PRISM Forum - Message Replies

Topic: PRISM Questions and Answers
Topic Posted by: SRC ( )
Organization: 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.

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Original Message:

Posted by: Alek
Date posted: Fri Sep 1 7:05:01 US/Eastern 2006
Subject: Prism Duty Cycle parameter
PRISM tool has a Duty Cycle parameter selector. Does it mean that a system failure rate calculated by Prism can be obtained in operating hours if I set the Duty Cycle value equal to 100% and the Cycling Rate equal to 0?


Subject: PRISM Duty Cycle
Reply Posted by: David Dylis ( )
Organization: Alion System Reliability Center (SRC)
Date Posted: Fri Sep 8 14:50:59 US/Eastern 2006
PRISM includes two parameters that address the on/off cycling of a piece of equipment:

Operational duty cycle
Cycling rate

The operational duty cycle is the amount of time that the unit actually operates while cycling rate is how many times a unit is turned on/off during a year (cycles/year). For instance an item can be turned on/off twice a day and operate 8 hours or could operate only 1 hour. Each would have a different duty cycle though the same cycling rate. The cycling rate affects the cycling portion of a PRISM part failure rate that uses the PRISM Component Prediction models. The more on/off cycles the higher the cycling failure rate. Duty cycle affects operational/non-operational failure rate values as they are tied to the amount of time that an item actually is in operation.

Traditionally, reliability prediction failure rate models have been based on the operating time of the part, and the values were typically stated in failures per million (or billion) operating hours. The PRISM component prediction models (and the empirical data contained in the databases used in the PRISM tool) predict the failure rate in units of failures per million calendar hours. This is necessary because it represents the common basis for all the failure rate contribution terms used in the PRISM methodology (e.g., operating, nonoperating, cycling, and induced). If an equivalent operating failure rate is desired (in units of failures per million operating hours) steps need to be taken to make this conversion.

To convert a PRISM analysis from Calendar to Operating hours the resulting PRISM predicted failure rate (based on the expected duty cycle) should be divided by the duty cycle in its decimal form (e.g., A 50% duty cycle = 0.5). It would be incorrect to assume a 100% duty cycle for the system (unless the system actually operates at 100% duty cycle) as factors that influence reliability, such as cycling and non-operation, would be not be properly addressed.

If you have any additional questions, feel free to contact me.

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