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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: 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: Reliability vs Availability Calculations
Reply Posted by: Clay Davis
Date Posted: Wed Oct 6 12:26:14 US/Eastern 1999
So how does one determine, from looking at a design and a preliminary BOM, what the availability (approximately) will be? In other words, we are trying to design a computer system that will have an availability of 99.5%. For the 1st year, that equals 43.8 minutes of downtime (24*7*.005). If the avg repair time is 15 minutes (a guess), what does the MTBF have to be? From the standard availability formula it looks like the MTBF would need to be 2,985 hrs - which does not seem correct. Also, say we do not want the system to fail more than twice a year (with the est 15 min repair time for each failure) - does that affect the calculations? Thanks again. --Clay

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