How is the PFDavg calculated for IEC 61511?

Reading time: 3 minutes - Difficulty: advanced
The most common method for calculating PFDavg in a safety chain is to use the Reliability Block Diagram (RBD) approach. Here's what it's all about.

What is Reliability Block Diagram (RDB)

It is nothing more than a diagram that provides the relationship between component states and the success or failure of a given system function.

So let us take a classic safety function consisting of sensor, logic solver and final elements:


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The probability of on demand failure is then calculated by independently considering the PFDavg for the three subsystems, and then adding them:

formula pfd iec 61511


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Calculation of the probability of failure of subsystems

So now the question for us is, how do we calculate the probability of failure of the various subsystems?

Here we are helped by IEC 61508-6 which provides formulas for calculating various configurations.

In fact, the result of the calculation for each individual subsystem depends on:

  • Architecture, and therefore any redundancies
  • The dangerous failure rate of each component
  • The effect of the diagnostic function, if any
  • Common cause failures
  • The frequency and effectiveness of the diagnostic proof test
  • The frequency and effectiveness of the proof test
  • MRT (Mean Repair Time)

It should not be forgotten that the calculation of PFDavg is not the only parameter to be considered in determining the SIL level of a safety function: in fact, architectural constraints and systematic capacity play a very important role.



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