Within a hazardous area, the maximum surface temperature of an electrical apparatus must not reach the ignition temperature of the potentially explosive atmosphere it operates in. Since each explosive substance has a defined auto ignition temperature (AIT), it can be assigned within a corresponding Temperature Class . Maximum Surface Temperature Codes indicate the maximum temperature a device's external enclosure can reach. Therefore, the equipment's temperature code marking shall not exceed the the ignition temperature of the specific gas, vapor, dust, fibre or flying to be encountered.

Always compare the auto ignition temperature of the specified hazard with the maximum surface temperature of the equipment you wish to install. The auto ignition temperature must be higher than the T-rating of the equipment. If they cross over, you may not install the equipment. Also compare the T-Code rating and the Group rating of the equipment together. For example, Ethyl Ether is rated as a ClassGroup C substance, with an AIT of 160°C (320°F) and Hydrogen is rated as a Class I,Group B substance with an AIT of 520°C (968°F). If a piece of equipment is rated foClass I, Group C & D with a T-Code of T-165°C (329°F) then it would not be suitable for either environment. However, if you were selecting the equipment just based on T-Code alone then it would incorrectly appear that it is suitable for a Hydrogen environment. Inversely, if you were selecting the equipment based on the Group rating alone, then it would incorrectly appear that it is suitable for an Ethyl Ether environment.