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Forum: Reliability & Maintainability Questions and Answers

Topic: Reliability & Maintainability Questions and Answers

Topic Posted by: Reliability & Maintainability Forum ( )
Organization: System Reliability Center
Date Posted: Mon Aug 31 12:47:36 US/Eastern 1998

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Posted by: Geoff Hampden-Smith ( )
Organization:Shell UK
Date posted: Fri Jun 24 5:58:23 US/Eastern 2005
Subject: Component Confidence levels
The target availability for a 3 component parallel system (valve shutdown system) is 99.95%. The three components have similar failure rates. However, the failures of the components are only revealed on testing. The question is what confidence limits should be chosen for the individual components to have a 95% chance of shutdown on demand for the parallel system. Any direction on performing the calculation would be appreciated.


Subject: Rel Conf Limits
Reply Posted by: Joe Dzekevich
Date Posted: Mon Jun 27 13:12:05 US/Eastern 2005
Geoff, Confidence limits apply to measured data. In general, for any item, Rl = alpha ^ 1/n. So, if you had 50 units on test, Rl (lower CL) at 95% would look something like: Rl = (1-.95)^1/50 = (.05)^1/50 = .9418 for a zero allowed failure test. the system repairable while it is operating? Then R is higher. My only thought is this: confidence limits depend on measured data and thus sample size (n). Do you really mean: what R do I need for each component so that I have a system R of .95? Joe

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