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: Alberto D'Amico ( )
Organization:Olivetti I@Jet
Date posted: Thu Jan 11 7:00:21 US/Eastern 2001
Subject: Duty cycle and calendar hours
Let's suppose we have a product, -> whose commercial life is 5 years (so 43600 calendar hours). -> It is plugged for 5 years (so the high voltage part of its Power supply unit works for 5 years with no interruption). -> It is on (in std-by mode, electronic board on) for -let's say- 2000 hrs every year (so 10000 hrs in its life) -> It's mechanical part's life have to be 60000 cycles (during the 5 years) -> If a single cycle lasts 60 seconds then, in the 5 years, there are 1000 fully operational hrs during its life. In such a situation, how do I set the duty cycles for the different modules of the product? how can I read the failure rate coming out from Prism? Since it is in calendar hours, is it correct to multiply it for the operational/calendar ratio (43600/1000 = 43.6, in our example)? A last question (sorry to bother you), how the duty cycles and usage profiles are taken into account if they are different within assemblies of the same system, and their conditions are different from the default ones for the system? Thank you Alberto


Subject: Duty cycle calculations
Reply Posted by: David Dylis ( )
Organization: Reliability Analysis Center (RAC)
Date Posted: Mon Jan 15 15:06:52 US/Eastern 2001
The duty cycle and cycling rate terms in PRISM modify the failure rate of parts that are calculated using PRISM RACRate models. The current PRISM tool contains RACRates models for integrated circuits, diodes, transistors, thyristors, capacitors, resistors and software. When generic failure rates (e.g., mechanical parts) are extracted from the reliability databases resident in the tool or when a user integrates data from another source, PRISM does not modify the failure rate of these components based on duty cycle. Therefore, it is recommended that the user integrate data that most closely represents the specific application whenever possible. PRISM allows the duty cycle to be set for individual parts or assemblies. Duty cycle can be adjusted under operating profile on the item information screen for each component or assembly. To determine the failure rate for components and assemblies, the information can be viewed using the failure rate tab for any component or assembly. In addition, several PRISM reports are available: A system level report displays the current system level summary information that includes the breakdown of System Level Model Parameters, Predecessor System Analysis and Observed Data. A Tree View report displays the current system tree with failure rates for each component and assembly included in the structure. The Assembly Breakdown Summary Report decomposes each Assembly to all of its child Assemblies. Component failure rates are not provided in this report. The Assembly Breakdown Detail Report displays the current system by Assembly. Each Assembly is decomposed to all of its child Assemblies and/or Components. The component detail report displays failure rates for all components used in the active System. Where applicable, RACRates Failure Rate Model Parameters are provided. The assembly pareto report displays the failure rates of Assemblies and Components existing directly below the active System or selected Branch. Failure Rates are rank ordered from highest failure rate to lowest. The component pareto report displays the failure rates of all Components existing in the active System. Failure Rates are rank ordered from highest failure rate to lowest. In your example, an item that only operates for 1000 hours over a period of 43,600 hours will have a duty cycle of approximately 2.3%. Since PRISM does not allow a duty cycle to be entered as a decimal value, I would recommend that either 2 or 3% be used as a duty cycle for this portion of your PRISM analysis.

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