Temperature Class (T1–T6)

The temperature class in an Ex marking states the maximum surface temperature the equipment can reach under worst-case conditions, so it can be matched against the auto-ignition temperature of the gases present. The scale runs from T1 (450 °C limit) through T2 (300 °C), T3 (200 °C), T4 (135 °C), T5 (100 °C) to T6 (85 °C) — higher T-number, cooler surface, wider safety margin. The rule is simple: the equipment’s maximum surface temperature must sit below the ignition temperature of the site’s gas, with T3 covering many fuels and T4 covering the great majority of industrial atmospheres. LED luminaires reach T4 and better far more easily than the halogen and HID fixtures they replace, which ran hot by design — one of the quieter safety arguments for LED conversion in hazardous plants. The class appears near the end of the code, e.g. Ex db IIC T4 Gb.