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Forum: Reliability & Maintainability Questions and Answers

Topic: Reliability & Maintainability Questions and Answers

Topic Posted by: Reliability & Maintainability Forum (src_forum@alionscience.com )
Organization: System Reliability Center
Date Posted: Mon Aug 31 12:47:36 US/Eastern 1998

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Original Message:

Posted by: Rob Poltz (getreliability@designanalytx.com )
Organization:Design Analytx International
Date posted: Tue Nov 5 12:37:02 US/Eastern 2002
Subject: FIT definition and derivation
Message:
There seems to be a misuse or misunderstanding of the term FITs and I was wondering its correct definition and derivation. I believe the proper definition is "Failure Units" and the Acronym "FIT" is based on the Mnemonic for Failure Unit, not Failures in Time, as a growing number of people have come to believe is correct. An explanation of FIT derivation is that it was originally construed the same way the term "Bit" was derived by taking certain letters (B from Binary, and last two letters from the word "digit"), to form "Bit", and this explanation appears in several documents dating back to the early 80s. The same powers of ten apply,i.e., (10e9) or billion units. Since "time" is not the only unit of measure when calculating failure (hazard) rates, it seems ludicrous to perpetrate a misnomer definition or at least standardize its correct usage. I realize that MH217 document was based on 10e6 hrs. for electronics systems, but mechanical systems are more apt to be calculated in number cycles, rotations, events, or some other operations. What do you think?


Reply:

Subject: FIT definition
Reply Posted by: Chiew Peng Chuah (c.p.chuah@sheffield.ac.uk )
Organization: University of Sheffield
Date Posted: Wed Nov 6 7:25:08 US/Eastern 2002
Message:
I have only recently starting to research on reliability issues for power drive applications. However, i did noticed the usage of FITs as Failure rate per unit time. This was openly used by manufacturers and various standards like MIL-HDBK217 and also the new standard set by RAC and their associated software PRISM although it was claimed that their term was per million calender hours.


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