6. Frequently asked questions
Does it make sense to talk about SIL for equipment?
Yes, as defined by IEC 61508. This is known as SIL capability.
Not within the meaning of IEC 61511. The estimated SIL level must be that of the FIS.
Can the SIL of a RIS be deduced from the SIL of its components?
No. At best, it is possible to say that the SIL of the RIS cannot be better than the lowest SIL of the equipment making up the RIS. For example, even if it doesn't really make sense, the association of a SIL 2 sensor with a SIL 2 PLC and a SIL 2 valve does not allow us to draw any other indication than that the RIS composed by the sensor, the PLC and the valve will at best be SIL 2. On the other hand, calculation of the PFD average could lead to SIL 1.
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Olivier Iddir – Principle for assessing the probability of failure of risk control measures. [SE 4 057] Safety and risk management (2009).
Olivier Iddir – LOPA method: principle and example of application. [SE 4 075] Safety and risk management (2012).
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