EN ISO 13849-1, the quantitative approach to machine safety begins with a qualitative process
EN ISO 13849-1 has now been fully in force since Jan. 1, 2012, without exception and EN 954 has fully been withdrawn. We’ve heard for four years or more that this change in direction equates to a paradigm shift for industry because it in part moves hazard level determination and mitigation from a qualitative process to a quantitative process. There’s more to this story!
It’s true that the Performance Level (PL) system of EN ISO 13849-1 is quantitative based at the completion of the process. I’ve read that the “fathers” of this standard determined that a quantitative approach was needed to both have a direct comparison to the Safety Integrity Level (SIL) system for hazards and to provide the basis for the validation requirements of EN ISO 13849-2. With this system engineers can now calculate the performance level of the safety related parts of the control system (SRP/CS) for each selected safety function. However, this entire process begins (in part) by determining a required performance level (PLr) for each safety function using a qualitative process. In fact, let’s look at the actual 4.3 clause of EN ISO 13849-1 below:
4.3 Determination of required performance level (PLr)
For each selected safety function to be carried out by a SRP/CS, a required performance level (PLr) shall be determined and documented (see Annex A for guidance on determining PLr). The determination of the required performance level is the result of the risk assessment and refers to the amount of the risk reduction to be carried out by the safety-related parts of the control system (see Figure 2).
The greater the amount of risk reduction required to be provided by the SRP/CS, the higher the PLr shall be.
In Annex A you will find two pages that guides you through the qualitative process of asking three questions; severity of injury, frequency of exposure, and possibility of avoidance. Doesn’t this sound eerily like the three qualitative questions used to determine the hazard Categories of B, 1, 2, 3, and 4? The risk graph below looks very similar to the Category graph with the exception of Performance Levels a, b, c, d, and e. The standard goes on to say that the Performance Level (PL) (calculated) must be equal or greater than the PLr.
So, for everyone who has already made the transition to EN ISO 13849-1 – please let us know your thoughts on how much the “fathers” have gained whereas it seems the basis can be argued we are still in a qualitative world of hazards!
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