I’ve written quite a bit recently on the topic of circuit architectures under ISO 13849–1, and one of my readers noticed an inconsistency between the text of the standard and Figure 5, the diagram that shows how the categories can span one or more Performance Levels.
If you look at Category 2 in Figure 5, you will notice that there are TWO bands, one for DCavg LOW and one for DCavg MED. However, reading the text of the definition for Category 2 gives (§6.2.5):
The diagnostic coverage (DCavg) of the total SRP/CS including fault-detection shall be low.
This leaves some confusion, because it appears from the diagram that there are two options for this architecture. This is backed up by the data in Annex K that underlies the diagram.
The same confusion exists in the text describing Category 3, with Figure 5 showing two bands, one for DCavg LOW and one for DCavg MED.
I contacted the ISO TC199 Secretariat, the people responsible for the content of ISO 13849–1, and pointed out this apparent conflict. They responded that they would pass the comment on to the TC for resolution, and would contact me if they needed additional information. As of this writing, I have not heard more.
So what should you do if you are trying to design to this standard? My advice is to follow Figure 5. If you can achieve a DCavg MED in your design, it is completely reasonable to claim a higher PL. Refer to the data in Annex K to see where your design falls once you have completed the MTTFd calculations.
Thanks to Richard Harris and Douglas Florence, both members of the ISO 13849 and IEC 62061 Group on LinkedIn for bringing this to my attention!
If you are interested in contacting the TC199 Secretariat, you can email the Secretary, Mr. Stephen Kennedy. More details on ISO TC199 can be found on the Technical Committee page on the ISO web Site.