This standard covers the details of construction@ performance@ and testing of portable@ mobile@ and stationary electrical instruments used for sensing the presence of combustible gas or vapor concentrations in ambient air; parts of such instruments may be installed or operated in Class I hazardous locations and gaseous mines in accordance with Codes specified by authorities having jurisdiction. (See Appendix I.) This standard applies to line-voltage-operated instruments rated at 600 V nominal or less@ and to portable@ mobile@ or stationary-type instruments utilizing a battery of a nonrechargeable (primary)-type or a rechargeable (secondary)- type. This standard covers combustible gas detection instruments intended to provide an indication and/or alarm@ the purpose of which is to give a warning of potential hazard. This standard does nor cover gas detection instruments of the laboratory- or scientific-type used for analysis or measurement@ instruments used for process control and process monitoring purposes@ or instruments used for residential purposes. This standard is written for gas detection instruments which are intended to measure gas concentrations in air in the range from zero up to the lower flammable or explosive limit. PURPOSE Part I of this standard provides minimum requirements for the perfonnance of combustible gas detection instruments and therefore enhances the safety of operations by employing these instruments. Part II of this standard* establishes user criteria for the installation@ operation@ and maintenance of combustible gas detection instruments.