Machinery Safety 101

Inconsistencies in ISO 13849 – 1:2006

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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