PRISM Forum - Message Replies
Topic: PRISM Questions and Answers
Topic Posted by: SRC
Date Posted: Wed Jan 12 8:33:33 US/Eastern 2000
Topic Description: Welcome to the PRISM forum! Please feel free to post your questions and comments about the PRISM assessment software here.
Posted by: Barry Ives
Organization:Lockheed Martin Systems Integration-Owego
Date posted: Tue Dec 10 12:33:27 US/Eastern 2002
Subject: Failure per Million Hours
Please explain how the failure rate per million hours increases as the duty cycle is decreased (when I divide by the duty cycle to convert from failures per million calendar hours). This seems to defy logic as I would expect the failure rate per million hours to decrease as I decrease the percentage of operating time.
Subject: PRISM Field and Predicted Comparison
Reply Posted by: David Dylis
Organization: Reliability Analysis Center (RAC)
Date Posted: Tue Dec 10 13:23:10 US/Eastern 2002
Failure rates in PRISM are presented in failures per million calendar hours and not in failures per million operating hours which adds to the confusion when a user is only interested in failure rates solely due to operation. I will answer the question with an example.
If an item has a total failure rate of 1 failure/million calendar hours, is operating at a 5% duty cycle, the operational failure rate is 0.99 failures/million operating hours and the non-operating failure rate is .01 failures/million non-operating hours, then we obtain:
1 Failures/Million Calendar Hours = (.05) x 0.99 Failures/Million Operating Hours + (0.95) x 0.01 Failures/Million Non-operating Hours
Now converting to Failures/Million Operating Hours
(.05 Operating/Calendar) x 0.99 Failures/Million Operating Hours = 1 Failures/Million Calendar Hours - (0.95 Non-operating/Calendar) x 0.01 Failures/Million Non-operating Hours
Operational Failure Rate at a 5% duty cycle = 20.010101 Failures/Million Operating Hours
Similarly, for a 50% duty cycle the failure rate would be: 1.91919
The PRISM predicted failure rate is based on calendar time. The duty cycle in PRISM identifies the amount of time that an item operates (e.g., 5% of the time) during the calendar period (e.g., 1 million calendar hours). Therefore, in a given calendar period, items that operate with a higher duty cycle are actually operating a greater amount of time then those at a lower duty cycle. When a conversion from calendar to operating time is made and there is no or minimal difference between two predicted failure rates with varying duty cycles, then the item with the lower duty cycle will have a higher predicted failure rate since the predicted value is based on a lower amount of operational hours.
If you have further questions, feel free to contact me.