{"id":3865,"date":"2023-01-04T17:58:24","date_gmt":"2023-01-04T16:58:24","guid":{"rendered":"https:\/\/www.gt-engineering.it\/normative-tecniche\/%tipologia_normativa%\/high-and-low-demand-operation-mode\/"},"modified":"2023-06-26T08:15:31","modified_gmt":"2023-06-26T07:15:31","slug":"high-and-low-demand-operation-mode","status":"publish","type":"normativa_tecnica","link":"https:\/\/www.gt-engineering.it\/en\/technical-standards\/en-iec-standards\/iec-61508\/high-and-low-demand-operation-mode\/","title":{"rendered":"High and Low Demand operation mode"},"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\/3\"\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\/01\/5-high-and-low-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=\"232\"\r\n                                srcset=\"https:\/\/www.gt-engineering.it\/wp-content\/uploads\/2023\/01\/5-high-and-low-300x232.jpg\"\r\n                                type=\"image\/jpeg\"\r\n                            \/>\r\n                                                                                <source\r\n                                media=\"(max-width: 768px)\"\r\n                                width=\"768\"\r\n                                height=\"595\"\r\n                                srcset=\"https:\/\/www.gt-engineering.it\/wp-content\/uploads\/2023\/01\/5-high-and-low-768x595.jpg\"\r\n                                type=\"image\/jpeg\"\r\n                            \/>\r\n                                                                                <source\r\n                                media=\"(max-width: 1024px)\"\r\n                                width=\"1024\"\r\n                                height=\"793\"\r\n                                srcset=\"https:\/\/www.gt-engineering.it\/wp-content\/uploads\/2023\/01\/5-high-and-low-1024x793.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\/01\/5-high-and-low-scaled.jpg\"\r\n                                                        title=\"5 high and low\"\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 style=\"margin: 0cm 0cm 8pt; text-align: justify;\"><span style=\"margin: 0cm 0cm 8pt; text-align: justify;\">IEC 61508-1 clarifies which target failure measure, or unreliability function F(t), should be used, <strong>depending upon the mode of operation<\/strong>:<\/span><\/p>\n<ul>\n<li style=\"text-align: justify; margin: 0cm 0cm 0cm 36pt;\"><span style=\"margin: 0cm 0cm 8pt; text-align: justify;\"><strong>PFD<sub>avg<\/sub> <\/strong>should be used for safety systems in low demand mode.<\/span><\/li>\n<li style=\"text-align: justify; margin: 0cm 0cm 8pt 36pt;\"><span style=\"margin: 0cm 0cm 8pt; text-align: justify;\"><strong>PFH<sub>D<\/sub> <\/strong>should be used for safety systems in high demand or in continuous mode.<\/span><\/li>\n<\/ul>\n<p>&nbsp;<\/p>\n<p>&nbsp;<\/p>\n<p>&nbsp;<\/p>\n<p>&nbsp;<\/p>\n<p style=\"margin: 0cm 28.3pt 0cm 0cm; text-align: justify;\"><em><strong><span style=\"margin-bottom: 0cm; margin: 0cm 0cm 8pt; text-align: justify;\">[IEC 61508-1] 7.6 Overall safety requirements allocation<\/span><\/strong><\/em><\/p>\n<blockquote>\n<p style=\"margin: 0cm 28.3pt 0cm 0cm; text-align: justify;\"><span style=\"margin: 0cm 28.3pt 0cm 0cm; text-align: justify;\">[\u2026] 7.6.2.9 When the allocation has sufficiently progressed, the safety integrity requirements, for each safety function allocated to the E\/E\/PE safety-related system(s), shall be specified in terms of the safety integrity level in accordance with Table 2 or Table 3 and shall indicate whether the target failure measure is, either: <\/span><\/p>\n<ul>\n<li style=\"list-style-type: none;\">\n<ul style=\"list-style-type: square;\">\n<li style=\"margin-top: 0cm; margin-right: 28.3pt; margin-bottom: 0cm; text-align: justify; margin: 0cm 0cm 0cm 36pt;\"><span style=\"margin: 0cm 28.3pt 0cm 0cm; text-align: justify;\">the average probability of dangerous failure on demand of the safety function, (PFD<sub>avg<\/sub>), for a low demand mode of operation (Table 2), or <\/span><\/li>\n<li style=\"margin-top: 0cm; margin-right: 28.3pt; margin-bottom: 0cm; text-align: justify; margin: 0cm 0cm 0cm 36pt;\"><span style=\"margin: 0cm 28.3pt 0cm 0cm; text-align: justify;\">the average frequency of a dangerous failure of the safety function [h<sup>-1<\/sup>], (PFH), for a high demand mode of operation (Table 3), or <\/span><\/li>\n<li style=\"margin-top: 0cm; margin-right: 1.0cm; margin-bottom: 6.0pt; text-align: justify; margin: 0cm 0cm 8pt 36pt;\"><span style=\"margin: 0cm 28.3pt 0cm 0cm; text-align: justify;\">the average frequency of a dangerous failure of the safety function [h<sup>-1<\/sup>], (PFH), for a continuous mode of operation (Table 3).<\/span><\/li>\n<\/ul>\n<\/li>\n<\/ul>\n<\/blockquote>\n<p style=\"margin: 0cm 28.3pt 0cm 0cm; text-align: justify;\"><span style=\"margin: 0cm 28.3pt 0cm 0cm; text-align: justify;\">The content of Table 2 of IEC 61508-1 is indicated hereafter, as well as the content of Table 3.<\/span><\/p>\n<\/div>            <\/div>\n        \n        \n                <div class=\"clear-both\"><\/div>\n        \n\n    <\/div>\n        <\/section>\n\n    \n\n\n\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:flex-row\">\n\n    \n\n        \n        \n    <\/div>\n        <\/section>\n\n    \n","protected":false},"excerpt":{"rendered":"<p>IEC 61508-1 clarifies which target failure measure, or unreliability function F(t), should be used, depending upon the mode of operation:<\/p>\n<p>\tPFDavg should be us&#8230;<\/p>\n","protected":false},"author":1,"featured_media":36221,"parent":3866,"menu_order":3,"template":"","meta":{"_acf_changed":false,"footnotes":""},"normative-tecniche":[58],"class_list":["post-3865","normativa_tecnica","type-normativa_tecnica","status-publish","has-post-thumbnail","hentry","tipologia_normativa-en-iec-standards"],"acf":{"sottotitolo":"","allegati":null},"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v27.2 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>High and Low Demand operation mode<\/title>\n<meta name=\"description\" content=\"IEC 61508-1 clarifies which target failure measure, or unreliability function F(t), should be used, depending upon the mode of operation: PFDavg should be us...\" \/>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" 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