1. What is a risk graph?

1.1 The principle

The risk graph is used to define the required SIL level based on the evaluation of four parameters:

• the impact of the risk on personnel or the environment (C) ;

• frequency of exposure to risk (F) ;

• the possibility of avoiding the hazard (P);

• the probability of hazard occurrence (W).

The "Calibration examples" table shows assessment criteria for the four parameters (C, F, P and W). These criteria are closely based on those presented in IEC 61508-5.