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Forum: Reliability & Maintainability Questions and Answers

Topic: Reliability & Maintainability Questions and Answers

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

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Original Message:

Posted by: JM Cloarec (jmcloarec@thsrc.com.tw )
Organization:IREG
Date posted: Tue Oct 28 22:57:59 US/Eastern 2003
Subject: MTBF assignment
Message:
Can we estimate a MTBF target like that : reliability performance system is any failure should occur less than once per year. As the system is used 18h per day, the MTBF must be equal or greater to 6,570 h i.e (8,760 / 24) * 18.


Reply:

Subject: MTBF Test Time
Reply Posted by: BWD (bdudley@alionscience.com )
Organization: RAC
Date Posted: Tue Nov 4 11:25:42 US/Eastern 2003
Message:
Given that the reliability time factor for performance is one year or 8760 hours, operating and non-operating time must be considered. So, lowering the time factor is not acceptable. My calculation for a 90% confidence level and one or less failure in one year is a mean-time-between- failure (MTBF) of 2,250 hours.


Reply:

Subject: MTBF chi square
Reply Posted by: JM Cloarec (jmcloarec@thsrc.com.tw )IREG
Date Posted: Tue Nov 4 23:25:59 US/Eastern 2003
Message:
Are you sure of your calculation? Using the Chi-square it gives me CHI(0.1, 4)/(2*8760)= 16,528 hours. Your calculation giving 2,250 hours is for 10% confidence level and not 90%. Am I wrong ????


Reply:

Subject: Confidence
Reply Posted by: BWD (bdudley@alionscience.com )
Organization: RAC
Date Posted: Wed Nov 5 8:57:20 US/Eastern 2003
Message:
In the chi-square equation and the table, alpha is the risk factor and is 1 - the confidence level. So, a 90% confidence is a 10% risk factor which you use in the table to obtain a estimate for the one sided lower level MTBF. This answers the question of what value you need to meet the required reliability at 90% confidence.


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