{"id":5504,"date":"2021-06-24T09:24:39","date_gmt":"2021-06-24T07:24:39","guid":{"rendered":"https:\/\/lorit-consultancy.com\/en\/?p=5504"},"modified":"2022-09-26T15:48:59","modified_gmt":"2022-09-26T13:48:59","slug":"dont-be-sheepish-about-rams","status":"publish","type":"post","link":"https:\/\/lorit-consultancy.com\/en\/2021\/06\/dont-be-sheepish-about-rams\/","title":{"rendered":"Don\u2019t be sheepish about RAMS"},"content":{"rendered":"\n

We have discussed in recent blogs failure in time (FIT) rates (Have you got your <\/a>diagnostics<\/a> covered?<\/a>) and their usage in functional safety activities. The topic of (functional) safety is one element of a much wider subject area – that of reliability, availability, maintainability and safety<\/strong> (RAMS). We operate as company mainly in two industries automotive and medical, but we dabble in other sectors from time to time and the industry that arguably has the most encompassing approach to RAMS is the rail sector \u2013 so we will take a look at some of the key points in EN 50216 and its approach to the subject. Automotive and medical device sectors address safety fairly comprehensively, but what about the R, the A and the M?<\/p>\n\n\n\n

Reliability<\/strong><\/h2>\n\n\n\n

Reliability is a key topic for any product development. Defining an expected service life is a requirement in most industry standards, but in some sectors, this can be a critical factor. Just think of devices installed in remote locations or devices that cannot be serviced readily. We have worked on projects where devices may be installed in a desert or the artic.<\/p>\n\n\n\n

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Reliability is defined as \u201cability to perform as required, without failure, for a given time interval, under given conditions\u201d.<\/em><\/p>\n<\/div><\/div>\n\n\n\n

One key technique for assessing the reliability of a system is a reliability block diagram (RBD)<\/strong>, which can be used to define the probability of the loss of the system function. Figure 1 illustrates a simple RBD:<\/p>\n\n\n\n

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Figure 1: Reliability Block Diagram<\/figcaption><\/figure>\n\n\n\n

The usage of FIT rates<\/strong> (denoted by lambda \u03bb) enables the reliability of a system to be calculated either mean time to failure MTTF for non-repairable systems or mean time between failures for repairable systems MTBF \u2248 1 \/\u03bb.<\/p>\n\n\n\n

Redundancy<\/strong><\/h2>\n\n\n\n

In Figure 1 on the basis that the FIT rate for TR1 is too high and the dominate failure mode of a transistor is short circuit, a redundant circuit TR2 and R2 can be added and hence yielding a more reliable system.<\/p>\n\n\n\n

However, one assumption often taken is that using redundancy will automatically yield a more reliable system. In the excellent reference from Alessandro Birolini \u2013 \u201cReliability Engineering\u201d <\/strong>– quite the contrary is suggested. Redundant elements have ever diminishing returns in producing a reliable system (with ever increasing cost), the key steps and priority sequence are as follows:<\/p>\n\n\n\n