ISO 13849 – 1 Analysis — Part 8: Fault Exclusion

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

Fault Consideration & Fault Exclusion

ISO 13849 – 1, Chapter 7 [1, 7] dis­cusses the need for fault con­sid­er­a­tion and fault exclu­sion. Fault con­sid­er­a­tion is the pro­cess of examin­ing the com­pon­ents and sub-​systems used in the safety-​related part of the con­trol sys­tem (SRP/​CS) and mak­ing a list of all the faults that could occur in each one. This a def­in­itely non-​trivial exer­cise!

Thinking back to some of the earli­er art­icles in this series where I men­tioned the dif­fer­ent types of faults, you may recall that there are detect­able and undetect­able faults, and there are safe and dan­ger­ous faults, lead­ing us to four kinds of fault:

  • Safe undetect­able faults
  • Dangerous undetect­able faults
  • Safe detect­able faults
  • Dangerous undetect­able faults

For sys­tems where no dia­gnostics are used, Category B and 1, faults need to be elim­in­ated using inher­ently safe design tech­niques. Care needs to be taken when clas­si­fy­ing com­pon­ents as “well-​tried” versus using a fault exclu­sion, as com­pon­ents that might nor­mally be con­sidered “well-​tried” might not meet those require­ments in every applic­a­tion.

For sys­tems where dia­gnostics are part of the design, i.e., Category 2, 3, and 4, the fault lists are used to eval­u­ate the dia­gnost­ic cov­er­age (DC) of the test sys­tems. Depending on the archi­tec­ture, cer­tain levels of DC are required to meet the rel­ev­ant PL, see [1, Fig. 5]. The fault lists are start­ing point for the determ­in­a­tion of DC, and are an input into the hard­ware and soft­ware designs. All of the dan­ger­ous detect­able faults must be covered by the dia­gnostics, and the DC must be high enough to meet the PLr. for the safety func­tion.

The fault lists and fault exclu­sions are used in the Validation por­tion of this pro­cess as well. At the start of the Validation pro­cess flow chart [2, Fig. 1], you can see how the fault lists and the cri­ter­ia used for fault exclu­sion are used as inputs to the val­id­a­tion plan.

The diagram shows the first few stages in the ISO 13849-2 Validation process. See ISO 13849-2, Figure 1.
Start of ISO 13849 – 2 Fig. 1

Faults that can be excluded do not need to val­id­ated, sav­ing time and effort dur­ing the sys­tem veri­fic­a­tion and val­id­a­tion (V & V). How is this done?

Fault Consideration

The first step is to devel­op a list of poten­tial faults that could occur, based on the com­pon­ents and sub­sys­tems included in SRP/​CS. ISO 13849 – 2 [2] includes lists of typ­ic­al faults for vari­ous tech­no­lo­gies. For example, [2, Table A.4] is the fault list for mech­an­ic­al com­pon­ents.

Mechanical fault list from ISO 13849-2
Table A.4 — Faults and fault exclu­sions — Mechanical devices, com­pon­ents and ele­ments
(e.g. cam, fol­low­er, chain, clutch, brake, shaft, screw, pin, guide, bear­ing)

[2] con­tains tables sim­il­ar to Table A.4 for:

  • Pressure-​coil springs
  • Directional con­trol valves
  • Stop (shut-​off) valves/​non-​return (check) valves/​quick-​action vent­ing valves/​shuttle valves, etc.
  • Flow valves
  • Pressure valves
  • Pipework
  • Hose assem­blies
  • Connectors
  • Pressure trans­mit­ters and pres­sure medi­um trans­ducers
  • Compressed air treat­ment — Filters
  • Compressed-​air treat­ment — Oilers
  • Compressed air treat­ment — Silencers
  • Accumulators and pres­sure ves­sels
  • Sensors
  • Fluidic Information pro­cessing — Logical ele­ments
  • etc.

As you can see, there are many dif­fer­ent types of faults that need to be con­sidered. Keep in mind that I did not give you all of the dif­fer­ent fault lists – this post would be a mile long if I did that! The point is that you need to devel­op a fault list for your sys­tem, and then con­sider the impact of each fault on the oper­a­tion of the sys­tem. If you have com­pon­ents or sub­sys­tems that are not lis­ted in the tables, then you need to devel­op your own fault lists for those items. Using Failure Modes and Effects Analysis (FMEA) tech­niques are usu­ally the best approach for these com­pon­ents [23], [24].

When con­sid­er­ing the faults to be included in the list there are a few things that should be con­sidered [1, 7.2]:

  • if after the first fault occurs oth­er faults devel­op due to the first fault, then you can group those faults togeth­er as a single fault
  • two or more single faults with a com­mon cause can be con­sidered as a single fault
  • mul­tiple faults with dif­fer­ent causes but occur­ring sim­ul­tan­eously is con­sidered improb­able and does not need to be con­sidered


A voltage reg­u­lat­or fails in a sys­tem power sup­ply so that the 24 Vdc out­put rises to an unreg­u­lated 36 Vdc (the intern­al power sup­ply bus voltage), and after some time has passed, two sensors fail, then all three fail­ures can be grouped and con­sidered as a single fault.

If a light­ning strike occurs on the power line and the res­ult­ing surge voltage on the 400 V mains causes an inter­pos­ing con­tact­or and the motor drive it con­trols to fail to danger, then these fail­ures may be grouped and con­sidered as one.

A pneu­mat­ic lub­ric­at­or runs out of lub­ric­ant and is not refilled, depriving down­stream pneu­mat­ic com­pon­ents of lub­ric­a­tion. The spool on the sys­tem dump valve sticks open because it is not cycled often enough. Neither of these fail­ures has the same cause, so there is no need to con­sider them as occur­ring sim­ul­tan­eously because the prob­ab­il­ity of both hap­pen­ing con­cur­rently is extremely small. One cau­tion: These two faults MAY have a com­mon cause – poor main­ten­ance. Even if this is true and you decide to con­sider them to be two faults with a com­mon cause, they could then be grouped as a single fault.

Fault Exclusion

Once you have your well-​considered fault lists togeth­er, the next ques­tion is “Can any of the lis­ted faults be excluded?” This is a tricky ques­tion! There are a few points to con­sider:

  • Does the sys­tem archi­tec­ture allow for fault exclu­sion?
  • Is the fault tech­nic­ally improb­able, even if it is pos­sible?
  • Does exper­i­ence show that the fault is unlikely to occur?*
  • Are there tech­nic­al require­ments related to the applic­a­tion and the haz­ard that might sup­port fault exclu­sion?

BE CAREFUL with this one!

Whenever faults are excluded, a detailed jus­ti­fic­a­tion for the exclu­sion needs to be included in the sys­tem design doc­u­ment­a­tion. Simply decid­ing that the fault can be excluded is NOT ENOUGH! Consider the risk a per­son will be exposed to in the event the fault occurs. If the sever­ity is very high, i.e., severe per­man­ent injury or death, you may not want to exclude the fault even if you think you could. Careful con­sid­er­a­tion of the res­ult­ing injury scen­ario is needed.

Basing a fault exclu­sion on per­son­al exper­i­ence is sel­dom con­sidered adequate, which is why I added the aster­isk (*) above. Look for good stat­ist­ic­al data to sup­port any decision to use a fault exclu­sion.

There is much more inform­a­tion avail­able in IEC 61508 – 2 on the sub­ject of fault exclu­sion, and there is good inform­a­tion in some of the books men­tioned below [0.2], [0.3], and [0.4]. If you know of addi­tion­al resources you would like to share, please post the inform­a­tion in the com­ments!


3.1.3 fault
state of an item char­ac­ter­ized by the inab­il­ity to per­form a required func­tion, exclud­ing the inab­il­ity dur­ing pre­vent­ive main­ten­ance or oth­er planned actions, or due to lack of extern­al resources
Note 1 to entry: A fault is often the res­ult of a fail­ure of the item itself, but may exist without pri­or fail­ure.
Note 2 to entry: In this part of ISO 13849, “fault” means ran­dom fault. [SOURCE: IEC 60050?191:1990, 05 – 01.]

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. Included in the last post of the series is the com­plete ref­er­ence list.

[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.

[4]     Safeguarding of Machinery. 2nd Edition. CSA Standard Z432. 2004.

[5]     Risk Assessment and Risk Reduction- A Guideline to Estimate, Evaluate and Reduce Risks Associated with Machine Tools. ANSI Technical Report B11.TR3. 2000.

[6]    Safety of machinery – Emergency stop func­tion – Principles for design. ISO Standard 13850. 2015.

[7]     Functional safety of electrical/​electronic/​programmable elec­tron­ic safety-​related sys­tems. 7 parts. IEC Standard 61508. Edition 2. 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.

[9]    Guidance on the applic­a­tion of ISO 13849 – 1 and IEC 62061 in the design of safety-​related con­trol sys­tems for machinery. IEC Technical Report TR 62061 – 1. 2010.

[10]     Safety of machinery – Functional safety of safety-​related elec­tric­al, elec­tron­ic and pro­gram­mable elec­tron­ic con­trol sys­tems. IEC Standard 62061. 2005.

[11]    Guidance on the applic­a­tion of ISO 13849 – 1 and IEC 62061 in the design of safety-​related con­trol sys­tems for machinery. IEC Technical Report 62061 – 1. 2010.

[12]    D. S. G. Nix, Y. Chinniah, F. Dosio, M. Fessler, F. Eng, and F. Schrever, “Linking Risk and Reliability — Mapping the out­put of risk assess­ment tools to func­tion­al safety require­ments for safety related con­trol sys­tems,” 2015.

[13]    Safety of machinery. Safety related parts of con­trol sys­tems. General prin­ciples for design. CEN Standard EN 954 – 1. 1996.

[14]   Functional safety of electrical/​electronic/​programmable elec­tron­ic safety-​related sys­tems – Part 2: Requirements for electrical/​electronic/​programmable elec­tron­ic safety-​related sys­tems. IEC Standard 61508 – 2. 2010.

[15]     Reliability Prediction of Electronic Equipment. Military Handbook MIL-​HDBK-​217F. 1991.

[16]     “IFA – Practical aids: Software-​Assistent SISTEMA: Safety Integrity – Software Tool for the Evaluation of Machine Applications”, Dguv​.de, 2017. [Online]. Available: http://​www​.dguv​.de/​i​f​a​/​p​r​a​x​i​s​h​i​l​f​e​n​/​p​r​a​c​t​i​c​a​l​-​s​o​l​u​t​i​o​n​s​-​m​a​c​h​i​n​e​-​s​a​f​e​t​y​/​s​o​f​t​w​a​r​e​-​s​i​s​t​e​m​a​/​i​n​d​e​x​.​jsp. [Accessed: 30- Jan- 2017].

[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.

[22]     “Field-​programmable gate array”, En​.wiki​pe​dia​.org, 2017. [Online]. Available: https://​en​.wiki​pe​dia​.org/​w​i​k​i​/​F​i​e​l​d​-​p​r​o​g​r​a​m​m​a​b​l​e​_​g​a​t​e​_​a​r​ray. [Accessed: 16-​Jun-​2017].

[23]     Analysis tech­niques for sys­tem reli­ab­il­ity – Procedure for fail­ure mode and effects ana­lys­is (FMEA). 2nd Ed. IEC Standard 60812. 2006.

[24]     “Failure mode and effects ana­lys­is”, En​.wiki​pe​dia​.org, 2017. [Online]. Available: https://​en​.wiki​pe​dia​.org/​w​i​k​i​/​F​a​i​l​u​r​e​_​m​o​d​e​_​a​n​d​_​e​f​f​e​c​t​s​_​a​n​a​l​y​sis. [Accessed: 16-​Jun-​2017].

How Risk Assessment Fails — Again. This time at DuPont.

This entry is part 6 of 8 in the series Risk Assessment

A recent report released by the US Chemical Safety Board (CSB) looks at a series of acci­dents that occurred over a 33-​hour peri­od on January 22 and 23, 2010 at the DuPont Corporation’s Belle, West Virginia, chem­ic­al man­u­fac­tur­ing plant.

A num­ber of sig­ni­fic­ant fail­ures occurred, but I want to focus on one pas­sage from the press release that is telling, par­tic­u­larly con­sid­er­ing that DuPont is seen as a class lead­er when it comes to work­er safety. I would encour­age you to read the entire release. You can also have a look at the DuPont invest­ig­a­tion details on the CSB site. CSB also pro­duced a video dis­cuss­ing the invest­ig­a­tion.

From the press release:

Internal DuPont doc­u­ments released with the CSB report indic­ate that in the 1980’s, com­pany offi­cials con­sidered increas­ing the safety of the area of the plant where phos­gene is handled by enclos­ing the area and vent­ing the enclos­ure through  a scrub­ber sys­tem to des­troy any tox­ic phos­gene gas before it entered the atmo­sphere. The ana­lys­is con­cluded that an enclos­ure was the safest option for both work­ers and the pub­lic.  However, the doc­u­ments indic­ate the com­pany was con­cerned with con­tain­ing costs and decided not to make the safety improve­ments. A DuPont employ­ee  wrote in 1988,  “It may be that in the present cir­cum­stances the busi­ness can afford $2 mil­lion for an enclos­ure; how­ever, in the long run can we afford to take such action which has such a small impact on safety and yet sets a pre­ced­ent for all highly tox­ic mater­i­al activities.[sic]”

The need for an enclos­ure was reit­er­ated in a 2004 pro­cess haz­ard ana­lys­is con­duc­ted by DuPont, but four exten­sions were gran­ted by DuPont man­age­ment between 2004 and 2009, and at the time of the January 2010 release, no safety enclos­ure or scrub­ber sys­tem had been con­struc­ted. CSB invest­ig­at­ors con­cluded that an enclos­ure, scrub­ber sys­tem, and routine require­ment for pro­tect­ive breath­ing equip­ment before per­son­nel entered the enclos­ure would have pre­ven­ted any per­son­nel expos­ures or injur­ies.”

The high­lighted pas­sage above shows one of the key fail­ure modes in risk assess­ment: fail­ure to act on the res­ults. So what’s the point of con­duct­ing risk assess­ments if they are going to be ignored? In a present­a­tion in 2010, a col­league of mine made this state­ment:

The risk assess­ment pro­cess is inten­ded to be used as a decision mak­ing tool that will help to pro­tect work­ers.” – Tom Doyle, 2010

This is a fun­da­ment­al truth. The risk assess­ment paper­work can­not pro­tect a work­er from a haz­ard, only action based on the report can do that.

When decision makers receive the res­ults from a risk assess­ment pro­cess and choose to ignore it, or as the press release stated, “…exten­sions were gran­ted by DuPont man­age­ment…”, man­age­ment is mak­ing a fun­da­ment­ally flawed decision. The risk assess­ment pro­cess inten­tion­ally exposes the haz­ards in the scope of the ana­lys­is, and expli­citly ana­lyzes the prob­able sever­ity of injury and occur­rence. Once the ana­lys­is is com­plete, choos­ing to ignore the res­ults, pre­sum­ing that there is no evid­ence that the res­ults are incor­rect, amounts to neg­li­gence in my opin­ion.

Does this mean that we should not con­duct risk assess­ments? Absolutely not! In the Western world, we are oblig­ated to pro­tect the safety of work­ers, includ­ing our col­leagues and employ­ees, as well as any­one else that may inten­tion­ally or unin­ten­tion­ally be exposed to the haz­ards cre­ated by our activ­it­ies. We are mor­ally and eth­ic­ally, as well as leg­ally, oblig­ated.

Used cor­rectly, risk assess­ment in any of its many forms provides a power­ful tool to pro­tect people. Like any oth­er power­ful tool, it also takes sig­ni­fic­ant cour­age and skill to use cor­rectly. Defaulting to the cost argu­ment alone, as it appears that DuPont did in this case, res­ults in the type of fatal fail­ures seen in this tra­gic series of events.

Special thanks to my col­league Bryan Hayward, the Safety Engineering Network Group on LinkedIn, and SafTEng​.net.

What is your exper­i­ence with imple­ment­ing risk assess­ment? Have you exper­i­enced this kind of res­ult in your work? Share your exper­i­ences by com­ment­ing on this post!

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Acknowledgements: US Chemical Safety Board for excerpts more…
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The purpose of risk assessment

This entry is part 4 of 8 in the series Risk Assessment

I’m often asked what seems like a pretty simple ques­tion: “Why do we need to do a risk assess­ment?” There are a lot of good reas­ons to do risk assess­ments, but ulti­mately, the pur­pose of risk assess­ment is best summed up in this quo­ta­tion:

Risk assess­ments, except in the simplest of cir­cum­stances, are not designed for mak­ing judge­ments, but to illu­min­ate them.”

Richard Wilson and E. A. C. Crouch, Science, Volume 236, 1987, pp.267