Temperature classes define the maximum surface temperature of equipment, ensuring equipment surfaces never exceed the auto-ignition temperature (AIT) of the surrounding explosive atmosphere. This classification is fundamental to safe equipment selection under the ATEX Directive.

The Six Temperature Classes

Equipment is classified into six temperature classes, each defining a maximum surface temperature:

• T1: 450°C maximum—suitable for substances with AIT above 450°C (hydrogen: 560°C, methane: 595°C, ammonia: 630°C)
• T2: 300°C maximum—suitable for substances with AIT above 300°C (ethanol: 365°C, acetone: 465°C, butane: 365°C)
• T3: 200°C maximum—suitable for substances with AIT above 200°C (petrol: 280°C, kerosene: 220°C, hexane: 225°C)
• T4: 135°C maximum—suitable for substances with AIT above 135°C (acetaldehyde: 140°C, diethyl ether: 160°C)
• T5: 100°C maximum—suitable for substances with AIT above 100°C (rarely used, limited industrial applications)
• T6: 85°C maximum—suitable for substances with AIT above 85°C (carbon disulphide: 90°C—the only common industrial substance requiring T6)

Selection Principle

Equipment temperature class must be equal to or lower than the T-class corresponding to the lowest AIT substance present. The selection process involves identifying all flammable substances that may be present (including process materials, cleaning solvents, maintenance chemicals), determining the lowest AIT among these substances, and selecting equipment with a T-class ensuring maximum surface temperature stays safely below that AIT.

For example, if a process area may contain both methane (AIT 595°C) and petrol vapour (AIT 280°C), equipment must be rated at least T3 (200°C maximum) to provide adequate margin below petrol's 280°C AIT.

Built-In Safety Margins

Temperature classes incorporate safety margins by design. T1 equipment (450°C max) used with hydrogen (AIT 560°C) provides a 110°C margin—approximately 20% safety factor. In practice, equipment surface temperatures rarely reach their rated maximum. A T4-rated motor might operate with actual surface temperature of 90°C under normal conditions, reaching 135°C only under thermal testing at maximum load and blocked ventilation.

Temperature Testing

Equipment manufacturers must verify temperature classification through thermal testing. Tests are conducted under worst-case conditions including maximum rated load, maximum ambient temperature (typically 40°C unless otherwise specified), any restricted cooling scenarios, and stabilised temperature conditions (typically 4 hours minimum). Multiple measurement points ensure the highest temperature anywhere on the equipment surface is captured.

Extended Ambient Temperature Ranges

Standard T-class ratings assume ambient temperatures of -20°C to +40°C. Equipment for higher ambient temperatures must be specifically rated—marking shows the extended range, for example "Ta -40°C to +60°C". At elevated ambients, T-class margins may be reduced; equipment rated T3 at 40°C ambient might only achieve T2 at 60°C ambient if surface temperature rises correspondingly.

Dust Considerations

For combustible dusts, temperature classification is more complex. Both cloud ignition temperature (MIT) and layer ignition temperature (LIT) must be considered. Equipment maximum surface temperature must not exceed two-thirds of the dust cloud MIT (in °C) or the dust layer ignition temperature minus 75°C, whichever is lower. For example, coal dust with MIT 600°C and LIT 250°C would limit surface temperature to 175°C (250°C - 75°C, the more restrictive value).

Relationship to Other Classifications

Temperature class is independent of equipment group, category, and protection type. All must be verified separately when selecting equipment. A Zone 2 classified area containing hydrogen would require IIC gas group and could use T1 temperature class, but the area's Zone 2 classification allows Category 3 equipment.