This is considered the core parameter of the explosion-proof marking.
- II: Application in non-mining explosive gas environments is indicated (distinguished from “Class I” for underground mines).
- B: The gas group is represented. Class II gases are divided into IIA, IIB, and IIC according to hazard levels. Ethylene and propylene are included in Class IIB gases, where a maximum experimental safe gap (MESG) of 0.5-0.9mm and a minimum igniting current ratio (MICR) of 0.45-0.8 are specified. A hazard level higher than IIA but lower than IIC is indicated.
- T4: The temperature class is denoted. It is indicated that a maximum surface temperature of 135°C is not exceeded by the equipment. It can be safely utilized in gas environments with an ignition temperature ≥135°C (such as acetaldehyde and ethanol). The temperature class is ranged from T1 (450°C) to T6 (85°C). The maximum permitted surface temperature is lowered and the application range is broadened as the number increases.
