Failure Rate (λ)

Definition:
Failure Rate (λ) is the expected number of failures per unit time for a system or component. This is a critical part of calculating probability of failure on demand (PFD) and risk reduction. It can be expressed as all failures or separately per the below.

There are four types of failure rate. A combination of

• dangerous or safe – meaning the failure impacts the ability of the safey function to perform its function, or to not.
• detected or undetected – meaning the diagnostics detects the error or not

Key Points:

• Expressed typically as failures/hour.
• Dangerous Detected (λ_DD), Safe Detected (λ_SD),
• Dangerous Undetected (λ_DU), Safe Undetected (λ_SU)
• Typically reported on the SIL Certificate.

Example:
A valve has a dangerous undetected failure rate (λ_DU) of 2E-6 failures/hour as reported on the SIL Certificate. This term is used in the PFD calculations.

See also: PFDavg, detected failure, undetected failure

Cited Source:

• IEC 61508-6:2010, Annex B.