Determination of the required PLr according to ISO 13849

Reading time: 4 minutes - Difficulty: Medium
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.

Do you want to help our page grow?

Follow us on Linkedin

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.