STL Levels
The STL level achieved by a safety function is determined by the probability of fail safe (PFS) of this safety function. The PFS value is determined by internal failures of the safety system that cause the safety function to be executed without a demand from the process. The table below demonstrates the PFS value and spurious trip reduction values of each STL level.
| STL level | PFSavg | STR |
|---|---|---|
| X | ≥10-(X+1) to <10-X | 10X |
| ... | ... | ... |
| 5 | ≥10−6 to <10−5 | 100000 |
| 4 | ≥10−5 to <10−4 | 10000 |
| 3 | ≥10−4 to <10−3 | 1000 |
| 2 | ≥10−3 to <10−2 | 100 |
| 1 | ≥10−2 to <10−1 | 10 |
Read more about this topic: Spurious Trip Level
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