Inconsistencies in ISO 13849 – 1:2006

This entry is part 7 of 8 in the series Circuit Architectures Explored

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.

ISO 13849-1 Figure 5
ISO 13849 – 1, Figure 5: Relationship between Categories, DC, MTTFd and PL

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.

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Author: Doug Nix

+DougNix is Managing Director and Principal Consultant at Compliance InSight Consulting, Inc. (http://www.complianceinsight.ca) in Kitchener, Ontario, and is Lead Author and Managing Editor of the Machinery Safety 101 blog.

Doug's work includes teaching machinery risk assessment techniques privately and through Conestoga College Institute of Technology and Advanced Learning in Kitchener, Ontario, as well as providing technical services and training programs to clients related to risk assessment, industrial machinery safety, safety-related control system integration and reliability, laser safety and regulatory conformity.

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