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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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Original Message:

Posted by: Clay Davis
Date posted: Tue Oct 5 17:12:16 US/Eastern 1999
Subject: Reliability vs Availability Calculations
I am trying to determine the availability for a new design for a computer system. Since availability is highly dependent of the software and the MTTR, I have decided to focus only on the hardware at this point. My question is, say for a system that has an MTBF of 10 yrs, is the availability (from a hardware standpoint) 0.9048 at the end of the first year? I calculated this number using the standard R=e(-lambda*t). Lambda = 1 / 87600 hours and t = 8760 hrs. This would lead me to believe the computer system is going to be un-available for more than 1 month during the first year, which cannot be correct. Any thought on what I am doing wrong? Thanks for your help. Clay Davis

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Subject: Availability Calculations
Reply Posted by: Patrick Hetherington
Date Posted: Mon Oct 11 7:15:19 US/Eastern 1999
Downtime during the first year assuming continual operation and 99.5% availability is 2628 minutes (24*365*60*.005). To determine MTBF solve Availabilty=MTBF/(MTBF+MTTR) for MTBF. In your case with a 15 minute (.25 hour) MTBF=A*MTTR/(1-A). MTBF = 49.75 hours (2985 minutes). By saying you want no more than 2 failures per year, you are implying a failure rate. Generally you would like to put confidence levels on this, but using the averages you are saying you would like a failure rate of 2/8760 or 0.00022 failures per hour. This equates to a MTBF of 4380 hours. Given you have the same MTTR (.25 hours) your availability is now 99.99%.

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