How to do a 13849–1 analysis: Complete Reference List

This entry is part 8 of 9 in the series How to do a 13849–1 analy­sis

An old book lying open with round eyeglasses lying on top.As promised in pre­vi­ous posts, here is the com­plete ref­er­ence list for the series “How to do a 13849–1 analy­sis”! If you have any addi­tion­al resources you think read­ers would find help­ful, please add them in the com­ments.

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.

References

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/ifa/praxishilfen/practical-solutions-machine-safety/software-sistema/index.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.wikipedia.org, 2017. [Online]. Avail­able: https://en.wikipedia.org/wiki/Field-programmable_gate_array. [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.wikipedia.org, 2017. [Online]. Avail­able: https://en.wikipedia.org/wiki/Failure_mode_and_effects_analysis. [Accessed: 16-Jun-2017].

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

Examples

#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!

Definitions

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.

References

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/ifa/praxishilfen/practical-solutions-machine-safety/software-sistema/index.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.wikipedia.org, 2017. [Online]. Avail­able: https://en.wikipedia.org/wiki/Field-programmable_gate_array. [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.wikipedia.org, 2017. [Online]. Avail­able: https://en.wikipedia.org/wiki/Failure_mode_and_effects_analysis. [Accessed: 16-Jun-2017].

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.

References

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.