ATEX Temperature Classes T1-T6: What Do They Mean for Exproof Fans?

Why is temperature class important?

The accessible or relevant surface temperature of equipment used in a hazardous area must remain within safe limits relative to the ignition temperature of the gas or vapour that may be present. For gas atmospheres, the T class expresses this maximum surface temperature with a standard marking.

This is especially important for fans because temperature behaviour is influenced not only by the electric motor, but also by bearings, possible mechanical friction, process-air temperature and the actual operating point. Temperature suitability cannot be determined from motor power or airflow alone.

Temperature class is not an alternative to the zone classification or gas group. All three must be evaluated together: the zone defines the required protection level, the gas group defines the applicable gas subgroup, and the temperature class limits the maximum permitted surface temperature.

What are the T1-T6 values?

Common temperature classes for gas and vapour atmospheres and their maximum surface temperatures are:

• T1: 450°C
• T2: 300°C
• T3: 200°C
• T4: 135°C
• T5: 100°C
• T6: 85°C

These values state the maximum surface temperature permitted for the corresponding class. For example, the T4 limit is 135°C. The required class is selected by considering the ignition characteristics of the gas or vapour present in the application.

Combustible-dust applications require a different assessment approach. Maximum surface temperature may need to be evaluated against the dust characteristics and possible layer accumulation, so the gas-oriented T1-T6 table should not simply be copied into dust applications.

What changes as the number increases?

As the T-class number increases, the permitted maximum surface temperature decreases. T6 therefore has a lower maximum surface temperature than T5, and T5 is lower than T4.

If an application requires T4, equipment marked T4, T5 or T6 may be considered provided all other certification conditions are also suitable. Equipment marked T3, however, allows a surface temperature up to 200°C and therefore does not satisfy a T4 requirement.

A higher T number should not automatically be interpreted as a universally better product. The equipment should match the actual site requirement. A more demanding class can affect cost, availability and product options. The engineering objective is to meet the documented requirement correctly and verifiably.

The fan and motor must be evaluated together

The motor nameplate is an important reference on an exproof fan, but the operating conditions of the complete fan must also be considered. Mechanical relationships between the impeller and casing, bearing condition, process-air temperature and ambient temperature can all influence actual operating temperatures.

The ambient-temperature limits within the certificate scope should be checked in particular. In addition to standard marking data, a product may have a specific minimum/maximum ambient-temperature range or special conditions of use. If the installation site falls outside those limits, checking only the T class is not sufficient.

For this reason, the fan nameplate, motor data, certificate and manufacturer technical documentation should be reviewed together before ordering.

Information required for correct selection

The required temperature class defined by the plant, or the relevant ignition data for the process gas or vapour, should be provided together with the zone and gas group. Fan performance data must then be added to the same selection file.

• Zone classification
• Gas name or gas group
• Required T class
• Ambient and conveyed-air temperature
• Required airflow and total pressure
• Installation type and operating conditions

This allows the hazardous-area classification and the aerodynamic duty to be verified in one selection. The next step is to read these requirements against the actual product label and ATEX marking.