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: Lanny Gerber
Date posted: Wed Jul 5 12:33:26 US/Eastern 2000
Subject: System Level Reliability Model
I just did a small system prediction. It appears that the system reliability model multiplier is being applied at each subassembly and also at the top system level(i.e. This mulitplier is applied at every level so the further down in a system the better the reliability would be). This seems odd to me. Should the system level models only be applied at the system level not the LRU level?
Thanks in Advance
Subject: PRISM System Level Model
Reply Posted by: (firstname.lastname@example.org
Organization: Reliability Analysis Center (RAC)
Date Posted: Mon Aug 14 14:11:52 US/Eastern 2000
The PRISM system level model uses a process grade factor to account for the factors contributing to system reliability. This process grade factor is applied at each level of assembly during a system analysis. Currently, if an assembly is part of a system or is a subassembly, a process grade factor is reapplied to the assembly modifying its failure rate. The intent of the PRISM process grade is to assess the manufacturer’s process associated with each part of the system. Therefore, once a process grade has been established for an assembly or system, the resultant failure rate should not be modified further by other process grade factors. When multiple levels of assemblies are incorporated in PRISM, assembly failure rates are not being assessed properly. With this in mind, PRISM is being modified as follows:
The failure rate of an assembly, with no subassemblies, is determined by summing the failure rates of the component hardware and applying the associated process grade factor to this value. The software failure rate is then added, which results in the total failure rate for the assembly. If this assembly is integrated into another assembly or into a system, the process grade of the higher assembly or system is not reapplied to this “subassembly” failure rate.
The process grade factor is only applied to hardware components that directly reside beneath the current assembly.
The values associated with the PRISM process grade factor have not changed. The PRISM defaults for the process grade factor and environment were normalized to approximately unity for an environment that has an operating temperature of 30oC, a dormant temperature of 14oC and a vibration level of 1. Therefore, selecting these values will approximate a PiE of 1 and a process grade factor of 1 in PRISM.
This PRISM modification will be released in Version 1.2 scheduled for the Fall of 2000.