Principles, Standards and Implementation

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System Design According to ISO/EN 13849-1:2006

Performance Level (PL)

When the design criteria are evaluated, the SRCS will be assigned a Performance Level. The performance level is a discrete level that specifies the ability of the safety related parts of the control system to perform a safety function.

In order to assess the PL achieved by an implementation of any of the five designated architectures, the following data is required for the system (or subsystem):

• MTTF_d (mean-time-to-dangerous failure of each channel)
  
• DC (diagnostic coverage)
  
• Architecture (the category)
  

Figure 157 shows a graphical method for determining the PL from the combination of these factors. Table 21 shows the tabular results of different Markov models that created the basis of this Figure 157. Refer to the table when more precise determination is needed.



Figure 157: Graphical method to determine PL

The reader will notice there is some overlap at the PL division lines. If MTTF is only provided in categorical terms (as low, medium or high), use Figure 158 to determine the PL.



Figure 158: Simplified graphical method

For example, an application uses the Category 3 architecture. If the DC is between 60% and 90%, and if the MTTF_d of each channel is between 10 and 30 years, then according to Figure 158, PLd is achieved.

Other factors must also be realized to satisfy the required PL. These requirements include the provisions for common cause failures, systematic failure, environmental conditions and mission time.

If the PFH_D of the system or subsystem is known, Table 20 (Annex K of the standard) can be used to derive the PL.