ISO 13849 – 1 Analysis — Part 6: CCF — Common Cause Failures

This entry is part 6 of 9 in the series How to do a 13849 – 1 analysis

What is a Common Cause Failure?

There are two similar-​sounding terms that people often get con­fused: Common Cause Failure (CCF) and Common Mode Failure. While these two types of fail­ures sound sim­il­ar, they are dif­fer­ent. A Common Cause Failure is a fail­ure in a sys­tem where two or more por­tions of the sys­tem fail at the same time from a single com­mon cause. An example could be a light­ning strike that causes a con­tact­or to weld and sim­ul­tan­eously takes out the safety relay pro­cessor that con­trols the con­tact­or. Common cause fail­ures are there­fore two dif­fer­ent man­ners of fail­ure in two dif­fer­ent com­pon­ents, but with a single cause.

Common Mode Failure is where two com­pon­ents or por­tions of a sys­tem fail in the same way, at the same time. For example, two inter­pos­ing relays both fail with wel­ded con­tacts at the same time. The fail­ures could be caused by the same cause or from dif­fer­ent causes, but the way the com­pon­ents fail is the same.

Common-​cause fail­ure includes com­mon mode fail­ure, since a com­mon cause can res­ult in a com­mon man­ner of fail­ure in identic­al devices used in a system.

Here are the form­al defin­i­tions of these terms:

3.1.6 com­mon cause fail­ure CCF

fail­ures of dif­fer­ent items, res­ult­ing from a single event, where these fail­ures are not con­sequences of each other

Note 1 to entry: Common cause fail­ures should not be con­fused with com­mon mode fail­ures (see ISO 12100:2010, 3.36). [SOURCE: IEC 60050?191-am1:1999, 04 – 23.] [1]


3.36 com­mon mode failures

fail­ures of items char­ac­ter­ized by the same fault mode

NOTE Common mode fail­ures should not be con­fused with com­mon cause fail­ures, as the com­mon mode fail­ures can res­ult from dif­fer­ent causes. [lEV 191 – 04-​24] [3]

The “com­mon mode” fail­ure defin­i­tion uses the phrase “fault mode”, so let’s look at that as well:

fail­ure mode
DEPRECATED: fault mode
man­ner in which fail­ure occurs

Note 1 to entry: A fail­ure mode may be defined by the func­tion lost or oth­er state trans­ition that occurred. [IEV 192 – 03-​17] [17]

As you can see, “fault mode” is no longer used, in favour of the more com­mon “fail­ure mode”, so it is pos­sible to re-​write the common-​mode fail­ure defin­i­tion to read, “fail­ures of items char­ac­ter­ised by the same man­ner of failure.”

Random, Systematic and Common Cause Failures

Why do we need to care about this? There are three man­ners in which fail­ures occur: ran­dom fail­ures, sys­tem­at­ic fail­ures, and com­mon cause fail­ures. When devel­op­ing safety related con­trols, we need to con­sider all three and mit­ig­ate them as much as possible.

Random fail­ures do not fol­low any pat­tern, occur­ring ran­domly over time, and are often brought on by over-​stressing the com­pon­ent, or from man­u­fac­tur­ing flaws. Random fail­ures can increase due to envir­on­ment­al or process-​related stresses, like cor­ro­sion, EMI, nor­mal wear-​and-​tear, or oth­er over-​stressing of the com­pon­ent or sub­sys­tem. Random fail­ures are often mit­ig­ated through selec­tion of high-​reliability com­pon­ents [18].

Systematic fail­ures include common-​cause fail­ures, and occur because some human beha­viour occurred that was not caught by pro­ced­ur­al means. These fail­ures are due to design, spe­cific­a­tion, oper­at­ing, main­ten­ance, and install­a­tion errors. When we look at sys­tem­at­ic errors, we are look­ing for things like train­ing of the sys­tem design­ers, or qual­ity assur­ance pro­ced­ures used to val­id­ate the way the sys­tem oper­ates. Systematic fail­ures are non-​random and com­plex, mak­ing them dif­fi­cult to ana­lyse stat­ist­ic­ally. Systematic errors are a sig­ni­fic­ant source of common-​cause fail­ures because they can affect redund­ant devices, and because they are often determ­in­ist­ic, occur­ring whenev­er a set of cir­cum­stances exist.

Systematic fail­ures include many types of errors, such as:

  • Manufacturing defects, e.g., soft­ware and hard­ware errors built into the device by the manufacturer.
  • Specification mis­takes, e.g. incor­rect design basis and inac­cur­ate soft­ware specification.
  • Implementation errors, e.g., improp­er install­a­tion, incor­rect pro­gram­ming, inter­face prob­lems, and not fol­low­ing the safety manu­al for the devices used to real­ise the safety function.
  • Operation and main­ten­ance, e.g., poor inspec­tion, incom­plete test­ing and improp­er bypassing [18].

Diverse redund­ancy is com­monly used to mit­ig­ate sys­tem­at­ic fail­ures, since dif­fer­ences in com­pon­ent or sub­sys­tem design tend to cre­ate non-​overlapping sys­tem­at­ic fail­ures, redu­cing the like­li­hood of a com­mon error cre­at­ing a common-​mode fail­ure. Errors in spe­cific­a­tion, imple­ment­a­tion, oper­a­tion and main­ten­ance are not affected by diversity.

Fig 1 below shows the res­ults of a small study done by the UK’s Health and Safety Executive in 1994 [19] that sup­ports the idea that sys­tem­at­ic fail­ures are a sig­ni­fic­ant con­trib­ut­or to safety sys­tem fail­ures. The study included only 34 sys­tems (n=34), so the res­ults can­not be con­sidered con­clus­ive. However, there were some start­ling res­ults. As you can see, errors in the spe­cific­a­tion of the safety func­tions (Safety Requirement Specification) res­ul­ted in about 44% of the sys­tem fail­ures in the study. Based on this small sample, sys­tem­at­ic fail­ures appear to be a sig­ni­fic­ate source of failures.

Pie chart illustrating the proportion of failures in each phase of the life cycle of a machine, based on data taken from HSE Report HSG238.
Figure 1 – HSG 238 Primary Causes of Failure by Life Cycle Stage

Handling CCF in ISO 13849 – 1

Now that we under­stand WHAT Common-​Cause Failure is, and WHY it’s import­ant, we can talk about HOW it is handled in ISO 13849 – 1. Since ISO 13849 – 1 is inten­ded to be a sim­pli­fied func­tion­al safety stand­ard, CCF ana­lys­is is lim­ited to a check­list in Annex F, Table F.1. Note that Annex F is inform­at­ive, mean­ing that it is guid­ance mater­i­al to help you apply the stand­ard. Since this is the case, you could use any oth­er means suit­able for assess­ing CCF mit­ig­a­tion, like those in IEC 61508, or in oth­er standards.

Table F.1 is set up with a series of mit­ig­a­tion meas­ures which are grouped togeth­er in related cat­egor­ies. Each group is provided with a score that can be claimed if you have imple­men­ted the mit­ig­a­tions in that group. ALL OF THE MEASURES in each group must be ful­filled in order to claim the points for that cat­egory. Here’s an example:

A portion of ISO 13849-1 Table F.1.
ISO 13849 – 1:2015, Table F.1 Excerpt

In order to claim the 20 points avail­able for the use of sep­ar­a­tion or segreg­a­tion in the sys­tem design, there must be a sep­ar­a­tion between the sig­nal paths. Several examples of this are giv­en for clarity.

Table F.1 lists six groups of mit­ig­a­tion meas­ures. In order to claim adequate CCF mit­ig­a­tion, a min­im­um score of 65 points must be achieved. Only Category 2, 3 and 4 archi­tec­tures are required to meet the CCF require­ments in order to claim the PL, but without meet­ing the CCF require­ment you can­not claim the PL, regard­less of wheth­er the design meets the oth­er cri­ter­ia or not.

One final note on CCF: If you are try­ing to review an exist­ing con­trol sys­tem, say in an exist­ing machine, or in a machine designed by a third party where you have no way to determ­ine the exper­i­ence and train­ing of the design­ers or the cap­ab­il­ity of the company’s change man­age­ment pro­cess, then you can­not adequately assess CCF [8]. This fact is recog­nised in CSA Z432-​16 [20], chapter 8. [20] allows the review­er to simply veri­fy that the archi­tec­tur­al require­ments, exclus­ive of any prob­ab­il­ist­ic require­ments, have been met. This is par­tic­u­larly use­ful for engin­eers review­ing machinery under Ontario’s Pre-​Start Health and Safety require­ments [21], who are fre­quently work­ing with less-​than-​complete design documentation.

In case you missed the first part of the series, you can read it here. In the next art­icle in this series, I’m going to review the pro­cess flow for sys­tem ana­lys­is as cur­rently out­lined in ISO 13849 – 1. Watch for it!

Book List

Here are some books that I think you may find help­ful on this journey:

[0]     B. Main, Risk Assessment: Basics and Benchmarks, 1st ed. Ann Arbor, MI USA: DSE, 2004.

[0.1]  D. Smith and K. Simpson, Safety crit­ic­al sys­tems hand­book. Amsterdam: Elsevier/​Butterworth-​Heinemann, 2011.

[0.2]  Electromagnetic Compatibility for Functional Safety, 1st ed. Stevenage, UK: The Institution of Engineering and Technology, 2008.

[0.3]  Overview of tech­niques and meas­ures related to EMC for Functional Safety, 1st ed. Stevenage, UK: Overview of tech­niques and meas­ures related to EMC for Functional Safety, 2013.


Note: This ref­er­ence list starts in Part 1 of the series, so “miss­ing” ref­er­ences may show in oth­er parts of the series. The com­plete ref­er­ence list is included in the last post of the series.

[1]     Safety of machinery — Safety-​related parts of con­trol sys­tems — Part 1: General prin­ciples for design. 3rd Edition. ISO Standard 13849 – 1. 2015.

[2]     Safety of machinery – Safety-​related parts of con­trol sys­tems – Part 2: Validation. 2nd Edition. ISO Standard 13849 – 2. 2012.

[3]      Safety of machinery – General prin­ciples for design – Risk assess­ment and risk reduc­tion. ISO Standard 12100. 2010.

[8]     S. Jocelyn, J. Baudoin, Y. Chinniah, and P. Charpentier, “Feasibility study and uncer­tain­ties in the val­id­a­tion of an exist­ing safety-​related con­trol cir­cuit with the ISO 13849 – 1:2006 design stand­ard,” Reliab. Eng. Syst. Saf., vol. 121, pp. 104 – 112, Jan. 2014.

[17]      “fail­ure mode”, 192 – 03-​17, International Electrotechnical Vocabulary. IEC International Electrotechnical Commission, Geneva, 2015.

[18]      M. Gentile and A. E. Summers, “Common Cause Failure: How Do You Manage Them?,” Process Saf. Prog., vol. 25, no. 4, pp. 331 – 338, 2006.

[19]     Out of Control — Why con­trol sys­tems go wrong and how to pre­vent fail­ure, 2nd ed. Richmond, Surrey, UK: HSE Health and Safety Executive, 2003.

[20]     Safeguarding of Machinery. 3rd Edition. CSA Standard Z432. 2016.

[21]     O. Reg. 851, INDUSTRIAL ESTABLISHMENTS. Ontario, Canada, 1990.

Series NavigationISO 13849 – 1 Analysis — Part 5: Diagnostic Coverage (DC)ISO 13849 – 1 Analysis — Part 7: Safety-​​Related Software

Author: Doug Nix

+DougNix is Managing Director and Principal Consultant at Compliance InSight Consulting, Inc. ( 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. Follow me on