ISO 13849–1 Analysis — Part 8: Fault Exclusion

This entry is part 9 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 detectable 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. [2, Annex A], Val­i­da­tion tools for mechan­i­cal sys­tems, dis­cuss­es the con­cepts of “Basic Safe­ty Prin­ci­ples”, “Well-Tried Safe­ty Prin­ci­ples”, and “Well-tried com­po­nents”.  [2, Annex A] also pro­vides exam­ples of faults and rel­e­vant fault exclu­sion cri­te­ria. There are sim­i­lar Annex­es that cov­er pneu­mat­ic sys­tems [2, Annex B], hydraulic sys­tems [2, Annex C], and elec­tri­cal sys­tems [2, Annex D].

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 flow­chart [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. Fail­ure Modes and Effects Analy­sis (FMEA) is usu­al­ly the best approach for devel­op­ing fault lists 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


#1 — Voltage Regulator

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. All three fail­ures can be grouped and con­sid­ered as a sin­gle fault because they orig­i­nate in a sin­gle fail­ure in the volt­age reg­u­la­tor.

#2 — Lightning Strike

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. Again, a sin­gle event caus­es all of the sub­se­quent fail­ures.

#3 — Pneumatic System Lubrication

3a — A pneu­mat­ic lubri­ca­tor runs out of lubri­cant and is not refilled, depriv­ing down­stream pneu­mat­ic com­po­nents of lubri­ca­tion.

3b — 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. 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 [0.1], [0.2], and [0.3]. 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”,, 2017. [Online]. Avail­able: [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”,, 2017. [Online]. Avail­able: [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”,, 2017. [Online]. Avail­able: [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

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