| Characteristics | PO/YEAR | Probability | Classe |
| Theoretically possible but unlikely to occure throughout the life of the installation | <10−4 | Extremely remote | A |
| Occurrence not expected over the lifetime of the facility | 10−3 a 10−4 | Remote | B |
| Low probability over the life of the installation | 10−2 a 10−3 | Unlikely | C |
| Expected occurrence once or twice throughout the life of the installation | 10−1 a 10−2 | Likely | D |
| Expected occurrence once or twice every 10 years | 100 a 10−1 | Frequent | E |
| Expected occurrence once or twice in each 1 year | >100 | Very frequent | F |
| Expected occurrence once or twice each month | >101 | Routine | G |