Six classes
| Class | Maximum permitted surface temperature |
|---|---|
| T1 | 450 °C |
| T2 | 300 °C |
| T3 | 200 °C |
| T4 | 135 °C |
| T5 | 100 °C |
| T6 | 85 °C |
The selection rule
The ignition temperature of the substance present must be higher than the equipment's maximum surface temperature. So the further down the classes you go (towards T6) the cooler the equipment stays and the more substances it can be used with. In mixtures and in areas holding several substances, the one with the lowest ignition temperature decides.
Ambient temperature is a limit as well
The marking also states an ambient temperature range. Outside that range the temperature class is not guaranteed: a housing on a hot line or in direct sun may run warmer than assumed, and be unsuitable on site while suitable on paper.
Why the dust side is different
In dust atmospheres the maximum surface temperature is stated directly rather than as a class, and it is judged against two thresholds: the ignition temperature of the dust cloud and — often lower — the smouldering temperature of the settled layer. As the layer thickens, the permitted surface temperature falls; here too, cleaning is a safety measure.