Scenario 2: 99% detection coverage

Calculation 1: No FGS present (i.e. detection failure = 1, FGS failure = 1, mitigation failure = 1)


Calculation 2: SIL1 FGS is used


Calculation 3: SIL2 FGS is used


Calculation 4: SIL3 FGS is used


Calculation 5: Summary of risk reduction achieved


The calculations show that when detection coverage is less than perfect (i.e. detection coverage = 0.99), the risk reduction that can be achieved by each FGS is significantly impacted. Calculation 5 shows that the risk reduction that is achievable by a SIL3 FGS is lowered to SIL1 (risk reduction < 100), the risk reduction that is achievable by a SIL2 FGS is also lowered to SIL1 (risk reduction < 100) and a SIL1 FGS will fall below the minimum SIL level.