Definition:
Spurious Trip Rate (STR) quantifies the frequency at which a safety function incorrectly trips or shuts down the process without an actual hazardous condition. Think of it as a nuisance trip that can take the whole process down. This concept relates to operational availability and less on actual safety.
STR is a rate, thus the units are spurious trip / time. SIL Safe keeps everything per hour by convention. Thus an STR could be 1.4E-6/hr. HOWEVER, this is hard for the brain to process, so the inverse of that and converting from hour to year is always communicated as well, which is MTTFsp. Here this would be 82 years.
Balancing PDFavg and STR is one of the core roles of the Functional Safety Engineer.
There is also a much more in depth blog article here discussing STR.
Key Points:
- Low STR improves operational availability.
- High STR can cause costly interruptions.
- Balancing PFDavg vs STR by using differing architectures is one of the roles of the Functional Safety Engineer.
- MTTFSP is the inverse of STR
Example:
A SIF with a spurious trip rate of once every 6 months is causing operational challenges. The facility makes a design change to update the architecture to lower MTTFSP to 2 year.
See also: Mean Time Between Failures (MTBF), PFD, MTTFsp, STR Blog article
Cited Source:
- IEC 61508-4:2010.