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.