SRC Forum - Message Replies
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
Posted by: Andrew Comons
Date posted: Mon Mar 4 20:47:32 US/Eastern 2002
Subject: MTBF and Redundent Systems
I am trying to write a system specification
for equipment which will be fielded in pairs, the second system will act as backup system. Each system will be required to have a spare parts package on-board, where the fulltime attending operator will be required to employ built in diagnostics and correct faults within a period of 5 minutes. If faults can't be corrected in 5 minutes the decision is to switch to the backup system. Mission duration is 120 hours. Reliability goal is 95%.
I need to determine MTBF of the system, including the redundent but I do not know how to account for a failure if it is correctable by the 5 minute fault detection/correction requirement.
Sample calcs, references or advice available?
Thanks in advance.
Subject: Repairable system reliability approximation
Reply Posted by: Larry George
Organization: Problem Solving Tools
Date Posted: Sat Mar 9 12:49:30 US/Eastern 2002
K.I.S.S. The following Markov chain model may be an adequate approximation for specifying reliability and MTBF. Build a spreadsheet to analyze the Markov chain shown in the transition diagram below for reliability and MTBF.
This model assumes failure and repair rates don't depend on time or age and that the reserve system won't fail while on standby.
[Sys1 up] mu<->lambda [Sys 1 dn] ->1-mu [Sys 2 up]
mu<->lambda [Sys 2 dn] ->1-mu [Failure]
lambda is the failure rate of either system
mu is the repair rate of either system. Use the probability that repair is completed within 5 minutes.
Let me know if you would like help with the spreadsheet model.