| 安全失效分数(SFF) | 硬件故障裕度要求(FTmin) | ||
| 0 | 1 | 2 | |
| 60% | 不允许 | SIL1 | SIL2 |
| 60%~90% | SIL1 | SIL2 | SIL3 |
| 90%~99% | SIL2 | SIL3 | SIL4 |
| 99% | SIL3 | SIL4 | SIL4 |