This European Standard is applicable to apparatus and systems for the detection and measurement of flammable or toxic gases or vapours or oxygen. This European Standard is a product standard which is based on EN 61508 (all parts) and for gas detection systems covers both low and high demand mode at SIL capabilities of 1, 2 or 3 only. Gas detection apparatus and gas detection systems are developed as generic products. This standard covers part of the phase 10 “realisation” of the overall safety lifecycle defined in Figure 2 of EN 61508-1:2010. Configuration and integration into specific applications is not covered by this standard. In the event of conflict between the requirements of this standard and those of EN 61508, EN 50402 will take precedence. NOTE 1 Applications requiring a SIL capability of 4 for a gas detection system are not practicable. NOTE 2 This European Standard is dedicated mainly to fixed apparatus. For portable gas detectors claiming a SIL higher than 1, this European Standard may be applied. 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 60079-29-1 or EN 60079-29-4), toxic gases (e.g. EN 45544) or oxygen (e.g. EN 50104). NOTE 3 These European Standards are called in the text "metrological standards". The examples above show the state of the standardisation for industrial applications at the time of publishing this European Standard. There may be other metrological standards covering other application fields, for which this European Standard is also applicable. EN 50271 specifies minimum requirements for apparatus using software and/or digital components. It also defines additional optional requirements for compliance with SIL 1 in low demand mode operation. EN 50402 includes all requirements of EN 50271. EN 50402 is also dedicated to apparatus and gas detection systems and/or components and should be used instead of EN 50271 in the following cases: - At SIL 1 when the system contains components not covered by EN 50271; - At SIL 2 and SIL 3; - At all SILs when non-digital based apparatus is used. Applying the above-mentioned metrological standards will ensure the measuring performance is adequate in normal operation of a gas detection system. Additionally the requirements of this European Standard address the functional safety of gas detection systems and encompass criteria for reliability, fault tolerance and avoidance of systematic failures. The avoidance and control of systematic failures will be covered by the requirements for the development processes and techniques and diagnostic measures chosen in the design. This European Standard will lead to the characterisation of the gas detection system by a SIL-capability and related hardware failure rate representing a hierarchical order of safety 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 (all parts). This European Standard is applicable for gas detection systems, which may consist of the following functional units: - gas-sampling; - sensor; - signal transmission; - input to control unit; - signal processing in control unit; - output from control unit. This European Standard does not specify requirements for the installation and maintenance of gas detection systems. It also does not specify the physical positioning of measuring points / locations. This European Standard does not specify which SIL-capability is sufficient for which application. NOTE 4 The SIL-capability required for an application will be specified by the user (see Clause 9 and Annex A ).