This European Standard is applicable to fixed gas detection systems for the detection and measurement of flammable or toxic1 gases or vapours or oxygen. This European Standard supplements the requirements of the European Standards for electrical apparatus for the detection and measurement of flammable gases, vapours ( e.g. EN 61779 or EN 50241), toxic gases (e.g. EN 45544) or oxygen (e.g. EN 50104). Applying the above mentioned metrological standards ensure the measuring performance in normal operation of the gas detection system. Additionally the requirements of this European Standard address the functional safety of gas detection systems and encompasses criteria for reliability, fault tolerance and avoidance of systematic faults. This European Standard will lead to the characterisation of the gas detection system by a TYPE representing a hierarchical order of failure levels. This will allow the user to incorporate the gas detection system into an overall safety system according to the safety integrity levels of EN 61508 or the categories of EN 954-1 (see Annex B). This European Standard is a product standard which is based on EN 61508 and includes additional requirements of EN 954-1. It covers part of the phase 9 .realisation. of the overall safety lifecycle defined in EN 61508-1. The scope is limited to gas detection systems used for a safety function operating in demand mode up to maximum of 20 demands per year.