How to Identify Temperature Groups (T-Code) in Zone System (ATEX/IECEx)

Temperature Class (T-Code) is a key parameter for explosion-proof electrical equipment. It indicates the equipment’s maximum surface temperature, which must stay below the auto-ignition temperature (AIT) of surrounding hazardous substances.

1.Temperature Groups Table (IEC/ATEX)
Temperature ClassMax Surface Temp.Suitable AIT RangeCommon ExamplesT1≤450°C>450°CHydrogen, Acetylene, MethaneT2≤300°C>300–450°CEthanol, PropaneT3≤200°C>200–300°CGasoline, DieselT4≤135°C>135–200°CDiethyl etherT5≤100°C>100–135°CCarbon disulfideT6≤85°C>85–100°CMost restrictive
For Dust: Usually marked with specific temperature (e.g. T135°C or T120°C) instead of T1–T6.

2.How to Select the Right T-Code
Find AIT of the substance (check MSDS/SDS or IEC 60079-20-1).
Apply Safety Margin: Equipment max surface temp must be lower than AIT. Recommend at least 20°C margin.
Example: AIT 155°C → Choose T4 or lower.
Common Examples:
Hydrogen (AIT ≈560°C) → T1~T4 (T4 most common)
Gasoline (AIT ≈280°C) → T3 or lower
Diethyl ether (AIT ≈160°C) → T4 or lower
Aluminum dust → Check specific limit (e.g. ≤135°C)
Marking Examples:
Ex db IIC T4 Gb → Max surface ≤135°C (Gas)
Ex tb IIIC T120°C Db → Max surface ≤120°C (Dust)

3.Important Notes
Gas: Use T1–T6 classes.
Dust: Use exact °C value.
Standard ambient rating: -20°C to +40°C (special versions for extremes).
Most Common: T5-T6 is widely used in oil & gas.

Quick Memory Aid:
T1 = Most lenient (450°C) → T6 = Most strict (85°C).
T5-T6 is the most popular choice for oil & gas applications.

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