Hazard and Risk Analysis
The standard requires that hazard and risk assessment should be carried out: 'The EUC (equipment under control) risk shall be evaluated, or estimated, for each determined hazardous event'.
The standard advises that 'Either qualitative or quantitative hazard and risk analysis techniques may be used' and offers guidance on a number of approaches. One of these, for the qualitative analysis of hazards, is a framework based on 6 categories of likelihood of occurrence and 4 of consequence.
Categories of likelihood of occurrence
| Category | Definition | Range (failures per year) |
|---|---|---|
| Frequent | Many times in system lifetime | > 10−3 |
| Probable | Several times in system lifetime | 10−3 to 10−4 |
| Occasional | Once in system lifetime | 10−4 to 10−5 |
| Remote | Unlikely in system lifetime | 10−5 to 10−6 |
| Improbable | Very unlikely to occur | 10−6 to 10−7 |
| Incredible | Cannot believe that it could occur | < 10−7 |
Consequence categories
| Category | Definition |
|---|---|
| Catastrophic | Multiple loss of life |
| Critical | Loss of a single life |
| Marginal | Major injuries to one or more persons |
| Negligible | Minor injuries at worst |
These are typically combined into a risk class matrix
| Consequence | ||||
| Likelihood | Catastrophic | Critical | Marginal | Negligible |
| Frequent | I | I | I | II |
| Probable | I | I | II | III |
| Occasional | I | II | III | III |
| Remote | II | III | III | IV |
| Improbable | III | III | IV | IV |
| Incredible | IV | IV | IV | IV |
Where:
- Class I: Unacceptable in any circumstance;
- Class II: Undesirable: tolerable only if risk reduction is impracticable or if the costs are grossly disproportionate to the improvement gained;
- Class III: Tolerable if the cost of risk reduction would exceed the improvement;
- Class IV: Acceptable as it stands, though it may need to be monitored.
Read more about this topic: IEC 61508
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