Temperature classes (T1-T6) define the maximum surface temperature that electrical equipment can reach in hazardous areas without igniting surrounding flammable gases or vapors. This classification ensures equipment safety in potentially explosive atmospheres.
The temperature classifications range from T1 (450°C maximum) to T6 (85°C maximum). T1 is suitable for gases like methane and propane, while T6 is required for carbon disulfide, which has the lowest auto-ignition temperature.
A key principle is that the equipment temperature class must be lower than the auto-ignition temperature (AIT) of the gas present. For example, methane has an AIT of 595°C, so even T1 devices are sufficient. T4-approved instruments are sufficient for most applications.
Carbon disulfide (CS2) is the only common substance with an AIT below 100°C, requiring T6-rated equipment. Ethers and aldehydes have AITs below 200°C, requiring at least T3-rated equipment.
When selecting equipment, the temperature class must be appropriate for the gases present. Equipment with T3 rating cannot be used for gases with AIT < 200°C. T4 devices are typically adequate unless CS2 is present.