This document gives guidance on the selection, installation, use and maintenance of electrical equipment used for the direct detection and direct concentration measurement of toxic gases and vapours in workplace atmospheres. The primary purpose of such equipment is to ensure safety of personnel and property by providing an indication of the concentration of a toxic gas or vapour and warning of its presence. This document is applicable to equipment whose purpose is to provide an indication, alarm and/or other output function to give a warning of the presence of a toxic gas or vapour in the atmosphere and in some cases to initiate automatic or manual protective actions. It is applicable to equipment in which the sensor automatically generates an electrical signal when gas is present. For the purposes of this document, equipment includes a) fixed equipment; b) transportable equipment, and c) portable equipment. This document is intended to cover equipment defined within ISO/IEC 62990-1, but may provide useful information for equipment not covered by that document.