Even if the probability of an explosive atmosphere being created is low, temperature classes and groups must be observed. These always play a role in the selection of devices in hazardous areas, as the specifications relate to potential sources of ignition in the form of hot surfaces or ignitable energy.
The maximum surface temperature of electrical equipment must always be lower than the ignition temperature of the gas/air or vapour/air mixture in which the device is used. Some devices are approved for T6 temperature class and therefore for operation in an environment with gases and vapours with ignition temperatures between +85 °C and +100 °C. In these environments, the surface temperature must not exceed +85 °C. The lowest temperature class, T1, permits use in areas with gases or vapours that can only be ignited at temperatures over +450 °C.
Gases, vapours, mists and dusts are subdivided into various groups according to their characteristics. To make this easier and to ensure the correct selection of equipment, these are assigned to relevant material groups. The equipment groups and temperature classes therefore describe the level of protection and the application range of the devices. According to IEC 60079-0, the devices are classified for all gas hazardous areas (except mines susceptible to firedamp) into groups IIA (e.g. propane), IIB (e.g. ethylene) and IIC (e.g. hydrogen). Both the temperature classes and the group classification must be observed in all zones.