Temperature class (also known as ‘T-rating’) defines the explosion hazard type for the Ex product. The class, from 1 to 6, indicates the highest temperature allowed without igniting anything in the area. Class defines risks of a zone in a given area.
The temperature class schema was created to help simplify the discussion around safe temperatures in Ex areas. Think of it as a standardized communication protocol, like SMS for text messaging. If you have an area classified as T4, you know you can use T4 rated products.
The classes are defined separately for gas and dust hazards by IEC standards 600079-10-1 (gas) and 600079-2 (dust). The temperature restrictions for gas apply to the whole product, inside and out. For dust, the classes only concern the hottest outer surface temperature. The international (IEC) classes are defined as follows:
An important side note here: currently the standards do not cover areas where both gas and dust explosion risks are present. The classes are however, related, to the auto-ignition temperatures of different types of gases and dusts.
The auto-ignition temperature is the temperature (under normal air pressure) at which the substance ignites without any additional energy source. Propane gas, for example, has an auto-ignition temperature of 493 °C. Dust from corn or oat grain will auto-ignite at around 430 °C. So, for Ex equipment, it is important that the T-rating maximum temperature is below the auto-ignition temperature of the dust or gas hazard present in the Ex Zone.