Temperature Classes T1 to T6

Temperature Classes T1 to T6

The temperature class of Ex equipment defines the maximum surface temperature the equipment can reach during operation. This temperature must always remain below the auto-ignition temperature (AIT) of the gases or dusts present in the hazardous area.

Temperature Classes for Gases (IEC 60079-0)

Class Max Surface Temp (°C) Suitable for gases with AIT above
T1 450 450°C
T2 300 300°C
T3 200 200°C
T4 135 135°C
T5 100 100°C
T6 85 85°C

Key Principle

  • Higher T-number = lower maximum surface temperature = safer for more gases
  • T6 equipment can be used in any temperature class environment (T1 through T6)
  • T1 equipment can only be used where the AIT exceeds 450°C
  • The hierarchy is: T6 > T5 > T4 > T3 > T2 > T1 (T6 covers all)

Common Gases and Their Temperature Classes

Gas AIT (°C) Required T-Class
Hydrogen 560 T1
Methane 537 T1
Carbon monoxide 605 T1
Propane 470 T1
Ethanol 365 T2
Ethylene 425 T2
Acetone 535 T1
Hydrogen sulphide 270 T3
n-Hexane 233 T3
Diethyl ether 175 T4
Acetaldehyde 140 T4
Carbon disulphide 95 T5
Ethyl nitrite 90 T6

The Hydrogen Paradox

Hydrogen has a very high AIT (560°C) — only T1 required — but it is in the most dangerous gas group (IIC). This is counterintuitive: hydrogen is extremely easy to ignite by a spark (0.017 mJ), but hard to ignite by a hot surface.

Conversely, carbon disulphide (CS₂) requires T5 class (max 100°C surface) but is also IIC. Equipment for CS₂ must handle both the stringent gas group AND low temperature class.

Temperature Considerations for Dust

Dust equipment does not use the T1–T6 classification. Instead, the maximum surface temperature is determined by two dust-specific properties:

1. Dust Cloud Ignition Temperature (T_CL)

The minimum temperature of a hot surface that will ignite a dust cloud.

Rule: Equipment max surface temp ≤ ⅔ × T_CL (in Kelvin, then convert back to °C)

Example: If T_CL = 450°C (723 K), max surface = ⅔ × 723 = 482 K = 209°C

2. Dust Layer Ignition Temperature (T_5mm)

The minimum temperature of a hot surface that will ignite a 5mm layer of settled dust.

Rule: Equipment max surface temp ≤ T_5mm − 75°C

Example: If T_5mm = 300°C, max surface = 300 − 75 = 225°C

Combined Rule

The maximum allowed surface temperature for dust equipment is the lower of:

  • ⅔ × T_CL (Kelvin) converted to °C
  • T_5mm − 75°C

For thicker dust layers (>5mm), additional derating applies — every 5mm of additional dust reduces the allowable surface temperature further. This is why housekeeping in dusty environments is critical.

Dust Temperature Examples

Dust T_CL (°C) T_5mm (°C) Max Surface (°C)
Wheat flour 380 340 162 (by T_CL rule)
Sugar 350 370 142 (by T_CL rule)
Aluminium 550 320 245 (by T_5mm rule)
Coal 610 225 150 (by T_5mm rule)
Wood dust 420 260 185 (by T_5mm rule)

Temperature Class in Equipment Marking

For gas equipment, the temperature class appears in the Ex marking:


Ex db IIC T4 Gb
              ^^
              Temperature class T4 (max 135°C)

For dust equipment, the actual maximum surface temperature is marked in °C:


Ex tb IIIC T125°C Db
                ^^^^
                Max surface temperature 125°C

Ambient Temperature Range

Standard Ex certification assumes an ambient temperature range of −20°C to +40°C. If equipment operates outside this range, it needs special certification marked with a "Ta" or "Tamb" range:


Ta = −40°C to +60°C

At higher ambient temperatures, the maximum surface temperature may be reached at lower power levels. At lower ambient temperatures, materials may become brittle or seals may fail.

Practical Considerations

  • Self-heating: Equipment in an enclosed space may reach higher surface temperatures than in open air. Ventilation around Ex equipment must be maintained.
  • Process temperature: If the equipment contacts a hot process (e.g., a temperature sensor in a 250°C pipe), the process temperature itself contributes to the surface temperature.
  • Solar radiation: Outdoor equipment in direct sunlight can experience additional heating. Dark-coloured enclosures in tropical climates may need derating.
  • Dust insulation effect: Dust layers act as thermal insulation, causing underlying surfaces to run hotter. Regular cleaning is essential.