Determining How Unsafe Control Actions Could Occur Step

4 Determining How Unsafe Control Actions Could Occur (Step 2) 

To create causal scenarios, the control structure diagram must include the process models for each component. If the system exists, then the content of these models should be easily determined by looking at the system functional design and its documentation. If the system does not yet exist, the analysis can start with a best guess and then be refined and changed as the analysis process proceeds. 

For the high power interlock example, the process model is simple and shown in figure 8.5. The general causal factors, shown in figure 4.8 and repeated here in figure 8.6 for convenience, are used to identify the scenarios. 

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