Definition:
Meantime to fail spurious (MTTFSP or MTFS), or mean time to trip spurious (MTTTSP) are all one term that relates to spurious trip rate (STR). STR is the core mathematical term that is always calculated, but it is hard for the human brain to process what that really means.
What people want to know is what is the average time between spurious trips. And that is exactly what MTTFSP is. Mathematically it’s the inverse of STR and you also have to convert your time unit in STR to years.
At times, a program may only use STR and not MTTFSP, which if fine. However, SIL Safe prefers using both.
Balancing PDFavg and STR is one of the core roles of the Functional Safety Engineer.
Key Points:
- MTTFSP is the inverse of STR, and is a easier way to think of the same concept.
- Balancing PFDavg vs STR by using differing architectures is one of the roles of the Functional Safety Engineer.
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: STR, PFD, STR blog article
Cited Source:
- IEC 61508-4:2010.