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. That concept is mathematically shown 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 stuck partially open not detected until functional testing. This would be dangerous or safe undetected, but likely a dangerous undetected.
See also: failure rate, detected failure
Cited Source:
- IEC 61508-4:2010, Clause 3.6.5.