Residual risk is the level of risk that remains after all safeguards and independent protection layers have been applied to a hazardous scenario. It is the counterpart to inherent risk — where inherent risk is the unmitigated starting point before any protection is credited, residual risk is the end state after all safeguards have been accounted for.
In IEC 61511 and LOPA, residual risk is calculated by dividing the inherent risk by the combined risk reduction factor (RRF) of all credited independent protection layers (IPLs). The result is then compared against the tolerable risk target. If residual risk exceeds the tolerable risk target, additional risk reduction is required — typically a safety instrumented function (SIF) with a defined target SIL.
Key Points:
- Residual risk = inherent risk ÷ combined RRF of all credited IPLs
- Must be at or below the tolerable risk target for the risk to be considered acceptable
- If residual risk exceeds the tolerable risk target, a SIF or additional IPL is required
- Distinct from inherent risk (before any safeguards) and tolerable risk (the acceptable threshold)
- The gap between residual risk and the tolerable risk target defines the required SIL for a new SIF
Example:
A process scenario has an inherent risk of 1E-2/year. Two IPLs are credited: a high-pressure shutdown with RRF = 100, and a pressure relief valve with RRF = 10. The combined RRF is 1,000, giving a residual risk of 1E-5/year. Since the tolerable risk target is 1E-4/year, the residual risk is acceptable and no SIF is required.
See Also: inherent risk, tolerable risk, LOPA, IPL, RRF, SIF, target SIL
Cited Source:
- IEC 61511-1:2016, Clause 9 (Risk Assessment and SIL Determination)
- CCPS — Layer of Protection Analysis: Tolerable Risk Guidelines for Chemical Processes
- ISA — IEC 61511 Functional Safety Standard