{"id":32858,"date":"2023-05-08T10:43:46","date_gmt":"2023-05-08T09:43:46","guid":{"rendered":"https:\/\/www.gt-engineering.it\/normative-tecniche\/%tipologia_normativa%\/en-iso-13849-1-performance-level-estimation\/ciao-2-2\/"},"modified":"2023-08-11T09:31:34","modified_gmt":"2023-08-11T08:31:34","slug":"the-simplified-approach","status":"publish","type":"normativa_tecnica","link":"https:\/\/www.gt-engineering.it\/en\/technical-standards\/en-iso-standards\/en-iso-13849-1-performance-level-estimation\/the-simplified-approach\/","title":{"rendered":"The Simplified Approach"},"content":{"rendered":"\n\n    \n        <section\n            data-name=\"Contenuto testuale con immagine\"\n            class=\"bow-gte-testo-media u-spacer-base\"\n            id=\"\"\n        >\n            \n<div class=\"o-text-media px-side-spacer sm:px-side-spacer-sm lg:px-side-spacer-tablet 2xl:px-side-spacer-desktop flex flex-wrap items-start md:block\">\n\n    \n        \n                    <div\n                class=\"h-auto mb-6 md:mb-4 lg:mb-8 xl:mb-9 float-left mr-6 md:mr-4 lg:mr-8 xl:mr-9  overflow-hidden shadow-content-image w-full md:w-1\/2\"\n            >\n                \r\n\r\n    \r\n        \r\n            \r\n                \r\n                    <picture\r\n                        class=\"\"\r\n                                            >\r\n                                                                                                        <source\r\n                                media=\"(max-width: 150px)\"\r\n                                width=\"150\"\r\n                                height=\"150\"\r\n                                srcset=\"https:\/\/www.gt-engineering.it\/wp-content\/uploads\/2023\/05\/approccio-semplificato-ISo-13849-1-1-150x150.jpg\"\r\n                                type=\"image\/jpeg\"\r\n                            \/>\r\n                                                                                <source\r\n                                media=\"(max-width: 300px)\"\r\n                                width=\"300\"\r\n                                height=\"225\"\r\n                                srcset=\"https:\/\/www.gt-engineering.it\/wp-content\/uploads\/2023\/05\/approccio-semplificato-ISo-13849-1-1-300x225.jpg\"\r\n                                type=\"image\/jpeg\"\r\n                            \/>\r\n                                                                                <source\r\n                                media=\"(max-width: 768px)\"\r\n                                width=\"768\"\r\n                                height=\"576\"\r\n                                srcset=\"https:\/\/www.gt-engineering.it\/wp-content\/uploads\/2023\/05\/approccio-semplificato-ISo-13849-1-1-768x576.jpg\"\r\n                                type=\"image\/jpeg\"\r\n                            \/>\r\n                                                                                <source\r\n                                media=\"(max-width: 1024px)\"\r\n                                width=\"1024\"\r\n                                height=\"768\"\r\n                                srcset=\"https:\/\/www.gt-engineering.it\/wp-content\/uploads\/2023\/05\/approccio-semplificato-ISo-13849-1-1-1024x768.jpg\"\r\n                                type=\"image\/jpeg\"\r\n                            \/>\r\n                                                                            <img\r\n                            class=\"a-image transparent border-image h-auto  w-full\"\r\n                                                        srcset=\"https:\/\/www.gt-engineering.it\/wp-content\/uploads\/2023\/05\/approccio-semplificato-ISo-13849-1-1-scaled.jpg\"\r\n                                                        title=\"approccio semplificato ISo 13849-1\"\r\n                                                        type=\"image\/jpeg\"\r\n                        \/>\r\n                    <\/picture>\r\n\r\n                \r\n\r\n                    \r\n    \r\n            <\/div>\n        \n                    <div class=\"relative w-full\">\n                \n<div class=\"a-text font-text text-base normal mt-8 first:mt-0 space-y-4\">\n    <p>The numerical quantification of the probability of failure of a subsystem can never be attained exactly, but only by approximation with the aid of statistical methods or other estimations are possible.<br \/>\nAny validated and recognized method can be used for this purpose. Such methods include reliability block diagrams (used in IEC 62061), fault tree analysis, Markov modelling (used in ISO 13849-1) or Petri nets.<br \/>\nHowever, in general, engineers lacking prior experience in quantification of the probability of failure of safety related control systems require some degree of support. This need was addressed, in ISO 13849-1, by developing a<strong> simplified approach<\/strong>, also called simplified method which, whilst being based upon sound scientific principles (Markov modelling), describes a simple method for quantification in successive steps.<br \/>\nThe starting point of the simplified method is the observation that the majority of safety related control systems <strong>can be grouped in very small number of basic types<\/strong>, or to combinations of these basic types.<br \/>\nThese types are, at one end of the spectrum, the <strong>single-channel untested system<\/strong> having components with different reliability level; in the middle of the spectrum, the same type, but enhanced by testing; and at the other end, the <strong>two-channel system<\/strong> featuring high quality testing. Systems with more than two channels are rare in machinery. Based upon that reasoning five different categories are defined in ISO 13849-1.<br \/>\nSubsystems designed according to ISO 13849-1 should therefore be in accordance with the requirements of <strong>one of those five categories<\/strong> that are fundamental to achieve a specific Performance Level. The categories describe the required behaviour of subsystems in respect of its resistance to faults, based upon the design considerations previously indicated (MTTFD, DCavg etc..).<br \/>\n<strong>Category B is the basic Category<\/strong> where the occurrence of a fault can lead to the loss of the safety function. In <strong>Category 1<\/strong> an improved resistance to faults is achieved by using high quality components.<br \/>\nWith Categories 2, 3 and 4, higher Reliability of the subsystem is achieved by improving fault tolerance (Category 3 and 4 only) and diagnostic measures. In <strong>Category 2<\/strong>, since there is no redundancy, higher reliability is achieved by periodically checking that the safety function is performed without faults (Diagnostic Coverage). In <strong>Categories 3 and 4<\/strong>, the Diagnostic Coverage works together with Redundant channels, so that a single fault will not lead to the loss of the safety function.<\/p>\n<p>Each category is therefore important to achieve a specific PL for a subsystem. However, the standard clarifies that they show a logical representation of the subsystem structure, which may differ from its physical one.<\/p>\n<blockquote><p><strong>[ISO 13849-1] 6.1.3.2 Designated Architectures \u2013 Specification of Categories<\/strong><br \/>\n6.1.3.2.1 <strong>General<\/strong>. [\u2026] The designated Architectures show a logical representation of the structure of the subsystems for each Category.<br \/>\nNOTE 1: For Categories 3 and 4, <strong>not all parts are necessarily physically redundant but there are redundant<\/strong> means of assuring that a single fault cannot lead to the loss of the sub-function. Therefore, the technical realization (for example, the circuit diagram) can differ from the logical representation of the architecture.<\/p><\/blockquote>\n<p>Another way to state the same concept is that each one of the 5 Categories of ISO 13849-1<strong> describes the required behaviour of the subsystem with respect to its resistance to faults<\/strong>.<\/p>\n<\/div>            <\/div>\n        \n        \n                <div class=\"clear-both\"><\/div>\n        \n\n    <\/div>\n        <\/section>\n\n    \n","protected":false},"excerpt":{"rendered":"<p>&nbsp;<\/p>\n<p>The Mean Time To Failure (MTTF) is a statistical parameter related to the reliability of the components used in the safety system.<\/p>\n<p>It represents the av&#8230;<\/p>\n","protected":false},"author":1,"featured_media":42035,"parent":3245,"menu_order":2,"template":"","meta":{"_acf_changed":false,"footnotes":""},"normative-tecniche":[57],"class_list":["post-32858","normativa_tecnica","type-normativa_tecnica","status-publish","has-post-thumbnail","hentry","tipologia_normativa-en-iso-standards"],"acf":{"sottotitolo":"Il Mean Time To Failure (MTTF)","allegati":null},"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v27.2 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>Annex C: MTTF calculation<\/title>\n<meta name=\"description\" content=\"\u00a0 The Mean Time To Failure (MTTF) is a statistical parameter related to the reliability of the components used in the safety system. 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