This International standard gives guidance for the design and implementation of a fixed gas detection system, including associated and/or peripheral gas detection equipment, for the detection of flammable gases/vapours and Oxygen when used in a safety-related application in accordance with IEC 61508 and IEC 61511. This International standard also applies to the detection of toxic gases. Other parts of this international standard and pertinent local, national and international standards separately specify the performance requirements of a gas detector and a gas detection control unit (logic solver). These standards are commonly known as Metrological Performance Standards and are concerned with the accuracy of the measured value, the overall system performance, but not the device or system integrity with respect to the safety function. This international standard applies to the integrity of the safety function. NOTE In certain jurisdictions, it can be a requirement for a Certification Body to certify the performance of equipment for the measurement of flammable gases, vapours, toxic gases and/or Oxygen used in life safety applications. This international standard sets out safety-related considerations of fixed gas detection systems, including associated and/or peripheral gas detection equipment in terms of the framework and philosophy of IEC 61508, and introduces the particular requirements demanded by a fixed gas detection system as shown in Figure 2. This international standard does not consider the Safety Integrity Level SIL 4. SIL 4 is assumed to be unrealistic to be achieved for gas detection systems. NOTE 3 It is rare for any risk study to determine a Safety Integrity higher than SIL 2 for a fixed gas detection system. This international standard is applicable for fixed gas detection systems, which might consist of the following hardware functional units • Gas sensor/transmitter • Gas detection control unit (logic solver) • Gas sampling (single and multiplexed streams) • Gas conditioning • Automatic gas calibration and adjustment • Output module (if not part of the control unit)