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What\u2019s dangerous and when can we detect it? PFDavg, PFH and Dangerous Failures in IEC 61508<\/h2>\n
The IEC 61508 approach uses two different quantities, depending on the demand placed on the specific safety function. PFDavg<\/strong> is the average probability of dangerous failure on demand and is used when low demands are placed on the safety function. PFH<\/strong> is the average frequency of dangerous failure and is used for higher demand rates of the safety function or continuous operation.<\/p>\nThe resulting PFDavg or PFH value provides an indication of the safety integrity level (SIL) that can be claimed for the product.<\/p>\n
IEC 61508 also divides failures into four groups: safe detected, safe undetected, dangerous detected and dangerous undetected<\/strong>. For this discussion, we are mainly concerned with the two dangerous failure groups.<\/p>\nA dangerous undetected failure<\/strong> is a failure that is not covered by our diagnostic coverage. A dangerous detected failure<\/strong>, on the other hand, is detected by the diagnostics, but can still become a concern if there is not sufficient time to repair the system.<\/p>\nIf the demand rate for the safety function is low (less than once a year), it is likely that a dangerous detected failure can be repaired before the next demand on the safety function. In this case, the dangerous detected failure can be disregarded from the calculation.<\/p>\n
The complexity of the PFD calculation depends on several factors, including the redundancy strategy (e.g. 1 out of 2, 2 out of 3), whether the device is repairable, and the relationship of common cause failures.<\/p>\n
The IEC 61508 approach ultimately provides a comprehensive summary of the probability of failure on demand or, in the case of PFH, the average frequency of dangerous failure.<\/p>\n
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Conclusion<\/h2>\n
Whichever measures we use as a yardstick for the quality of our system, and more specifically the hardware, we need to be clear about the context and what we are trying to estimate. Whether the device is repairable or not is one key question (and not a consideration for ISO 26262). The quality of the diagnostics in the architecture is another key point.<\/p>\n
Ultimately, the more complex the system, the higher the failure rate will be. However, the level of diagnostic coverage also needs to be considered when determining whether the resulting metric is acceptable.<\/p>\n
We do not have apples-to-apples comparisons across industries and standards. Ultimately, architectural metrics provide useful data on the quality of the design, while PFD or PFH gives us a comprehensive figure for failure in IEC 61508. ISO 26262 PMHF provides more of a benchmark for comparing different items and is less useful for defining a quantitative figure for the overall failure rate.<\/p>\n
By Alastair Walker<\/a>, Owner \/ Consultant<\/p>\n","protected":false},"excerpt":{"rendered":"Understanding Safety Metrics behind IEC 61508 and ISO 26262 As a consultancy we spend a fair amount of our time working with hardware metrics both for safety and reliability engineering. A few key questions crop up here: What is the objective of hardware metrics? What do the numbers tell us? What benefit do the metrics […]<\/p>\n","protected":false},"author":11,"featured_media":9262,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[90,91,93],"tags":[],"class_list":["post-9254","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-reliability","category-safety","category-automotive"],"acf":[],"_links":{"self":[{"href":"https:\/\/lorit-consultancy.com\/en\/wp-json\/wp\/v2\/posts\/9254","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/lorit-consultancy.com\/en\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/lorit-consultancy.com\/en\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/lorit-consultancy.com\/en\/wp-json\/wp\/v2\/users\/11"}],"replies":[{"embeddable":true,"href":"https:\/\/lorit-consultancy.com\/en\/wp-json\/wp\/v2\/comments?post=9254"}],"version-history":[{"count":7,"href":"https:\/\/lorit-consultancy.com\/en\/wp-json\/wp\/v2\/posts\/9254\/revisions"}],"predecessor-version":[{"id":9274,"href":"https:\/\/lorit-consultancy.com\/en\/wp-json\/wp\/v2\/posts\/9254\/revisions\/9274"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/lorit-consultancy.com\/en\/wp-json\/wp\/v2\/media\/9262"}],"wp:attachment":[{"href":"https:\/\/lorit-consultancy.com\/en\/wp-json\/wp\/v2\/media?parent=9254"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/lorit-consultancy.com\/en\/wp-json\/wp\/v2\/categories?post=9254"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/lorit-consultancy.com\/en\/wp-json\/wp\/v2\/tags?post=9254"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}
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