Definition:
Confidence Level represents the assurance that risk assessment results and SIL calculations are accurate, considering uncertainties in data and modeling.
Used wherever quantitative data is generated or interpreted in the SIS safety lifecycle, including: hazard and risk analysis (LOPA frequencies), SIL selection, SIL verification, and periodic revalidation using updated field failure data.
Key Points:
- Higher confidence levels support stronger risk-based decisions. However, it may require more conservative assumptions, longer operating hours, or better-quality failure data (for example, Route 2H justification, long FMEDA data sets, or robust proof‑test records)
- Often expressed as a percentage (such as 70%, or 99%) that the true value of a parameter (such as failure rate or PFDavg) is at or below the calculated value.
Example:
During SIL verification of a level high‑high trip targeting SIL 2, the calculated mean PFDavg is 8E-3, which lies inside the SIL 2 band (1E-3 to 1E-2).
However, when uncertainty is evaluated, the 70% upper confidence limit for PFDavg is 1.3E-2, which is above the SIL 2 limit of 1.0E-2, so the SIF cannot be claimed as SIL 2 at 70% confidence and the design team must make improvements. Such as either improve the design or tightening proof testing.
Cited Source:
- IEC 61508-6:2010, Annex B.