Failure rates are the number of failures per unit time for a piece of equipment which are usually assumed to be a constant value. They can be broken down into several categories, such as safe and dangerous, detected and undetected, and independent/normal and common cause. Failure rates are often expressed by λ and in the unit of measurement of FITs

Both IEC 61508 and IEC 61511 define failure rates, use them in assessments and equations, and emphasize the following:

“The reliability data used when quantifying the effect of random failures shall be 

documented and 

(clause 11.9.3)

This strong language reinforces what any good reliability engineer has long understood: No matter how good your calculations or software might be, if you put failure rates that are not credible or justified for your application, they will be worthless. This updated language will hopefully lead to the end of unrealistic failure rates. 


Optimistic failure rates and data leads to unsafe designs

  • Insufficient redundancy 
  • Insufficient testing

Required risk reduction will not be reached

SIL invalid

To check if your failure rates are too optimistic, go to   

Tagged as:     silsafe     SIL     Loren Stewart     IEC 61511     IEC 61508     FITS     Failure Rates  

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