The Greek symbol lambda, λ, represents failure rates in functional safety, usually expressed in the unit of measurement of FITS.

λ can be expressed as a total failure rate for a device (λT), or it can be broken down into more specific groupings:

  • Safe detected (λSD)
  • Safe undetected (λSU)
  • Dangerous detected (λDD)
  • Dangerous undetected (λDU)
  • No effect (λNE)
  • Annunciated (λA) failures

The easiest way to know a device’s λ or failure rate, is from a certificate or FMEDA (Failure Mode Effects and Diagnostic Analysis) report. 

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 
justified "  (clause 11.9.3)

Because failure rates are entered into significant end user equations such as PFDavg and other SIL Verification tasks, it is imperative that they are not only applied to the right application, operation, and demand, but also that they are realistic and correct. The new editions of both standards now highlight this importance.

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

Related Items

Back to Basics: Failure Rates (Introduction)

Back to Basics: Failure Rates - FITS

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