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Posted 2017-02-27 2018-10-27 Doug Nix
Under­stand­ing Dia­gnost­ic Cov­er­age (DC) as it is used in ISO 13849 – 1 [1] is crit­ic­al to ana­lys­ing the design of any safety func­tion assessed using this stand­ard. In case you missed a pre­vi­ous part of the series, you can read it here.
In the last instal­ment of this series dis­cuss­ing MTTFD, I brought up the fact that everything fails even­tu­ally, and so everything has a nat­ur­al fail­ure rate. The bathtub curve shown at the top of this post shows a typ­ic­al fail­ure rate curve for most products. Fail­ure rates tell you the aver­age time (or some­times the mean time) it takes for com­pon­ents or sys­tems to fail. Fail­ure rates are expressed in many ways, MTTFD and PFHd being the ways rel­ev­ant to this dis­cus­sion of ISO 13849 ana­lys­is. MTTFD is giv­en in years, and PFHd is giv­en in frac­tion­al hours (1/h). As a remind­er, PFHd stands for “Prob­ab­il­ity of dan­ger­ous Fail­ure per Hour”.
Three of the stand­ard archi­tec­tures include auto­mat­ic dia­gnost­ic func­tions, Cat­egor­ies 2, 3 and 4. As soon as we add dia­gnostics to the sys­tem, we need to know what faults the dia­gnostics can detect and how many of the dan­ger­ous fail­ures rel­at­ive to the total num­ber of fail­ures that rep­res­ents. Dia­gnost­ic Cov­er­age (DC) rep­res­ents the ratio of dan­ger­ous fail­ures that can be detec­ted to the total dan­ger­ous fail­ures that could occur, expressed as a per­cent­age. There will be some fail­ures that do not res­ult in a dan­ger­ous fail­ure, and those fail­ures are excluded from DC because we don’t need to worry about them – if they occur, the sys­tem will not fail into a dan­ger­ous state.
Note 1 to entry: Dia­gnost­ic cov­er­age can exist for the whole or parts of a safety-related sys­tem. For example, dia­gnost­ic cov­er­age could exist for sensors and/or logic sys­tem and/or final ele­ments. [SOURCE: IEC 61508 – 4:1998, 3.8.6, mod­i­fied.]
Note 4 to entry: Fail­ures which only affect the avail­ab­il­ity of the pro­cess under con­trol are out­side of the scope of this part of ISO 13849. [SOURCE: IEC 60050 – 191:1990, 04 – 01.]
Note 1 to entry: Wheth­er or not the poten­tial is real­ized can depend on the chan­nel archi­tec­ture of the sys­tem; in redund­ant sys­tems a dan­ger­ous hard­ware fail­ure is less likely to lead to the over­all dan­ger­ous or fail-to- func­tion state.
Just as a remind­er, SRP/CS stands for “safety-related parts of con­trol sys­tems”.
To do any cal­cu­la­tions, we need data, and this is true for fail­ure rates as well. ISO 13849 – 1 provides some tables in the annexes that list some com­mon types of com­pon­ents and their asso­ci­ated fail­ure rates, and there are more fail­ure rate tables in ISO 13849 – 2. A word of cau­tion here: Do not mix sources of fail­ure rate data, as the con­di­tions under which that data is true won’t match the data in ISO 13849. There are a few good sources of fail­ure rate data out there, for example, MIL-HDBK-217, Reli­ab­il­ity Pre­dic­tion of Elec­tron­ic Equip­ment [15], as well as the data­base main­tained by Exida. In any case, use a single source for your fail­ure rate data.
IEC 61508 [7] defines a num­ber of vari­ables related to fail­ure rates. The lower­case Greek let­ter lambda,
, is used to denote fail­ures.
Of these vari­ables, we only need to con­cern ourselves with
. To under­stand how these vari­ables are used, we can express their rela­tion­ship as
Fol­low­ing on that idea, the Dia­gnost­ic Cov­er­age can be expressed as a per­cent­age like this:
If you want to actu­ally cal­cu­late DC%, you have some work ahead of you. Rather than going into the details here, I am going to refer you hard­core types to IEC 61508 – 2, Func­tion­al safety of electrical/electronic/programmable elec­tron­ic safety-related sys­tems – Part 2: Require­ments for electrical/electronic/programmable elec­tron­ic safety-related sys­tems. This stand­ard goes into some depth on how to determ­ine fail­ure rates and how to cal­cu­late the “Safe Fail­ure Frac­tion,” a num­ber which is related to DC but is not the same.
For every­one else, the good news is that you can use the table in Annex E to estim­ate the DC%. It’s worth not­ing here that Annex E is “Inform­at­ive.” In stand­ards-speak, this means that the inform­a­tion in the annex is not part of the “norm­at­ive” text, which means that it is simply inform­a­tion to help you use the norm­at­ive part of the stand­ard. The design must con­form to the require­ments in the norm­at­ive text if you want to claim con­form­ity to the stand­ard. The fact that [1, Annex E] is inform­at­ive gives you the option to cal­cu­late the DC% value rather than select­ing it from Table E.1. Using the cal­cu­lated value would not viol­ate the require­ments in the norm­at­ive text.
ISO 13849 – 1, Table 5 Dia­gnost­ic cov­er­age (DC)
When you have mul­tiple safety func­tions that make up a com­plete safety sys­tem, for example, an emer­gency stop func­tion and a guard inter­lock­ing func­tion, the DC val­ues need to be aver­aged to determ­ine the over­all DC for the com­plete sys­tem. [1, Annex E] provides you with a meth­od to do this in Equa­tion E.1.
ISO 13849 – 1‑2015 Equa­tion E.1
That’s it for this art­icle. The next part will cov­er Com­mon Cause Fail­ures (CCF). Look for it on 20-Mar-17!
[0.1] D. Smith and K. Simpson, Safety crit­ic­al sys­tems hand­book, 3rd Ed. Ams­ter­dam: Elsevi­er­/But­ter­worth-Heine­mann, 2011.
Acknow­ledge­ments: IEC and ISO as cited
Series Nav­ig­a­tion ISO 13849 – 1 Ana­lys­is — Part 4: MTTFD – Mean Time to Dan­ger­ous Fail­ure”>ISO 13849 – 1 Ana­lys­is — Part 4: MTTFD – Mean Time to Dan­ger­ous Fail­ureISO 13849 – 1 Ana­lys­is — Part 6: CCF — Com­mon Cause Fail­ures”>ISO 13849 – 1 Ana­lys­is — Part 6: CCF — Com­mon Cause Fail­ures
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ISO 13849 – 1 Analysis — Part 6: CCF — Common Cause Failures⟶
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