New Directions in Plastics Machinery

Canada’s Participation in ISO TC 270

SCC Standards Council of Canada LogoIn February of 2016, Canada formed the SCC Mirror Committee (SMC) to ISO TC 270, Rubber and Plastics Machinery. This inter­na­tion­al tech­nic­al com­mit­tee is cur­rently devel­op­ing ISO 20430, the first inter­na­tion­al plastic injec­tion mould­ing machine stand­ard. Until the pub­lic­a­tion of ISO 20430, two stand­ards have been fight­ing for dom­in­ance: EN 201, Plastics and rub­ber machines — Injection mould­ing machines — Safety require­ments, and ANSI B151.1, American National Standard for Plastics Machinery – Horizontal Injection Moulding Machines – Safety Requirements for man­u­fac­ture, Care and Use.

Canada has a strong plastic and rub­ber industry, with key equip­ment man­u­fac­tur­ers like Athena Automation, Husky Injection Molding Systems, Mold Masters and GN Plastics among oth­ers pro­du­cing world class machinery for the industry. The industry is rep­res­en­ted nation­ally by the Canadian Plastics Industry Association. Despite this, Canada has nev­er had its own stand­ard for this type of machinery.

Involvement in ISO TC 270 allows Canada’s plastics industry to have a voice in devel­op­ing the inter­na­tion­al stand­ards for the machinery they design and build, and which ever more com­monly, they buy and use.

The com­mit­tee needs your help to know which way Canadian industry wants us to focus our efforts as the work on ISO 20430 wraps up in com­ing months. We have a short sur­vey, just three ques­tions long, where you can rank five pos­sible top­ics we can focus on. We will be sub­mit­ting our com­mit­tee vote in early August on the top­ic, so you have a month or so to answer the ques­tion­naire. Let us know your pref­er­ences.

Why now?

ISO LogoUntil the pub­lic­a­tion of ISO 20430, two stand­ards have been fight­ing for dom­in­ance: EN 201 in Europe, and ANSI B151.1 in North America. Until the rel­at­ively recent form­a­tion of ISO TC 270 in 2012, there were NO inter­na­tion­al stand­ards for this type of machinery. While there have been some efforts to har­mon­ise the European and ANSI stand­ards, there are still some sig­ni­fic­ant gaps between these stand­ards. In addi­tion, ANSI’s B151 com­mit­tee has a num­ber of addi­tion­al stand­ards for aux­il­i­ary equip­ment for items like robots designed to unload molds, that are not dir­ectly addressed in EN stand­ards.

Canada was giv­en a chance to par­ti­cip­ate through our ongo­ing friend­ship with ANSI and the USA, so between 2012 and 2015, Canadian del­eg­ates atten­ded ISO TC 270 work­ing group meet­ings inform­ally, and put Canada’s per­spect­ive for­ward through the US ANSI TAG com­mit­tee, but in 2016 it became clear that we needed to form our own com­mit­tee. If you are involved in the industry and you are a mem­ber of one of these gen­er­al groups and would like to get involved with stand­ards devel­op­ment, please go to our recruit­ing page and join us!

Committee Membership Matrix

Matrix Category Min Max Current
Total  15  25  6
Producer Interest (PI) 3 5 3
User Interest, Management (UM) 3 5 1
User Interest, Labour (UL) 3 5 0
Regulatory Authority (RA) 3 5 1
General Interest (GI) 3 5 1

As you can see from the table, we need mem­bers in every group except the pro­du­cers to meet our inten­ded bal­ance.

Definitions of the Categories

Producer Interest (PI) — Machine build­ers, Auxiliary Equipment Manufacturers, Consultants, and Engineering Companies provid­ing for-​profit ser­vices related to plastics and rub­ber machinery.

User Interest, Labour (UL) — Canadian labour uni­ons, labour organ­iz­a­tions, and indi­vidu­al work­ers loc­ated at Canadian work­places.

User Interest, Management (UM) — Trade asso­ci­ations, com­pan­ies, con­tract­ors, and organ­iz­a­tions rep­res­ent­ing com­pan­ies engaged in work per­formed in Canada.

Regulatory Authorities (RA) — OHS pro­vin­cial and fed­er­al reg­u­lat­ory bod­ies (labour and elec­tric­al).

General Interest (GI) — Safety asso­ci­ations, research organ­iz­a­tions, insti­tu­tions, and non-​commercial con­sult­ants who have expert­ise in the sub­ject area.

We need your help!

CAC ISO TC 270 needs your help!

Can you volun­teer some time? Sign up!

Can you help dir­ect us? Answer our ques­tion­naire!

Need more inform­a­tion? Contact Doug Nix!

Testing Emergency Stop Systems

This entry is part 11 of 13 in the series Emergency Stop

Emergency Stop on machine consoleI’ve had a num­ber of ques­tions from read­ers regard­ing test­ing of emer­gency stop sys­tems, and par­tic­u­larly with the fre­quency of test­ing. I addressed the types of tests that might be needed in anoth­er art­icle cov­er­ing Checking Emergency Stop Systems. This art­icle will focus on the fre­quency of test­ing rather than the types of tests.

The Problem

Emergency stop sys­tems are con­sidered to be “com­ple­ment­ary pro­tect­ive meas­ures” in key machinery safety stand­ards like ISO 12100 [1], and CSA Z432 [2]; this makes emer­gency stop sys­tems the backup to the primary safe­guards. Complementary pro­tect­ive meas­ures are inten­ded to per­mit “avoid­ing or lim­it­ing the harm” that may res­ult from an emer­gent situ­ation. By defin­i­tion, this is a situ­ation that has not been fore­seen by the machine build­er, or is the res­ult of anoth­er fail­ure. This could be a fail­ure of anoth­er safe­guard­ing sys­tem, or a fail­ure in the machine that is not con­trolled by oth­er means., e.g., a work­piece shat­ters due to a mater­i­al flaw, and the broken pieces dam­age the machine, cre­at­ing new, uncon­trolled, fail­ure con­di­tions in the machine.

Emergency stop sys­tems are manu­ally triggered, and usu­ally infre­quently used. The lack of use means that func­tion­al test­ing of the sys­tem doesn’t hap­pen in the nor­mal course of oper­a­tion of the machinery. Some types of faults may occur and remain undetec­ted until the sys­tem is actu­ally used, i.e., con­tact blocks fall­ing off the back of the oper­at­or device. Failure at that point may be cata­stroph­ic, since by implic­a­tion the primary safe­guards have already failed, and thus the fail­ure of the backup elim­in­ates the pos­sib­il­ity of avoid­ing or lim­it­ing harm.

To under­stand the test­ing require­ments, it’s import­ant to under­stand the risk and reli­ab­il­ity require­ments that drive the design of emer­gency stop sys­tems, and then get into the test fre­quency ques­tion.

Requirements

In the past, there were no expli­cit require­ments for emer­gency stop sys­tem reli­ab­il­ity. Details like the col­our of the oper­at­or device, or the way the stop func­tion worked were defined in ISO 13850 [3], NFPA 79 [4], and IEC 60204 – 1 [5]. In the soon-​to-​be pub­lished 3rd edi­tion of ISO 13850, a new pro­vi­sion requir­ing emer­gency stop sys­tems to meet at least PLc will be added [6], but until pub­lic­a­tion, it is up to the design­er to determ­ine the safety integ­rity level, either PL or SIL, required. To determ­ine the require­ments for any safety func­tion, the key is to start at the risk assess­ment. The risk assess­ment pro­cess requires that the design­er under­stand the stage in the life cycle of the machine, the task(s) that will be done, and the spe­cif­ic haz­ards that a work­er may be exposed to while con­duct­ing the task. This can become quite com­plex when con­sid­er­ing main­ten­ance and ser­vice tasks, and also applies to fore­see­able fail­ure modes of the machinery or the pro­cess. The scor­ing or rank­ing of risk can be accom­plished using any suit­able risk scor­ing tool that meets the min­im­um require­ments in [1]. There are some good examples giv­en in ISO/​TR 14121 – 2 [7] if you are look­ing for some guid­ance. There are many good engin­eer­ing text­books avail­able as well. Have a look at our Book List for some sug­ges­tions if you want a deep­er dive.

Reliability

Once the ini­tial unmit­ig­ated risk is under­stood, risk con­trol meas­ures can be spe­cified. Wherever the con­trol sys­tem is used as part of the risk con­trol meas­ure, a safety func­tion must be spe­cified. Specification of the safety func­tion includes the Performance Level (PL), archi­tec­tur­al cat­egory (B, 1 – 4), Mean Time to Dangerous Failure (MTTFd), and Diagnostic Coverage (DC) [6], or Safety Integrity Level (SIL), and Hardware Fault Tolerance (HFT), as described in IEC 62061 [8], as a min­im­um. If you are unfa­mil­i­ar with these terms, see the defin­i­tions at the end of the art­icle.

Referring to Figure 1, the “Risk Graph” [6, Annex A], we can reas­on­ably state that for most machinery, a fail­ure mode or emer­gent con­di­tion is likely to cre­ate con­di­tions where the sever­ity of injury is likely to require more than basic first aid, so select­ing “S2″ is the first step. In these situ­ations, and par­tic­u­larly where the fail­ure modes are not well under­stood, the highest level of sever­ity of injury, S2, is selec­ted because we don’t have enough inform­a­tion to expect that the injur­ies would only be minor. As soon as we make this selec­tion, it is no longer pos­sible to select any com­bin­a­tion of Frequency or Probability para­met­ers that will res­ult in any­thing lower than PLc.

It’s import­ant to under­stand that Figure 1 is not a risk assess­ment tool, but rather a decision tree used to select an appro­pri­ate PL based on the rel­ev­ant risk para­met­ers. Those para­met­ers are:

Table 1 – Risk Parameters
Severity of Injury fre­quency and/​or expos­ure to haz­ard pos­sib­il­ity of avoid­ing haz­ard or lim­it­ing harm
S1 – slight (nor­mally revers­ible injury) F1 – seldom-​to-​less-​often and/​or expos­ure time is short P1 – pos­sible under spe­cif­ic con­di­tions
S2 – ser­i­ous (nor­mally irre­vers­ible injury or death) F2 – frequent-​to-​continuous and/​or expos­ure time is long P2 – scarcely pos­sible
Decision tree used to determine PL based on risk parameters.
Figure 1 – “Risk Graph” for determ­in­ing PL

PLc can be accom­plished using any of three archi­tec­tures: Category 1, 2, or 3. If you are unsure about what these archi­tec­tures rep­res­ent, have a look at my series cov­er­ing this top­ic.

Category 1 is single chan­nel, and does not include any dia­gnostics. A single fault can cause the loss of the safety func­tion (i.e., the machine still runs even though the e-​stop but­ton is pressed). Using Category 1, the reli­ab­il­ity of the design is based on the use of highly reli­able com­pon­ents and well-​tried safety prin­ciples. This approach can fail to danger.

Category 2 adds some dia­gnost­ic cap­ab­il­ity to the basic single chan­nel con­fig­ur­a­tion and does not require the use of “well-​tried” com­pon­ents. This approach can also fail to danger.

Category 3 archi­tec­ture adds a redund­ant chan­nel, and includes dia­gnost­ic cov­er­age. Category 3 is not sub­ject to fail­ure due to single faults and is called “single-​fault tol­er­ant”. This approach is less likely to fail to danger, but still can in the pres­ence of mul­tiple, undetec­ted, faults.

A key concept in reli­ab­il­ity is the “fault”. This can be any kind of defect in hard­ware or soft­ware that res­ults in unwanted beha­viour or a fail­ure. Faults are fur­ther broken down into dan­ger­ous and safe faults, mean­ing those that res­ult in a dan­ger­ous out­come, and those that do not. Finally, each of these classes is broken down into detect­able and undetect­able faults. I’m not going to get into the math­em­at­ic­al treat­ment of these classes, but my point is this: there are undetect­able dan­ger­ous faults. These are faults that can­not be detec­ted by built-​in dia­gnostics. As design­ers, we try to design the con­trol sys­tem so that the undetect­able dan­ger­ous faults are extremely rare, ideally the prob­ab­il­ity should be much less than once in the life­time of the machine.

What is the life­time of the machine? The stand­ards writers have settled on a default life­time of 20 years, thus the answer is that undetect­able dan­ger­ous fail­ures should hap­pen much less than once in twenty years of 24/​7/​365 oper­a­tion. So why does this mat­ter? Each archi­tec­tur­al cat­egory has dif­fer­ent require­ments for test­ing. The test rates are driv­en by the “Demand Rate”. The Demand Rate is defined in [6]. “SRP/​CS” stands for “Safety Related Part of the Control System” in the defin­i­tion:

3.1.30
demand rate (rd) – fre­quency of demands for a safety-​related action of the SRP/​CS

Each time the emer­gency stop but­ton is pressed, a “demand” is put on the sys­tem. Looking at the “Simplified Procedure for estim­at­ing PL”, [6, 4.5.4], we find that the stand­ard makes the fol­low­ing assump­tions:

  • mis­sion time, 20 years (see Clause 10);
  • con­stant fail­ure rates with­in the mis­sion time;
  • for cat­egory 2, demand rate <= 1/​100 test rate;
  • for cat­egory 2, MTTFDTE lar­ger than half of MTTFDL.

NOTE When blocks of each chan­nel can­not be sep­ar­ated, the fol­low­ing can be applied: MTTFD of the sum­mar­ized test chan­nel (TE, OTE) lar­ger than half MTTFD of the sum­mar­ized func­tion­al chan­nel (I, L, O).

So what does all that mean? The 20-​year mis­sion time is the assumed life­time of the machinery. This num­ber under­pins the rest of the cal­cu­la­tions in the stand­ard and is based on the idea that few mod­ern con­trol sys­tems last longer than 20 years without being replaced or rebuilt. The con­stant fail­ure rate points at the idea that sys­tems used in the field will have com­pon­ents and con­trols that are not sub­ject to infant mor­tal­ity, nor are they old enough to start to fail due to age, but rather that the sys­tem is oper­at­ing in the flat por­tion of the stand­ard­ized fail­ure rate “bathtub curve”, [9]. See Figure 2. Components that are sub­ject to infant mor­tal­ity failed at the fact­ory and were removed from the sup­ply chain. Those fail­ing from “wear-​out” are expec­ted to reach that point after 20 years. If this is not the case, then the main­ten­ance instruc­tions for the sys­tem should include pre­vent­at­ive main­ten­ance tasks that require repla­cing crit­ic­al com­pon­ents before they reach the pre­dicted MTTFd.

Diagram of a standardized bathtub-shaped failure rate curve.
Figure 2 – Weibull Bathtub Curve [9]
For sys­tems using Category 2 archi­tec­ture, the auto­mat­ic dia­gnost­ic test rate must be at least 100x the demand rate. Keep in mind that this test rate is nor­mally accom­plished auto­mat­ic­ally in the design of the con­trols, and is only related to the detect­able safe or dan­ger­ous faults. Undetectable faults must have a prob­ab­il­ity of less than once in 20 years, and should be detec­ted by the “proof test”. More on that a bit later.

Finally, the MTTFD of the func­tion­al chan­nel must be at least twice that of the dia­gnost­ic sys­tem.

Category 1 has no dia­gnostics, so there is no guid­ance in [6] to help us out with these sys­tems. Category 3 is single fault tol­er­ant, so as long as we don’t have mul­tiple undetec­ted faults we can count on the sys­tem to func­tion and to alert us when a single fault occurs; remem­ber that the auto­mat­ic tests may not be able to detect every fault. This is where the “proof test” comes in. What is a proof test? To find a defin­i­tion for the proof test, we have to look at IEC 61508 – 4 [10]:

3.8.5
proof test
peri­od­ic test per­formed to detect fail­ures in a safety-​related sys­tem so that, if neces­sary, the sys­tem can be restored to an “as new” con­di­tion or as close as prac­tic­al to this con­di­tion

NOTE – The effect­ive­ness of the proof test will be depend­ent upon how close to the “as new” con­di­tion the sys­tem is restored. For the proof test to be fully effect­ive, it will be neces­sary to detect 100% of all dan­ger­ous fail­ures. Although in prac­tice 100% is not eas­ily achieved for oth­er than low-​complexity E/​E/​PE safety-​related sys­tems, this should be the tar­get. As a min­im­um, all the safety func­tions which are executed are checked accord­ing to the E/​E/​PES safety require­ments spe­cific­a­tion. If sep­ar­ate chan­nels are used, these tests are done for each chan­nel sep­ar­ately.

The 20-​year life cycle assump­tion used in the stand­ards also applies to proof test­ing. Machine con­trols are assumed to get at least one proof test in their life­time. The proof test should be designed to detect faults that the auto­mat­ic dia­gnostics can­not detect. Proof tests are also con­duc­ted after major rebuilds and repairs to ensure that the sys­tem oper­ates cor­rectly.

If you know the archi­tec­ture of the emer­gency stop con­trol sys­tem, you can determ­ine the test rate based on the demand rate. It would be con­sid­er­ably easi­er if the stand­ards just gave us some min­im­um test rates for the vari­ous archi­tec­tures. One stand­ard, ISO 14119 [11] on inter­locks does just that. Admittedly, this stand­ard does not include emer­gency stop func­tions with­in its scope, as its focus is on inter­locks, but since inter­lock­ing sys­tems are more crit­ic­al than the com­ple­ment­ary pro­tect­ive meas­ures that back them up, it would be reas­on­able to apply these same rules. Looking at the clause on Assessment of Faults, [9, 8.2], we find this guid­ance:

For applic­a­tions using inter­lock­ing devices with auto­mat­ic mon­it­or­ing to achieve the neces­sary dia­gnost­ic cov­er­age for the required safety per­form­ance, a func­tion­al test (see IEC 60204 – 1:2005, 9.4.2.4) can be car­ried out every time the device changes its state, e.g. at every access. If, in such a case, there is only infre­quent access, the inter­lock­ing device shall be used with addi­tion­al meas­ures, because between con­sec­ut­ive func­tion­al tests the prob­ab­il­ity of occur­rence of an undetec­ted fault is increased.

When a manu­al func­tion­al test is neces­sary to detect a pos­sible accu­mu­la­tion of faults, it shall be made with­in the fol­low­ing test inter­vals:

  • at least every month for PLe with Category 3 or Category 4 (accord­ing to ISO 13849 – 1) or SIL 3 with HFT (hard­ware fault tol­er­ance) = 1 (accord­ing to IEC 62061);
  • at least every 12 months for PLd with Category 3 (accord­ing to ISO 13849 – 1) or SIL 2 with HFT (hard­ware fault tol­er­ance) = 1 (accord­ing to IEC 62061).

NOTE It is recom­men­ded that the con­trol sys­tem of a machine demands these tests at the required inter­vals e.g. by visu­al dis­play unit or sig­nal lamp. The con­trol sys­tem should mon­it­or the tests and stop the machine if the test is omit­ted or fails.

In the pre­ced­ing, HFT=1 is equi­val­ent to say­ing that the sys­tem is single-​fault tol­er­ant.

This leaves us then with recom­men­ded test fre­quen­cies for Category 2 and 3 archi­tec­tures in PLc, PLd, and PLe, or for SIL 2 and 3 with HFT=1. We still don’t have a test fre­quency for PLc, Category 1 sys­tems. There is no expli­cit guid­ance for these sys­tems in the stand­ards. How can we determ­ine a test rate for these sys­tems?

My approach would be to start by examin­ing the MTTFd val­ues for all of the sub­sys­tems and com­pon­ents. [6] requires that the sys­tem has HIGH MTTFd value, mean­ing 30 years <= MTTFd <= 100 years [6, Table 5]. If this is the case, then the once-​in-​20-​years proof test is the­or­et­ic­ally enough. If the sys­tem is con­struc­ted, for example, as shown Figure 2 below, then each com­pon­ent would have to have an MTTFd > 120 years. See [6, Annex C] for this cal­cu­la­tion.

Basic Stop/Start Circuit
Figure 2 – Basic Stop/​Start Circuit

PB1 – Emergency Stop Button

PB2 – Power “ON” Button

MCR – Master Control Relay

MOV – Surge Suppressor on MCR Coil

M1 – Machine prime mover (motor)

Note that the fuses are not included, since they can only fail to safety, and assum­ing that they were spe­cified cor­rectly in the ori­gin­al design, are not sub­ject to the same cyc­lic­al aging effects as the oth­er com­pon­ents.

M1 is not included since it is the con­trolled por­tion of the machine and is not part of the con­trol sys­tem.

If a review of the com­pon­ents of the sys­tem shows that any single com­pon­ent falls below the tar­get MTTFD, then I would con­sider repla­cing the sys­tem with a high­er cat­egory design. Since most of these com­pon­ents will be unlikely to have MTTFD val­ues on the spec sheet, you will likely have to con­vert from total life val­ues (B10). This is out­side the scope of this art­icle, but you can find guid­ance in [6, Annex C]. More fre­quent test­ing, i.e., more than once in 20 years, is always accept­able.

Where manu­al test­ing is required as part of the design for any cat­egory of sys­tem, and par­tic­u­larly in Category 1 or 2 sys­tems, the con­trol sys­tem should alert the user to the require­ment and not per­mit the machine to oper­ate until the test is com­pleted. This will help to ensure that the requis­ite tests are prop­erly com­pleted.

Need more inform­a­tion? Leave a com­ment below, or send me an email with the details of your applic­a­tion!

Definitions

3.1.9 [8]
func­tion­al safety
part of the over­all safety relat­ing to the EUC and the EUC con­trol sys­tem which depends on the cor­rect func­tion­ing of the E/​E/​PE safety-​related sys­tems, oth­er tech­no­logy safety-​related sys­tems and extern­al risk reduc­tion facil­it­ies
3.2.6 [8]
electrical/​electronic/​programmable elec­tron­ic (E/​E/​PE)
based on elec­tric­al (E) and/​or elec­tron­ic (E) and/​or pro­gram­mable elec­tron­ic (PE) tech­no­logy

NOTE – The term is inten­ded to cov­er any and all devices or sys­tems oper­at­ing on elec­tric­al prin­ciples.

EXAMPLE Electrical/​electronic/​programmable elec­tron­ic devices include

  • elec­tromech­an­ic­al devices (elec­tric­al);
  • solid-​state non-​programmable elec­tron­ic devices (elec­tron­ic);
  • elec­tron­ic devices based on com­puter tech­no­logy (pro­gram­mable elec­tron­ic); see 3.2.5
3.5.1 [8]
safety func­tion
func­tion to be imple­men­ted by an E/​E/​PE safety-​related sys­tem, oth­er tech­no­logy safety-​related sys­tem or extern­al risk reduc­tion facil­it­ies, which is inten­ded to achieve or main­tain a safe state for the EUC, in respect of a spe­cif­ic haz­ard­ous event (see 3.4.1)
3.5.2 [8]
safety integ­rity
prob­ab­il­ity of a safety-​related sys­tem sat­is­fact­or­ily per­form­ing the required safety func­tions under all the stated con­di­tions with­in a stated peri­od of time
NOTE 1 – The high­er the level of safety integ­rity of the safety-​related sys­tems, the lower the prob­ab­il­ity that the safety-​related sys­tems will fail to carry out the required safety func­tions.
NOTE 2 – There are four levels of safety integ­rity for sys­tems (see 3.5.6).
3.5.6 [8]
safety integ­rity level (SIL)
dis­crete level (one out of a pos­sible four) for spe­cify­ing the safety integ­rity require­ments of the safety func­tions to be alloc­ated to the E/​E/​PE safety-​related sys­tems, where safety integ­rity level 4 has the highest level of safety integ­rity and safety integ­rity level 1 has the low­est
NOTE – The tar­get fail­ure meas­ures (see 3.5.13) for the four safety integ­rity levels are spe­cified in tables 2 and 3 of IEC 61508 – 1.
3.6.3 [8]
fault tol­er­ance
abil­ity of a func­tion­al unit to con­tin­ue to per­form a required func­tion in the pres­ence of faults or errors
NOTE – The defin­i­tion in IEV 191 – 15-​05 refers only to sub-​item faults. See the note for the term fault in 3.6.1.
[ISO/​IEC 2382 – 14-​04 – 061]
3.1.1 [6]
safety – related part of a con­trol sys­tem (SRP/​CS)
part of a con­trol sys­tem that responds to safety-​related input sig­nals and gen­er­ates safety-​related out­put sig­nals
NOTE 1 The com­bined safety-​related parts of a con­trol sys­tem start at the point where the safety-​related input sig­nals are ini­ti­ated (includ­ing, for example, the actu­at­ing cam and the roller of the pos­i­tion switch) and end at the out­put of the power con­trol ele­ments (includ­ing, for example, the main con­tacts of a con­tact­or).
NOTE 2 If mon­it­or­ing sys­tems are used for dia­gnostics, they are also con­sidered as SRP/​CS.
3.1.2 [6]
cat­egory
clas­si­fic­a­tion of the safety-​related parts of a con­trol sys­tem in respect of their res­ist­ance to faults and their sub­sequent beha­viour in the fault con­di­tion, and which is achieved by the struc­tur­al arrange­ment of the parts, fault detec­tion and/​or by their reli­ab­il­ity
3.1.3 [6]
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 A fault is often the res­ult of a fail­ure of the item itself, but may exist without pri­or fail­ure.
[IEC 60050 – 191:1990, 05 – 01]

NOTE 2 In this part of ISO 13849, “fault” means ran­dom fault.

3.1.4 [6]
fail­ure
ter­min­a­tion of the abil­ity of an item to per­form a required func­tion

NOTE 1 After a fail­ure, the item has a fault.

NOTE 2 “Failure” is an event, as dis­tin­guished from “fault”, which is a state.

NOTE 3 The concept as defined does not apply to items con­sist­ing of soft­ware only.
[IEC 60050 – 191:1990, 04 – 01]

NOTE 4 Failures which only affect the avail­ab­il­ity of the pro­cess under con­trol are out­side of the scope of this part of ISO 13849.

3.1.5 [6]
dan­ger­ous fail­ure
fail­ure which has the poten­tial to put the SRP/​CS in a haz­ard­ous or fail-​to-​function state

NOTE 1 Whether or not the poten­tial is real­ized 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 Adapted from IEC 61508 – 4:1998, defin­i­tion 3.6.7.

3.1.20 [6]
safety func­tion
func­tion of the machine whose fail­ure can res­ult in an imme­di­ate increase of the risk(s)
[ISO 12100 – 1:2003, 3.28]
3.1.21 [6]
mon­it­or­ing
safety func­tion which ensures that a pro­tect­ive meas­ure is ini­ti­ated if the abil­ity of a com­pon­ent or an ele­ment to per­form its func­tion is dimin­ished or if the pro­cess con­di­tions are changed in such a way that a decrease of the amount of risk reduc­tion is gen­er­ated
3.1.22 [6]
pro­gram­mable elec­tron­ic sys­tem (PES)
sys­tem for con­trol, pro­tec­tion or mon­it­or­ing depend­ent for its oper­a­tion on one or more pro­gram­mable elec­tron­ic devices, includ­ing all ele­ments of the sys­tem such as power sup­plies, sensors and oth­er input devices, con­tact­ors and oth­er out­put devices

NOTE Adapted from IEC 61508 – 4:1998, defin­i­tion 3.3.2.

3.1.23 [6]
per­form­ance level (PL)
dis­crete level used to spe­cify the abil­ity of safety-​related parts of con­trol sys­tems to per­form a safety func­tion under fore­see­able con­di­tions

NOTE See 4.5.1.

3.1.25 [6]
mean time to dan­ger­ous fail­ure (MTTFd)
expect­a­tion of the mean time to dan­ger­ous fail­ure

NOTE Adapted from IEC 62061:2005, defin­i­tion 3.2.34.

3.1.26 [6]
dia­gnost­ic cov­er­age (DC)
meas­ure of the effect­ive­ness of dia­gnostics, which may be determ­ined as the ratio between the fail­ure rate of detec­ted dan­ger­ous fail­ures and the fail­ure rate of total dan­ger­ous fail­ures

NOTE 1 Diagnostic cov­er­age can exist for the whole or parts of a safety-​related sys­tem. For example, dia­gnost­ic cov­er­age could exist for sensors and/​or logic sys­tem and/​or final ele­ments.

NOTE 2 Adapted from IEC 61508 – 4:1998, defin­i­tion 3.8.6.

3.1.33 [6]
safety integ­rity level (SIL)
dis­crete level (one out of a pos­sible four) for spe­cify­ing the safety integ­rity require­ments of the safety func­tions to be alloc­ated to the E/​E/​PE safety-​related sys­tems, where safety integ­rity level 4 has the highest level of safety integ­rity and safety integ­rity level 1 has the low­est
[IEC 61508 – 4:1998, 3.5.6]

Acknowledgements

Thanks to my col­leagues Derek Jones and Jonathan Johnson, both from Rockwell Automation, and mem­bers of ISO TC199. Their sug­ges­tion to ref­er­ence ISO 14119 clause 8.2 was the seed for this art­icle.

I’d also like to acknow­ledge Ronald Sykes, Howard Touski, Mirela Moga, Michael Roland, and Grant Rider for ask­ing the ques­tions that lead to this art­icle.

References

[1]     Safety of machinery — General prin­ciples for design — Risk assess­ment and risk reduc­tion. ISO 12100. International Organization for Standardization (ISO). Geneva 2010.

[2]    Safeguarding of Machinery. CSA Z432. Canadian Standards Association. Toronto. 2004.

[3]    Safety of machinery – Emergency stop – Principles for design. ISO 13850. International Organization for Standardization (ISO). Geneva 2006.

[4]    Electrical Standard for Industrial Machinery. NFPA 79. National Fire Protection Association (NFPA). Batterymarch Park. 2015

[5]    Safety of machinery – Electrical equip­ment of machines – Part 1: General require­ments. IEC 60204 – 1. International Electrotechnical Commission (IEC). Geneva. 2009.

[6]    Safety of machinery — Safety-​related parts of con­trol sys­tems — Part 1: General prin­ciples for design.  ISO 13849 – 1. International Organization for Standardization (ISO). Geneva. 2006.

[7]    Safety of machinery — Risk assess­ment — Part 2: Practical guid­ance and examples of meth­ods. ISO/​TR 14121 – 2. International Organization for Standardization (ISO). Geneva. 2012.

[8]   Safety of machinery – Functional safety of safety-​related elec­tric­al, elec­tron­ic and pro­gram­mable elec­tron­ic con­trol sys­tems. IEC 62061. International Electrotechnical Commission (IEC). Geneva. 2005.

[9]    D. J. Wilkins (2002, November). “The Bathtub Curve and Product Failure Behavior. Part One – The Bathtub Curve, Infant Mortality and Burn-​in”. Reliability Hotline [Online]. Available: http://​www​.weibull​.com/​h​o​t​w​i​r​e​/​i​s​s​u​e​2​1​/​h​o​t​t​o​p​i​c​s​2​1​.​htm. [Accessed: 26-​Apr-​2015].

[10] Functional safety of electrical/​electronic/​programmable elec­tron­ic safety-​related sys­tems – Part 4: Definitions and abbre­vi­ations. IEC 61508 – 4. International Electrotechnical Commission (IEC). Geneva. 1998.

[11] Safety of machinery — Interlocking devices asso­ci­ated with guards — Principles for design and selec­tion. ISO 14119. International Organization for Standardization (ISO). Geneva. 2013.

Sources for Standards

CANADA

Canadian Standards Association sells CSA, ISO and IEC stand­ards to the Canadian Market.

USA

ANSI offers stand­ards from most US Standards Development Organizations. They also sell ISO and IEC stand­ards into the US mar­ket.


International

International Organization for Standardization (ISO).

International Electrotechnical Commission (IEC).

Europe

Each EU mem­ber state has their own stand­ards body. For reas­ons unknown to me, each stand­ards body can set their own pri­cing for the doc­u­ments they sell. All offer English lan­guage cop­ies, in addi­tion to cop­ies in the offi­cial language(s) of the mem­ber state. My best advice is to shop around a bit. Prices can vary by as much as 10:1.

British Standards Institute (BSi) $$$

Danish Standards (DS) $

Estonian Standards (EVS) $

German stand­ards (DIN) – Beuth Verlag GmbH

Inconsistencies in ISO 13849 – 1:2006

This entry is part 7 of 8 in the series Circuit Architectures Explored

I’ve writ­ten quite a bit recently on the top­ic of cir­cuit archi­tec­tures under ISO 13849 – 1, and one of my read­ers noticed an incon­sist­ency between the text of the stand­ard and Figure 5, the dia­gram that shows how the cat­egor­ies can span one or more Performance Levels.

ISO 13849-1 Figure 5
ISO 13849 – 1, Figure 5: Relationship between Categories, DC, MTTFd and PL

If you look at Category 2 in Figure 5, you will notice that there are TWO bands, one for DCavg LOW and one for DCavg MED. However, read­ing the text of the defin­i­tion for Category 2 gives (§6.2.5):

The dia­gnost­ic cov­er­age (DCavg) of the total SRP/​CS includ­ing fault-​detection shall be low.

This leaves some con­fu­sion, because it appears from the dia­gram that there are two options for this archi­tec­ture. This is backed up by the data in Annex K that under­lies the dia­gram.

The same con­fu­sion exists in the text describ­ing Category 3, with Figure 5 show­ing two bands, one for DCavg LOW and one for DCavg MED.

I con­tac­ted the ISO TC199 Secretariat, the people respons­ible for the con­tent of ISO 13849 – 1, and poin­ted out this appar­ent con­flict. They respon­ded that they would pass the com­ment on to the TC for res­ol­u­tion, and would con­tact me if they needed addi­tion­al inform­a­tion. As of this writ­ing, I have not heard more.

So what should you do if you are try­ing to design to this stand­ard? My advice is to fol­low Figure 5. If you can achieve a DCavg MED in your design, it is com­pletely reas­on­able to claim a high­er PL. Refer to the data in Annex K to see where your design falls once you have com­pleted the MTTFd cal­cu­la­tions.

Thanks to Richard Harris and Douglas Florence, both mem­bers of the ISO 13849 and IEC 62061 Group on LinkedIn for bring­ing this to my atten­tion!

If you are inter­ested in con­tact­ing the TC199 Secretariat, you can email the Secretary, Mr. Stephen Kennedy. More details on ISO TC199 can be found on the Technical Committee page on the ISO web Site.