Definition:
An Undetected Failure is a failure that is not automatically revealed by the system’s diagnostics. You can have dangerous-undetected (λDU) and safe undetected (λSU).
The “dangerous” means that the protective feature may be compromised (not dangerous in the sense that it causes a occupational safety risk). These λDU are the most concerning as these will not be caught until the next proof test and the SIF is non operable.
As simple of a concept this is, it is still often confused. Undetected does NOT mean a failure that is undetected in a proof test. This concept is mathematically represented as proof test coverage (Cpt).
Key Points:
- Major contributor to PFD calculations.
- λDU requires proof testing to detect and is the most dangerous condition.
Example:
A shutdown valve with no diagnostics is stuck partially open. It is not detected until functional testing. This would be an “undetected failure”. It could be safe or dangerous, but likely dangerous as it is a valve.
See Also: failure rate, detected failure
Cited Source:
- IEC 61508-4:2010, Clause 3.6.5.