The PLr (required) must be determined by first defining the risk to be mitigated, as indicated in Annex A of standard ISO 13849, applying the Risk Graph method.
What is the Risk Graph method?
Annex A (information) of EN ISO 13849-1 illustrates a method, based on the Risk Graph, for defining the PLr based on the severity of the risk to be mitigated.
The precise risk to be mitigated is point 1 of the graph.
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This method, which is a simplified version of the one of ISO 12100, involves the assessment of 3 factors, namely:
- Severity, called «S», which can be S1 or S2 (where S1 is Light and S2 is Severe)
- The Frequency and/or Exposure to the hazard, referred to as «F» and also divided into F1 or F2 (where F1 is rare/infrequent and with short exposure, while F2 is frequent/continuous with long exposure)
- The possibility of avoiding danger or limiting damage, called «P», which can be P1 or P2 (where P1 is possible, while P2 is hardly possible)
Starting from point 1, the risk assessment will determine which path to take, considering severity, frequency/exposure and the possibility of avoiding danger or damage, up to a very precise required PLr value, usually shown on the right.
It is clear that depending on the “path” a PLr can be obtained that contributes differently to reduction of the risk.
Application according to ISO 13849
For optimal application of the Risk Graph method, it should be remembered that:
- The assessment must be carried out without considering the mitigation measures related to the SRP/CS (but those related to other methods can be considered)
- Each risk scenario, even if potentially connected or generated by the same hazard, must be assessed separately
- This method is advisable but not mandatory; other methods, also derived from the risk analysis according to EN ISO 12100:2010, can be used for determination of the PLr
Finally, there are specific risk scenarios with a predetermined Performance Level. These scenarios are listed in the type C standards.