| Safety Integrity Level | Low Demand Mode of Operation (Probability of failure to perform safety functions on demand) | Continuous/High-demand mode of operation (Probability of dangerous failure per hour) |
| 4 | >= 10^{-5} to 10^{-4} | >= 10^{-9} to 10^{-8} |
| 3 | >= 10^{-4} to 10^{-3} | >= 10^{-8} to 10^{-7} |
| 2 | >= 10^{-3} to 10^{-2} | >= 10^{-7} to 10^{-6} |
| 1 | >= 10^{-2} to 10^{-1} | >= 10^{-6} to 10^{-5} |