What is common cause failure and how is it addressed in SIS design?

Common cause failure (CCF) is a failure affecting multiple redundant components from a single cause - defeating redundancy benefits. Examples: calibration errors affecting all sensors, software bugs in all logic solvers, environmental conditions (vibration, corrosion), common mode design flaws. CCF is modeled using beta factor (fraction of failures that are common cause, typically 2-10%). Mitigation: diversity (different manufacturers, technologies, or measurement principles), physical separation, different maintenance teams, environmental protection, and independence of utilities. CCF limits achievable SIL even with high redundancy. Analysis per IEC 61511 Annex F with scoring system.