If the test interval for periodic inspection and testing is increased, what happens to the PFDavg calculation for a safety instrumented function?

If the test interval for periodic inspection and testing is increased, what happens to the PFDavg calculation for a safety instrumented function?

• A. Increase
• B. Decrease
• C. Remain the same
• D. It varies randomly

Answer: (A)

When the test interval for periodic inspection and testing is increased, it generally leads to an increase in the average probability of failure on demand (PFDavg) for a safety instrumented function. This is because the longer the interval between tests, the more likelihood there is for undetected failures to accumulate in the system. In functional safety, safety instrumented systems (SIS) are designed to perform specific safety functions to prevent or mitigate hazardous events. Regular testing is essential to ensure that these systems remain reliable and effective. When the testing interval is extended, the system may experience a higher average probability of failing to respond correctly when a demand for safety arises. This increase in the possible number of failures that can go undetected over a longer period contributes to a higher PFDavg. The reliability of the system diminishes as it remains untested for a longer duration, reflecting a decreased confidence in its performance during an actual demand scenario. Therefore, with a longer test interval, the average probability of failure tends to rise, leading to a less favorable safety condition in terms of the PFDavg.

When the test interval for periodic inspection and testing is increased, it generally leads to an increase in the average probability of failure on demand (PFDavg) for a safety instrumented function. This is because the longer the interval between tests, the more likelihood there is for undetected failures to accumulate in the system.

In functional safety, safety instrumented systems (SIS) are designed to perform specific safety functions to prevent or mitigate hazardous events. Regular testing is essential to ensure that these systems remain reliable and effective. When the testing interval is extended, the system may experience a higher average probability of failing to respond correctly when a demand for safety arises. This increase in the possible number of failures that can go undetected over a longer period contributes to a higher PFDavg.

The reliability of the system diminishes as it remains untested for a longer duration, reflecting a decreased confidence in its performance during an actual demand scenario. Therefore, with a longer test interval, the average probability of failure tends to rise, leading to a less favorable safety condition in terms of the PFDavg.