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.

Series Nav­i­ga­tionInter­lock Archi­tec­tures Pt. 6 — Com­par­ing North Amer­i­can and Inter­na­tion­al Sys­temsYOU ready?”>31-Dec-2011 — Are YOU ready?

Author: Doug Nix

Doug Nix is Managing Director and Principal Consultant at Compliance InSight Consulting, Inc. ( in Kitchener, Ontario, and is Lead Author and Senior 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. For more see Doug's LinkedIn profile.