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/​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].

ISO 13849 – 1 Analysis — Part 3: Architectural Category Selection

This entry is part 3 of 9 in the series How to do a 13849 – 1 ana­lys­is

At this point, you have com­pleted the risk assess­ment, assigned required Performance Levels to each safety func­tion, and developed the Safety Requirement Specification for each safety func­tion. Next, you need to con­sider three aspects of the sys­tem design: Architectural Category, Channel Mean Time to Dangerous Failure (MTTFD), and Diagnostic Coverage (DCavg). In this part of the series, I am going to dis­cuss select­ing the archi­tec­tur­al cat­egory for the sys­tem.

If you missed the second instal­ment in this series, you can read it here.

Understanding Performance Levels

To under­stand ISO 13849 – 1, it helps to know a little about where the stand­ard ori­gin­ated. ISO 13849 – 1 is a sim­pli­fied meth­od for determ­in­ing the reli­ab­il­ity of safety-​related con­trols for machinery. The basic ideas came from IEC 61508 [7], a seven-​part stand­ard ori­gin­ally pub­lished in 1998. IEC 61508 brought for­ward the concept of the Average Probability of Dangerous Failure per Hour, PFHD (1/​h). Dangerous fail­ures are those fail­ures that res­ult in non-​performance of the safety func­tion, and which can­not be detec­ted by dia­gnostics. Here’s the form­al defin­i­tion from [1]:

3.1.5

dan­ger­ous fail­ure
fail­ure which has the poten­tial to put the SRP/​CS in a haz­ard­ous or fail-​to-​function state

Note 1 to entry: Whether or not the poten­tial is real­ised can depend on the chan­nel archi­tec­ture of the sys­tem; in redund­ant sys­tems a dan­ger­ous hard­ware fail­ure is less likely to lead to the over­all dan­ger­ous or fail-​to-​function state.

Note 2 to entry: [SOURCE: IEC 61508 – 4, 3.6.7, mod­i­fied.]

The Performance Levels are simply bands of prob­ab­il­it­ies of Dangerous Failures, as shown in [1, Table 2] below.

Table 2 from ISO 13849-2:2015 showing the five Performance levels and the corresponding ranges of PFHd values.
Performance Levels as bands of PFHd ranges

The ranges shown in [1, Table 2] are approx­im­ate. If you need to see the spe­cif­ic lim­its of the bands for any reas­on, see [1, Annex K] describes the full span of PFHD, in table format.

There is anoth­er way to describe the same char­ac­ter­ist­ics of a sys­tem, this one from IEC. Instead of using the PL sys­tem, IEC uses Safety Integrity Levels (SILs). [1, Table 3] shows the cor­res­pond­ence between PLs and SILs. Note that the cor­res­pond­ence is not exact. Where the cal­cu­lated PFHd is close to either end of one of the PL or SIL bands, use the table in [1, Annex K] or in [9] to determ­ine to which band(s) the per­form­ance should be assigned.

IEC pro­duced a Technical Report [10] that provides guid­ance on how to use ISO 13849 – 1 or IEC 62061. The fol­low­ing table shows the rela­tion­ship between PLs, PFHd and SILs.

Table showing the correspondence between the PL, PFHd, and SIL.
IEC/​TR 62061 – 1:2010, Table 1

IEC 61508 includes SIL 4, which is not shown in [10, Table 1] because this level of per­form­ance exceeds the range of PFHD pos­sible using ISO 13849 – 1 tech­niques. Also, you may have noticed that PLb and PLc are both with­in SIL1. This was done to accom­mod­ate the five archi­tec­tur­al cat­egor­ies that came from EN 954 – 1 [12].

Why PL and not just PFHD? One of the odd things that humans do when we can cal­cu­late things is the devel­op­ment of what has been called “pre­ci­sion bias” [12]. Precision bias occurs when we can com­pute a num­ber that appears very pre­cise, e.g., 3.2 x 10-6, which then makes us feel like we have a very pre­cise concept of the quant­ity. The prob­lem, at least in this case, is that we are deal­ing with prob­ab­il­it­ies and minus­cule prob­ab­il­it­ies at that. Using bands, like the PLs, forces us to “bin” these appar­ently pre­cise num­bers into lar­ger groups, elim­in­at­ing the effects of pre­ci­sion bias in the eval­u­ation of the sys­tems. Eliminating pre­ci­sion bias is the same reas­on that IEC 61508 uses SILs – bin­ning the cal­cu­lated val­ues helps to reduce our tend­ency to devel­op a pre­ci­sion bias. The real­ity is that we just can’t pre­dict the beha­viour of these sys­tems with as much pre­ci­sion as we would like to believe.

Getting to Performance Levels: MTTFD, Architectural Category and DC

Some aspects of the sys­tem design need to be con­sidered to arrive at a Performance Level or make a pre­dic­tion about fail­ure rates in terms of PFHd.

First is the sys­tem archi­tec­ture: Fundamentally, single chan­nel or two chan­nel. As a side note, if your sys­tem uses more than two chan­nels there are ways to handle this in ISO 13849 – 1 that are work­arounds, or you can use IEC 62061 or IEC 61508, either of which will handle these more com­plex sys­tems more eas­ily. Remember, ISO 13849 – 1 is inten­ded for rel­at­ively simple sys­tems.

When we get into the ana­lys­is in a later art­icle, we will be cal­cu­lat­ing or estim­at­ing the Mean Time to Dangerous Failure, MTTFD, of each chan­nel, and then of the entire sys­tem. MTTFD is expressed in years, unlike PFHd, which is expressed in frac­tion­al hours (1/​h). I have yet to hear why this is the case as it seems rather con­fus­ing. However, that is cur­rent prac­tice.

Architectural Categories

Once the required PL is known, the next step is the selec­tion of the archi­tec­tur­al cat­egory. The basic archi­tec­tur­al cat­egor­ies were intro­duced ini­tially in EN 954 – 1:1996 [12].  The Categories were car­ried for­ward unchanged into the first edi­tion of ISO 13849 – 1 in 1999. The Categories were main­tained and expan­ded to include addi­tion­al require­ments in the second and third edi­tions in 2005 and 2015.

Since I have explored the details of the archi­tec­tures in a pre­vi­ous series, I am not going to repeat that here. Instead, I will refer you to that series. The archi­tec­tur­al Categories come in five fla­vours:

Architecture Basics
Category Structure Basic Requirements Safety Princple
For full require­ments, see [1, Cl. 6]
B Single chan­nel Basic cir­cuit con­di­tions are met (i.e., com­pon­ents are rated for the cir­cuit voltage and cur­rent, etc.) Use of com­pon­ents that are designed and built to the rel­ev­ant com­pon­ent stand­ards. [1, 6.2.3] Component selec­tion
1 Single chan­nel Category B plus the use of “well-​tried com­pon­ents” and “well-​tried safety prin­ciples” [1, 6.2.4] Component selec­tion
2 Single chan­nel Category B plus the use of “well-​tried safety prin­ciples” and peri­od­ic test­ing [1, 4.5.4] of the safety func­tion by the machine con­trol sys­tem. [1, 6.2.5] System Structure
3 Dual chan­nel Category B plus the use of “well-​tried safety prin­ciples” and no single fault shall lead to the loss of the safety func­tion.

Where prac­tic­able, single faults shall be detec­ted. [1, 6.2.6]

System Structure
4 Dual chan­nel Category B plus the use of “well-​tried safety prin­ciples” and no single fault shall lead to the loss of the safety func­tion.

Single faults are detec­ted at or before the next demand on the safety sys­tem, but where this is not pos­sible an accu­mu­la­tion of undetec­ted faults will not lead to the loss of the safety func­tion. [1, 6.2.7]

System Structure

[1, Table 10] provides a more detailed sum­mary of the require­ments than the sum­mary table above provides.

Since the Categories can­not all achieve the same reli­ab­il­ity, the PL and the Categories are linked as shown in [1, Fig. 5]. This dia­gram sum­mar­ises te rela­tion­ship of the three cent­ral para­met­ers in ISO 13849 – 1 in one illus­tra­tion.

Figure relating Architectural Category, DC avg, MTTFD and PL.
Relationship between cat­egor­ies, DCavg, MTTFD of each chan­nel and PL

Starting with the PLr from the Safety Requirement Specification for the first safety func­tion, you can use Fig. 5 to help you select the Category and oth­er para­met­ers neces­sary for the design. For example, sup­pose that the risk assess­ment indic­ates that an emer­gency stop sys­tem is needed. ISO 13850 requires that emer­gency stop func­tions provide a min­im­um of PLc, so using this as the basis you can look at the ver­tic­al axis in the dia­gram to find PLc, and then read across the fig­ure. You will see that PLc can be achieved using Category 1, 2, or 3 archi­tec­ture, each with cor­res­pond­ing dif­fer­ences in MTTFD and DCavg. For example:

  • Cat. 1, MTTFD = high and DCavg = none, or
  • Cat. 2, MTTFD = Medium to High and DCavg = Low to Medium, or
  • Cat. 3, MTTFD = Low to High and DCavg = Low to Medium.

As you can see, the MTTFD in the chan­nels decreases as the dia­gnost­ic cov­er­age increases. The design com­pensates for lower reli­ab­il­ity in the com­pon­ents by increas­ing the dia­gnost­ic cov­er­age and adding redund­ancy. Using [1, Fig. 5] you can pin down any of the para­met­ers and then select the oth­ers as appro­pri­ate.

One addi­tion­al point regard­ing Category 3 and 4: The dif­fer­ence between these Categories is increased Diagnostic Coverage. While Category 3 is Single Fault Tolerant, Category 4 has addi­tion­al dia­gnost­ic cap­ab­il­it­ies so that addi­tion­al faults can­not lead to the loss of the safety func­tion. This is not the same as being mul­tiple fault tol­er­ant, as the sys­tem is still designed to oper­ate in the pres­ence of only a single fault, it is simply enhanced dia­gnost­ic cap­ab­il­ity.

It is worth not­ing that ISO 13849 only recog­nises struc­tures with single or dual chan­nel con­fig­ur­a­tions. If you need to devel­op a sys­tem with more than single redund­ancy (i.e., more than two chan­nels), you can ana­lyse each pair of chan­nels as a dual chan­nel archi­tec­ture, or you can move to using IEC 62061 or IEC 61508, either of which per­mits any level of redund­ancy.

The next step in this pro­cess is the eval­u­ation of the com­pon­ent and chan­nel MTTFD, and then the determ­in­a­tion of the com­plete sys­tem MTTFD. Part 4 of this series pub­lishes on 13-​Feb-​17.

In case you missed the first part of the series, you can read it here.

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. ISO Standard 13849 – 1. 2015.

[7]     Functional safety of electrical/​electronic/​programmable elec­tron­ic safety-​related sys­tems. IEC Standard 61508. 2nd Edition. Seven Parts. 2010.

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

[10]    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.

[11]    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.

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

ISO 13849 – 1 Analysis — Part 2: Safety Requirement Specification

This entry is part 2 of 9 in the series How to do a 13849 – 1 ana­lys­is

Developing the Safety Requirement Specification

The Safety Requirement Specification sounds pretty heavy, but actu­ally, it is just a big name for a way to organ­ise the inform­a­tion you need to have to ana­lyse and design the safety sys­tems for your machinery. Note that I am assum­ing that you are doing this in the “right” order, mean­ing that you are plan­ning the design before­hand, rather than try­ing to back-​fill the doc­u­ment­a­tion after com­plet­ing the design. In either case, the pro­cess is the same, but get­ting the inform­a­tion you need can be much harder after the fact, than before the doing the design work. Doing some aspects in a review mode is impossible, espe­cially if a third party to whom you have no access did the design work [8].

If you missed the first instal­ment in this series, you can read it here.

What goes into a Safety Requirements Specification?

For ref­er­ence, chapter 5 of ISO 13849 – 1 [1] cov­ers safety require­ment spe­cific­a­tions to some degree, but it needs some cla­ri­fic­a­tion I think. First of all, what is a safety func­tion?

Safety func­tions include any func­tion of the machine that has a dir­ect pro­tect­ive effect for the work­er using the machinery. However, using this defin­i­tion, it is pos­sible to ignore some import­ant func­tions. Complementary pro­tect­ive meas­ures, like emer­gency stop, can be missed because they are usu­ally “after the fact”, i.e., the injury occurs, and then the E-​stop is pressed, so you can­not say that it has a “dir­ect pro­tect­ive effect”. If we look at the defin­i­tions in [1], we find:

3.1.20

safety func­tion

func­tion of the machine whose fail­ure can res­ult in an imme­di­ate increase of the risk(s)
[SOURCE: ISO 12100:2010, 3.30.]

Linking Risk to Functional Safety

Referring to the risk assess­ment, any risk con­trol that pro­tects work­ers from some aspect of the machine oper­a­tion using a con­trol func­tion like an inter­locked gate, or by main­tain­ing a tem­per­at­ure below a crit­ic­al level or speed at a safe level, is a safety func­tion. For example: if the tem­per­at­ure in a pro­cess rises too high, the pro­cess will explode; or if a shaft speed is too high (or too low) the tool may shat­ter and eject broken pieces at high speed. Therefore, the tem­per­at­ure con­trol func­tion and the speed con­trol func­tion are safety func­tions. These func­tions may also be pro­cess con­trol func­tions, but the poten­tial for an imme­di­ate increase in risk due to a fail­ure is what makes these func­tions safety func­tions no mat­ter what else they may do.

[1, Table 8] gives you some examples of vari­ous kinds of safety func­tions found on machines. The table is not inclus­ive – mean­ing there are many more safety func­tions out there than are lis­ted in the table. Your job is to fig­ure out which ones live in your machine. It is a bit like Pokemon – ya gotta catch ‘em all!

Basic Safety Requirement Specification

Each safety func­tion must have a Performance Level or a Safety Integrity Level assigned as part of the risk assess­ment. For each safety func­tion, you need to devel­op the fol­low­ing inform­a­tion:

Basic Safety Requirement Specification
Item Description
Safety Function Identification Name or oth­er ref­er­ences, e.g. “Access Gate Interlock” or “Hazard Zone 2.”
Functional Characteristics
  • Intended use or fore­see­able mis­use of the machine rel­ev­ant to the safety func­tion
  • Operating modes rel­ev­ant to the safety func­tion
  • Cycle time of the machine
  • Response time of the safety func­tion
Emergency Operation Is this an emer­gency oper­a­tion func­tion? If yes, what types of emer­gen­cies might be mit­ig­ated by this func­tion?
Interactions What oper­at­ing modes require this func­tion to be oper­a­tion­al? Are there modes where this func­tion requires delib­er­ate bypass? These could include nor­mal work­ing modes (auto­mat­ic, manu­al, set-​up, changeover), and fault-​finding or main­ten­ance modes.
Behaviour How you want the sys­tem to behave when the safety func­tion is triggered, i.e., Power is imme­di­ately removed from the MIG weld­er using an IEC 60204 – 1 Category 0 stop func­tion, and robot motions are stopped using IEC 60204 – 1 Category 1 stop func­tion through the robot safety stop input.

or

All hori­zont­al pneu­mat­ic motions stop in their cur­rent pos­i­tions. Vertical motions return to the raised or retrac­ted pos­i­tions.

Also to be con­sidered is a power loss con­di­tion. Should the sys­tem behave in the same way as if the safety func­tion was triggered, not react at all, or do some­thing else? Consider ver­tic­al axes that might require hold­ing brakes or oth­er mech­an­isms to pre­vent power loss caus­ing unex­pec­ted motion.

Machine State after trig­ger­ing What is the expec­ted state of the machine after trig­ger­ing the safety func­tion? What is the recov­ery pro­cess?
Frequency of Operation How often do you expect this safety func­tion to be used? A reas­on­able estim­ate is needed. More on this below.
Priority of Operation If sim­ul­tan­eous trig­ger­ing of mul­tiple safety func­tions is pos­sible, which function(s) takes pre­ced­ence? E.g., Emergency Stop always takes pre­ced­ence over everything else. What hap­pens if you have a safe speed func­tion and a guard inter­lock that are asso­ci­ated because the inter­lock is part of a guard­ing func­tion cov­er­ing a shaft, and you need to troubleshoot the safe speed func­tion, so you need access to the shaft where the encoders are moun­ted?
Required Performance Level I sug­gest record­ing the S, F, and P val­ues selec­ted as well as the PLr value selec­ted for later ref­er­ence.

Here’s an example table in MS Word format that you can use as a start­ing point for your SRS doc­u­ments. Note that SRS can be much more detailed than this. If you want more inform­a­tion on this, read IEC 61508 – 1, 7.10.2.

So, that is the min­im­um. You can add lots more inform­a­tion to the min­im­um require­ments, but this will get you star­ted. If you want more inform­a­tion on devel­op­ing the SRS, you will need to get a copy of IEC 61508 [7].

What’s Next?

Next, you need to be able to make some design decisions about sys­tem archi­tec­ture and com­pon­ents. Circuit archi­tec­tures have been dis­cussed at some length on the MS101 blog in the past, so I am not going to go through them again in this series. Instead, I will show you how to choose an archi­tec­ture based on your design goals in the next instal­ment. In case you missed the first part of the series, you can read it here.

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.

[7]     Functional safety of electrical/​electronic/​programmable elec­tron­ic safety-​related sys­tems. Seven 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.