What does the statement "On-line testing is required to sustain the integrity of each SIF within a SIS" imply?

• A. Undetected failures are not likely to cause a problem
• B. Undetected failures will increase the PFDavg over time
• C. On-line testing might not require record keeping
• D. Batteries are replaced during each on-line test

Answer: (B)

The statement "On-line testing is required to sustain the integrity of each Safety Instrumented Function (SIF) within a Safety Integrity System (SIS)" implies that undetected failures can accumulate, leading to an increasing probability of failure on demand (PFDavg) over time. On-line testing serves as a proactive measure to identify and mitigate these undetected failures, ensuring that the SIFs remain functional and meet safety performance requirements.

When there are undetected failures within a SIF, the effectiveness of that function diminishes. If regular testing is not conducted, the likelihood of these failures going unnoticed increases, which can lead to a higher PFDavg. Therefore, performing on-line testing is crucial to identify any potential issues that could affect the reliability of safety systems, thereby maintaining safety standards and operational integrity.

Other choices do not correctly acknowledge the relationship between undetected failures and the overall integrity of the system: for instance, some options may incorrectly suggest that undetected failures are manageable or do not require rigorous follow-up, which neglects the critical role that constant monitoring and testing play in safety systems.