According to NEC Groups and MESGs

For flame arrester selection, gases are classified according to two methods Nadonal Electrical Code (NEC) gronps or the Maximnm Experimental Safe Gap (MESG). 

The NEC group method for classifying flame arresters is similar to that used for electrical area classification. NFPA 497 (1997) provides the criteria for classilying gases into NEC groups for suitability for electrical area classification. 

Originally the classification of materials was derived from tests of proprietary explosion-proof (flameproof) enclosures. There were no published criteria. Equipment was approved relative to the lowest ignition temperature of any material in the group (Magison 1987). In about 1965 the U.S. Coast Guard asked the National Academy of Sciences (NAS) to form a panel to classify 200 materials of commerce. The Electrical Hazards Panel of the Committee on Hazardous Materials was formed by the NAS. The Panel studied many ways to estimate the hazard classification of materials. The Panel finally reported to die U.S. Coast Guard in 1970 that no workable, predicdve scheme could be defined, and it then proceeded to assign tentative classifications to the 200 materials. 

More recently, the NFPA has used the MESG and the MIC ratio for classifying explosion-proof electrical equipment (NFPA 497 1997), and this approach can also be used for classilying flame arresters. In this method, NEC Class I combustible materials (vapors or gases) are divided into four groups  

In Europe, rather than Groups A through D, gases and vapors are classified in Groups IIA through IIC. A comparison of the U.S. and European groupings is as follows  

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