What is spurious trip rate and how is it managed in SIS design?

Spurious trip rate (STR) is the rate of unwanted shutdowns caused by SIS failures. Safe failures (false trips) cause production loss, economic impact, and potential startup hazards. STR is calculated from safe failure rates of all SIF components in series. Management strategies: use 2oo3 or 2oo4 voting (require multiple inputs for trip), high-reliability components (lower safe failure rate), diagnostic coverage (detect failures before they cause trips), proper maintenance (prevent degradation), and redundant final elements with voting. Balance STR against SIL - architectures that improve safety (1oo2 sensors) may increase STR. Target STR often <0.1 per year for critical processes.