An ignition may occur when an ignitable gas mixture, vapour, mist or dust cloud collide with an object whose temperature equal or is higher than the ignition temperature of this combustible gas, vapour, mist or dust, (for example, generated static electricity, heating elements, electric motors, etc.).
With mixtures of different gases or substances, the gas or dust with the lowest ignition temperature is always decisive, unless further details are known. The highest surface temperature must be lower than the ignition temperature of the gas, vapour, mist or dust mixture to prevent ignition. Equipment classified in a certain temperature class may therefore be used for gases with an ignition temperature higher than the temperature associated with that group. Below is a table with an overview of all temperature classes.
| Temperature class | Maximum surface temperature | Ignition temperature of the flammable dust or gas |
|---|---|---|
| T1 | 450 °C | Boven 450 °C |
| T2 | 300 °C | 300 – 450 °C |
| T3 | 200 °C | 200 – 300 °C |
| T4 | 135°C | 135 – 200 °C |
| T5 | 100 °C | 100 – 135 °C |
| T6 | 85 °C | 85 – 100 °C |
The manufacturer is already forced to build in a safety margin; for example, for T6 to T4 there is a safety margin of 5°C and for T3 to T1 of 10°C. In practice, this means that a T6 marked product can never have a (hot spot) surface temperature higher than 80°C.
For dust explosion-proof equipment, the classification is not mandatory in temperature classes, but it is mandatory to regulated it on the equipment by a maximum surface temperature as an absolute value.
In the case of dust explosion safety, not only the ignite temperature but also the smouldering temperature (glimmering temperature) applies as a selection criterion. The ignition temperature is the temperature at which a whirlwind dust cloud inflames. The smouldering temperature is the temperature at which a 5mm thick layer on a hot surface begins to burn.