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SIL assessment of safety instrumented functions (SIF) – Calculations

To justify a SIL level, in addition to PFD or PFH requirements, IEC 61511-1 defines minimum requirements. According to the standard, "for safety instrumented functions, sensors, logic units and terminal elements shall have a minimum hardware fault tolerance (HFT)".

To say that a device has a minimum hardware fault tolerance of 1 means that there is a second device to perform the function.

This requirement depends on :

• SFF (Safe Failure Fraction) for logic processing;

• equipment "characteristics" for sensors and terminal elements.

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