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: Ankur
Date posted: Fri Mar 25 11:13:20 US/Eastern 2005
Subject: Failure Rates
I am doing FMEA analysise of a benchmark power supply as for trial. For this I have to calculate RPN which is (RPN= SEVERITY x PROBABILITY OF FAILURE X DETECTION). I have found out the failure rates of each components using prism. That gives me failures/million hr. How can relate this failure rate with the severity or prbability of occurence. Can you suggest on this. I'll be really kind
if you can help,
Subject: Risk Priority Numbers
Reply Posted by: bwd
Date Posted: Wed Mar 30 11:58:37 US/Eastern 2005
Risk Priority Number (RPN)
Commercial organizations use the Risk Priority Number (RPN) system for evaluating failure modes. RPN is a subjective ranking number system assigned to a specific category. Three most widely used categories are:
• Occurrence Ranking
• Severity Ranking
• Detection Ranking
The scale used for each category is a range from 1 (low) to 10 (high). RPN is computed by taking the product of all three numbers to yield a value from a minimum of 1 to a maximum of 1000. High RPN's will be associated with failure modes demanding attention.
The Occurrence ranking number is a subjective estimate of the frequency of occurrence of an independent failure mode. The occurrence ranking is determined by consensus of the team. The ranking numbers range from Remote (1) to almost certain (10).
Associated with the ranking is a criteria and a subjective probability that may be expected with the independent failure mode.
The Severity Ranking number is a subjective estimate of the severity of the effect of failure, based on the assumption that an independent failure mode has occurred. The severity ranking is determined by consensus of the team. The ranking numbers range from negligible (1) to catastrophic (10). Associated with the ranking is a criteria that reflects the severity of the effect of failure to the user of customer. The Severity classifications used in system safety engineering are blended with the criteria for severity ranking given below. For example, category I – Minor will be equivalent to a ranking range from 1 to 3. Category II – Marginal will be equated to ranking range from 4 to 6. Category III – Critical will be equated to 7 to 8. Category IV – Catastrophic will be equated to a ranking range from 9 to 1.
The detection (D) ranking is a subjective estimate of detecting an independent failure mode. The detection ranking number is determined by consensus of the team. The ranking numbers range from excellent chance (1) of detecting a failure mode to a remote chance (10) of detecting a failure mode.
Associated with the ranking is a criteria and subjective probability of detection that might be expected with the independent failure mode.