As explained in the definitions of the NEMA Types 7 and 9. The groups (indicated by letters), which follow the type class, indicate the gas or vapor atmospheres within the hazardous locations.

• 在NEMA类型7和9定义中以加以说明。组(用字母标明)，标记在类性和类别之后，表示危险区域内的气体或蒸汽环境。