Signaling in hazardous locations – classifications, groupings, and temperature codes
Signaling devices used in hazardous locations must be selected and installed in accordance with the National Electrical Code® (NEC). Proper classification ensures that audible and visual signaling equipment does not become an ignition source in the presence of flammable gases, vapors, combustible dusts, or fibers.
Hazardous locations are classified using the Class and Division system, which defines:
- The type of hazard present (Class)
- The likelihood of the hazard being present (Division)
- The specific gas or dust characteristics (Group)
- The maximum allowable equipment surface temperature (Temperature Code / T-Code)
Hazardous location classes
Class I – Flammable gases and vapors
Locations where flammable gases or vapors may be present in sufficient quantities to produce an ignitable or explosive mixture.
Typical applications:
- Oil and gas facilities
- Refineries
- Chemical plants
- Fuel storage areas
Class II – Combustible dusts
Locations made hazardous by the presence of combustible dusts, including dusts that may be electrically conductive or capable of ignition.
Typical applications:
- Grain handling
- Food processing
- Sugar plants
- Coal handling
- Metal processing
Class III – Fibers and flyings
Locations where easily ignitable fibers or flyings are present, but not likely to be suspended in the air in quantities sufficient to produce ignitable mixtures.
Typical applications:
- Textile mills
- Woodworking facilities
- Cotton processing
Hazardous location divisions
Division 1
A location in which a hazardous atmosphere:
- Exists under normal operating conditions, or
- May exist frequently due to maintenance, repair, or leakage
Division 1 represents the highest level of hazard.
Division 2
A location in which hazardous materials are:
- Normally contained, but
- May become hazardous under abnormal conditions such as equipment failure or accidental release
Hazardous location installations may be designed using either the Class / Division system or the Class / Zone system.
| Class / Zone classification US / Canada | Class / Division classification US / Canada |
| N/A | Mining |
| Class 1, Zone 0 | Class I, Division 1 |
| Class 1, Zone 1 | |
| Class 1, Zone 2 | Class I, Division 2 |
| Class 1, Zone 20 | Class II, Division 1 |
| Class 11, Zone 21 | |
| Class 11, Zone 22 | Class II, Division 2, Class III |
Gas and dust groups
| US / Canada zones system | US / Canada class / division system | ||
| Gas / dust group | Typical Substances | Gas / dust group | Typical Substances |
| IIC | Acetylene | Class I, Group A | Acetylene |
| IIB + H2 | Hydrogen | Class I, Group B | Hydrogen |
| IIB | Ethylene | Class I, Group C | Ethylene |
| IIA | Propane | Class I, Group D | Propane |
| IIIC | Conductive dust | Class II, Group E | Combustible metal dust |
| IIIB | Non-conductive dust | Class II, Group F | Combustible carbonaceous dust |
| Class II, Group G | Combustible dusts not in group E or F | ||
Temperature classifications (T-Codes)
In addition to Class, Division, and Group, equipment installed in hazardous locations must comply with temperature classification (T-code) requirements.
The T-code defines the maximum allowable surface temperature of the equipment and must always be below the ignition temperature of the gas or dust present.
Temperature codes - Class I, II & III
| Group II & III | Class I, II & III | ||||
| Max. surface temp. of equipment (°C/°F) | Temperature | Ignition temperature of gas / | Max. surface temp. of | Temperature class | Ignition temperature of gas / |
| class | dust (°C) | equipment (°C) | dust (°C) | ||
| 450°C (842°F) | T1 | >450°C (>842°F) | 450°C (842°F) | T1 | >450°C (>842°F) |
| 300°C (572°F) | T2 | >300–450°C (>572–842°F) | 300°C (572°F) | T2 | >300–450°C (>572–842°F) |
| — | — | — | 280°C (536°F) | T2A | >280–300°C (>536–572°F) |
| — | — | — | 260°C (500°F) | T2B | >260–280°C (>500–536°F) |
| — | — | — | 230°C (446°F) | T2C | >230–260°C (>446–500°F) |
| — | — | — | 215°C (419°F) | T2D | >215–230°C (>419–446°F) |
| 200°C (392°F) | T3 | >200–300°C (>392–572°F) | 200°C (392°F) | T3 | >200–215°C (>392–419°F) |
| — | — | — | 180°C (356°F) | T3A | >180–200°C (>356–392°F) |
| — | — | — | 165°C (329°F) | T3B | >165–180°C (>329–356°F) |
| — | — | — | 160°C (320°F) | T3C | >160–165°C (>320–329°F) |
| 135°C (275°F) | T4 | >135–200°C (>275–392°F) | 135°C (275°F) | T4 | >135–160°C (>275–320°F) |
| — | — | — | 120°C (248°F) | T4A | >120–135°C (>248–275°F) |
| 100°C (212°F) | T5 | >100–135°C (>212–275°F) | 100°C (212°F) | T5 | >100–120°C (>212–248°F) |
| 85°C (185°F) | T6 | >85–100°C (>185–212°F) | 85°C (185°F) | T6 | >85–100°C (>185–212°F) |
Mining applications
For Group I Mining apparatus is subject to fixed maximum surface temperature limits rather than standard NEC / UL T-codes:
- Coal dust: 150°C (302°F) maximum surface temperature
- Methane: 450°C (842°F) maximum surface temperature
Equipment apparatus group selection
Equipment must be specified for:
- The Class
- The Division (or Zone)
- The Group
- The Temperature Code (T-Code)
The apparatus group rating must match or exceed the severity of the hazard present.
Key selection rules
- Equipment rated for Group B may be used in Groups C and D, but not vice versa
- Conductive dusts (Group E) represent the most severe dust hazard
- Apparatus group selection must always be combined with correct temperature code and area classification