Definition:
A Detected Failure is a failure mode that is automatically revealed by the system’s diagnostics. You can have dangerous-detected (λDD) and safe detected (λSD).
As simple of a concept this is, it is still often confused. Detected does NOT mean a failure that is detected in a proof test. That concept is mathematically shown as proof test coverage (1-Cpt).
How a detected failure is communicated matters: a field device often signals one by driving its 4-20 mA output out of range per NAMUR recommendation NE 43, so the logic solver recognizes the fault instead of acting on a bad reading.
Key Points:
- Major contributor to PFD calculations.
- Detected failures could trigger various actions depending on the facility design of the particular SIF and SRS.
Example:
A SC 2 HART enabled pressure transmitter has automatic diagnostics. It detected a plugged impulse line. This would almost certainly be a dangerous detected failure.
See Also: failure rate, undetected failure
Cited Source:
- IEC 61511-1:2016, Clause 3.2.13