Definition:
Beta Factor (β) is the method for quantifying the contribution of Common Cause Failures (CCF) in redundant systems. It is the mathematical representation of CCF. It is the fraction of all dangerous failures that are CCF related. Therefore, 1-β represents the independent failures.
This term is part of most PFDavg calculations. Note that failure rate values include both common cause and independent. β only covers hardware failures. It is scored per subsystem using IEC 61508-6 Annex D, and the result is a 1oo2 value that Table D.5 adjusts for other architectures.
Deep dive: For a much more in-depth treatment, see Beta Factor Estimation – Checking the Certificate Value Against Your Installation.
Key Points:
- Common cause failures occur when multiple components fail simultaneously due to a shared cause.
- Beta factor represents the ratio of total dangerous failures attributable to common cause.
Example:
If one technician miscalibrates both redundant pressure instruments against the same bad reference, this is treated as a common cause failure. A shared power supply is a common cause too, but it is modeled as its own component rather than through β.
See Also: common cause failure, applying the beta factor, CMF
Cited Source:
- IEC 61508-6:2010, Annex D.