Devices which are intended for use in potentially explosive atmospheres are divided into two groups. Cf: IEC Standard 60079-0:20
- Group I: Electrical operating facilities for mine workings susceptible to firedamp
- Group II: Electrical operating facilities for use in all other potentially explosive areas with the exception of mine workings susceptible to firedamp
- Group II is divided into sub-groups IIA, IIB and IIC. The letter indicates the maximum experimental safe gap (maximum width of the gap on a flashback arrester through which ignition cannot be transmitted). This depends on the explosive material. The degree of danger increases from A to C.
- Group III stands for electrical operating facilities for potentially explosive dust atmospheres.