Temperature Classes T1 to T6 — Complete Guide
The temperature class (T-class) of Ex equipment defines the maximum surface temperature that the equipment will reach under normal operating conditions. This maximum surface temperature must not exceed the auto-ignition temperature (AIT) of the flammable gas or vapour present in the area.
Temperature Class Table
| T-Class | Maximum Surface Temperature |
|---|---|
| T1 | 450°C |
| T2 | 300°C |
| T3 | 200°C |
| T4 | 135°C |
| T5 | 100°C |
| T6 | 85°C |
Selecting the Correct T-Class
The rule is simple: the equipment T-class maximum surface temperature must be less than the AIT of the gas present. For example, if hydrogen sulphide (AIT 260°C) is present, you need T-class T2 or lower (T3 maximum surface temperature of 200°C would be too high).
When in doubt, choose a lower T-class number (higher protection level). T4 is the most common T-class on process plant — it is suitable for most common hydrocarbons. Hydrogen sulphide requires T2 or T1. Hydrogen itself has an AIT of 500°C, so any T-class is technically acceptable for hydrogen — but IIC gas group is the more critical requirement.
T-Class and Inspection
Temperature class is a certification characteristic — it is set at the design stage and cannot be changed in the field. However, inspection must verify that the equipment is operating as intended. Signs that equipment is running hotter than expected — discolouration, paint blistering, excessive heat output — warrant investigation as the actual surface temperature may be exceeding the certified T-class limit.
Interactive inspection simulations, equipment selection decisions, and 1,185+ practice questions all referenced to IEC 60079. Try the platform free — no card required.
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