities to produce explosive or ignitable mixtures. |
| Class III | Hazard due to presence (or possible presence) of ignitable fibers or volatile particles in sufficient quantities to produce explosive or ignitable mixtures. |
Divisions
Section titled “Divisions”The Division defines the probability that the hazardous material is present in an ignitable concentration in the atmosphere. The system establishes two probability levels, unlike the Zone system which uses three.
| Division | Probability of presence of hazardous material |
|---|---|
| Division 1 | The material has a high probability of producing an explosive or ignitable mixture. It is present continuously, intermittently, periodically, or originates from the equipment itself during normal operating conditions. |
| Division 2 | The material has a low probability of producing an explosive or ignitable mixture. It is only present during abnormal conditions for short periods, such as container failures or system breakdowns. |
Groups
Section titled “Groups”The Group defines the specific type of hazardous material present. Groups A, B, C, and D correspond to Class I gases and vapors, while Groups E, F, and G correspond to Class II dusts and Class III fibers. Group classification considers properties such as MESG (Maximum Experimental Safe Gap) and the MIC (Minimum Igniting Current) ratio.
| Group | Type of hazardous material |
|---|---|
| Group A | Atmospheres that contain acetylene. |
| Group B | Atmospheres with flammable gases or vapors with MESG ≤ 0,45 mm / ≤ 0.018 in or MIC ratio ≤ 0,40. Includes hydrogen, butadiene, ethylene oxide, propylene oxide, and acrolein. |
| Group C | Atmospheres with flammable gases or vapors with MESG > 0,75 mm / > 0.030 in or MIC ratio between 0,40 and 0,80. Includes carbon monoxide, ether, hydrogen sulfide, cyclopropane, and ethylene. |
| Group D | Atmospheres with flammable gases or vapors with MESG > 0,75 mm / > 0.030 in or MIC ratio > 0,80. Includes gasoline, acetone, ammonia, benzene, butane, ethanol, methane, natural gas, naphtha, and propane. |
| Group E | Atmospheres with combustible metal dusts: aluminum, magnesium, bronze, chromium, titanium, zinc, and their commercial alloys. |
| Group F | Atmospheres with carbonaceous dusts: carbon black, charcoal, coke, or coal dusts with more than 8 % trapped volatiles. |
| Group G | Atmospheres with combustible dusts not included in Groups E or F: flour, grain, starch, sugar, wood, plastics, and chemicals. |
Classification Criteria by Division
Section titled “Classification Criteria by Division”Class I, Division 1
Section titled “Class I, Division 1”A Class I, Division 1 area is defined by three conditions according to NEC 500.5(B)(1): ignitable concentrations of flammable gases or vapors can exist under normal operations; they can exist due to repair, maintenance, or leakage tasks; or a failure in the equipment can release ignitable concentrations while the electrical equipment acts as an ignition source. Typical areas include the interior of closed storage tanks, flammable liquid transfer areas, and zones immediately adjacent to tank vents.
Class I, Division 2
Section titled “Class I, Division 2”A Class I, Division 2 area is defined according to NEC 500.5(B)(2) when flammable gases or vapors are normally confined within closed containers and escape only in case of accidental rupture or abnormal equipment failure; when ignitable concentrations are normally prevented by positive mechanical ventilation that could fail; or in areas adjacent to a Class I, Division 1 location where concentrations may occasionally migrate without adequate positive ventilation.
Class II, Division 1
Section titled “Class II, Division 1”According to NEC 500.5(C)(1), a Class II, Division 1 area occurs when under normal operating conditions combustible dust is suspended in the air continuously or periodically in sufficient quantities to produce explosive mixtures; when equipment failures release ignitable dust mixtures and the equipment becomes an ignition source; or when Group E dusts may be present in hazardous quantities under normal or abnormal conditions.
Class II, Division 2
Section titled “Class II, Division 2”A Class II, Division 2 area is defined in NEC 500.5(C)(2) when combustible dust may be present in the air in sufficient quantities to produce explosive mixtures only due to abnormal operations, or when dust accumulation is normally insufficient to interfere with normal operation of electrical equipment but could become ignitable due to malfunction failures.
Class III, Division 1
Section titled “Class III, Division 1”A Class III, Division 1 area covers locations where easily ignitable fibers or materials that produce combustible volatile particles are handled, manufactured, or used. It includes areas in textile plants such as bale opening, carding, and cotton spinning, where suspended fibers can form ignitable mixtures under normal operating conditions.
Class III, Division 2
Section titled “Class III, Division 2”A Class III, Division 2 area comprises locations where easily ignitable fibers are stored or handled differently from the manufacturing process. In these areas, the material is confined in containers or packages, and would only be released in hazardous concentrations during abnormal conditions such as package breakage or extraction system failures.
Quick Class/Division Comparison
Section titled “Quick Class/Division Comparison”| Classification | Material | Ignition risk | Typical examples |
|---|---|---|---|
| Class I, Div 1 | Gases/vapors | High — present in normal operation | Interior of storage tanks, liquid transfer areas |
| Class I, Div 2 | Gases/vapors | Low — only in abnormal conditions | Surroundings of valves and flanges, pump rooms with forced ventilation |
| Class II, Div 1 | Dusts | High — present in normal operation | Interior of grain silos, flour mills, dust conveyors |
| Class II, Div 2 | Dusts | Low — only in abnormal conditions | Bag storage areas, vicinity of sealed systems |
| Class III, Div 1 | Fibers/particles | High — present in normal operation | Textile plants: carding, spinning, bale opening areas |
| Class III, Div 2 | Fibers/particles | Low — only in abnormal conditions | Cotton bale warehouses, sewing and garment assembly areas |
Frequently asked questions
Section titled “Frequently asked questions”What is the main difference between Class I Division 1 and Division 2?
Section titled “What is the main difference between Class I Division 1 and Division 2?”The difference lies in the probability of presence of the hazardous material. In Division 1 there is a high probability of explosive mixtures during normal operation, with continuous, intermittent, or periodic presence (500.5(B)(1)), while in Division 2 the explosive mixture only occurs during abnormal conditions and for short periods, such as a container failure at 0,45 mm / 0.018 in separation (500.5(B)(2)). This distinction determines the allowed protection methods: explosion-proof equipment in Division 1 versus equipment with less restrictive protections in Division 2.
What types of dusts belong to Group E and why do they require extreme caution?
Section titled “What types of dusts belong to Group E and why do they require extreme caution?”Group E includes combustible metal dusts such as aluminum, magnesium, bronze, chromium, titanium, and zinc, whose particles are typically smaller than 420 µm / 0.0165 in. These dusts are particularly dangerous because the electrical conductivity and abrasiveness of their particles can penetrate seals and cause internal short circuits in electrical equipment, generating ignition temperatures reaching 760 °C / 1400 °F for magnesium. The NEC requires extreme caution to prevent ignition and explosion under any condition, normal or abnormal.
How many material groups does the Class/Division system define?
Section titled “How many material groups does the Class/Division system define?”The Class/Division system defines 7 groups in total: 4 for Class I gases and vapors (Groups A, B, C, and D) and 3 for Class II and III dusts and fibers (Groups E, F, and G). The grouping criteria for Class I use the MESG value, for example Group B with MESG ≤ 0,45 mm / ≤ 0.018 in and Group C with MESG > 0,75 mm / > 0.030 in, while Groups E, F, and G are differentiated by the type of particle: metallic, carbonaceous, or organic respectively.
How does the MESG value relate to Group classification in Class I?
Section titled “How does the MESG value relate to Group classification in Class I?”The MESG (Maximum Experimental Safe Gap) is the maximum clearance between two parallel metal surfaces that prevents the propagation of an explosion. Measured under specified test conditions, Group B requires MESG ≤ 0,45 mm / ≤ 0.018 in due to the high hazard of gases such as hydrogen or butadiene, while Groups C and D are assigned with MESG > 0,75 mm / > 0.030 in according to the MIC ratio. This mechanism directly relates the group classification to the mechanical design of explosion-proof enclosures.
What documentation does NEC Article 500.4 require for classified areas?
Section titled “What documentation does NEC Article 500.4 require for classified areas?”NEC Article 500.4 requires that all areas designated as hazardous be documented by means of area classification drawings. This documentation must be available to those who design, install, inspect, maintain, or operate the electrical equipment. The documentation includes the class, division, and group of each location, as well as the autoignition temperatures of the materials present, which can exceed 280 °C / 536 °F for Group D gases such as gasoline.
How do the Class/Division and Zone systems of the NEC differ?
Section titled “How do the Class/Division and Zone systems of the NEC differ?”Both systems coexist in the NEC: the Class/Division system in Article 500 and the Zone system in Articles 505 and 506. The fundamental difference is in the probability levels: the Class/Division system uses 2 levels (Division 1 and 2), while the Zone system uses 3 levels for gases (Zones 0, 1, and 2) and 3 for dusts, following the international IEC method. Zone 0 corresponds to continuous presence, Zone 2 to presence only for short periods, and Zone 1 to the intermediate equivalent of Division 1.
Sources Consulted
Section titled “Sources Consulted”- engineeringtoolbox.com: https://www.engineeringtoolbox.com/hazardous-areas-classification-d_345.html
- mikeholt.com: https://www.mikeholt.com/newsletters.php?action=display&letterID=2697