Which term describes the capability of a safety function to detect faults within a system?

• A. System availability.
• B. Mean time between failures.
• C. Redundancy effectiveness.
• D. Diagnostic coverage.

Answer: (D)

Diagnostic coverage measures how well the safety function's diagnostics detect faults inside the system. It expresses the extent to which built-in self-checks, tests, and monitoring can reveal faults that could prevent the safety function from operating safely. A higher diagnostic coverage means more potential faults are caught, reducing the risk of a dangerous undetected fault and allowing the system to switch to a safe state when needed. This concept is distinct from system availability (uptime), mean time between failures (reliability), and redundancy effectiveness (how well duplicates protect against failure without necessarily reflecting fault detection capability).