What must be calculated to achieve a specific Safety Integrity Level (SIL)?

• A. The Safe Failure Fraction (SFF)
• B. The probability of failure on demand (PFDavg)
• C. The complexity of the device used
• D. The historical data of equipment failures

Answer: (B)

To achieve a specific Safety Integrity Level (SIL), the probability of failure on demand (PFDavg) must be calculated. PFDavg measures the likelihood that a safety system will fail to perform its intended function when needed. It is crucial for establishing the effectiveness of safety functions and is an essential parameter in determining the SIL according to safety standards such as IEC 61508.

The SIL is a measure of the level of risk reduction required for a safety function, and it is classified into four levels (SIL1 to SIL4), with each level corresponding to specific ranges of PFDavg. Thus, by calculating the PFDavg, one can determine whether a system meets the necessary criteria for the desired SIL.

While other factors, such as the Safe Failure Fraction (SFF) and historical data of equipment failures, are related to safety assessments and system reliability, they do not directly represent the probability of failure on demand or define SIL levels. Complexity of the device used is also not a direct measure related to SIL determination.