Tag Archives: EN ISO 13849-1 - Page 2

Interlock Architectures – Pt. 5: Category 4 — Control Reliable

Posted by on September 26, 2011 Comments Off
This entry is part 5 of 8 in the series Circuit Architectures Explored

The most reli­able of the five sys­tem archi­tec­tures, Category 4 is the only archi­tec­ture that uses multiple-​​fault tol­er­ant tech­niques to help ensure that com­po­nent fail­ures do not result in an unac­cept­able expo­sure to risk. This post will delve into the depths of this archi­tec­ture in this install­ment on sys­tem archi­tec­tures. The def­i­n­i­tions and require­ments dis­cussed in this arti­cle come from ISO 13849–1, Edition 2 (2006) and ISO 13849–2, Edition 1 (2003).

As with pre­ced­ing arti­cles in this series, I’ll be build­ing on con­cepts dis­cussed in those arti­cles. If you need more infor­ma­tion, you should have a look at the pre­vi­ous arti­cles to see if I’ve answered your ques­tions there.

The Definition

The Category 4 def­i­n­i­tion builds on both Category B and Category 3. As you read, recall that “SRP/​CS” stands for “Safety Related Parts of the Control System”. Here is the com­plete definition:

6.2.7 Category 4
For cat­e­gory 4, the same require­ments as those accord­ing to 6.2.3 for cat­e­gory B shall apply. “Well-​​tried safety prin­ci­ples” accord­ing to 6.2.4 shall also be fol­lowed. In addi­tion, the fol­low­ing applies.
SRP/​CS of cat­e­gory 4 shall be designed such that

  • a sin­gle fault in any of these safety-​​related parts does not lead to a loss of the safety func­tion, and
  • the sin­gle fault is detected at or before the next demand upon the safety func­tions, e.g. imme­di­ately, at switch on, or at end of a machine oper­at­ing cycle, but if this detec­tion is not pos­si­ble, then an accu­mu­la­tion of unde­tected faults shall not lead to the loss of the safety function.

The diag­nos­tic cov­er­age (DCavg) of the total SRP/​CS shall be high, includ­ing the accu­mu­la­tion of faults. The MTTFd of each of the redun­dant chan­nels shall be high. Measures against CCF shall be applied (see
Annex F).

NOTE 1 Category 4 sys­tem behav­iour allows that

  • when a sin­gle fault occurs the safety func­tion is always performed,
  • the faults will be detected in time to pre­vent the loss of the safety function,
  • accu­mu­la­tion of unde­tected faults is taken into account.

NOTE 2 The dif­fer­ence between cat­e­gory 3 and cat­e­gory 4 is a higher DCavg in cat­e­gory 4 and a required MTTFd of each chan­nel of “high” only.

In prac­tice, the con­sid­er­a­tion of a fault com­bi­na­tion of two faults may be sufficient.

5% Discount on ISO and IEC Standards with code: CC2011

Breaking it down

For cat­e­gory 4, the same require­ments as those accord­ing to 6.2.3 for cat­e­gory B shall apply. “Well-​​tried safety prin­ci­ples” accord­ing to 6.2.4 shall also be followed.

The first two sen­tences give the basic require­ment for all the cat­e­gories from 2 through 4. Sound com­po­nent selec­tion based on the appli­ca­tion require­ments for volt­age, cur­rent, switch­ing capa­bil­ity and life­time must be con­sid­ered. In addi­tion, using well tried safety prin­ci­ples, such as switch­ing the +V rail side of the coil cir­cuit for con­trol com­po­nents is required. If you aren’t sure about what con­sti­tutes a “well-​​tried safety prin­ci­ple”, see the arti­cle on Category 2 where this is dis­cussed. Don’t con­fuse “well-​​tried safety prin­ci­ples” with “well-​​tried com­po­nents”. There is no require­ment in Category 4 for the use of well-​​tried com­po­nents, although you can use them for addi­tional reli­a­bil­ity if the design require­ments warrant.

In addi­tion, the fol­low­ing applies.
SRP/​CS of cat­e­gory 4 shall be designed such that

  • a sin­gle fault in any of these safety-​​related parts does not lead to a loss of the safety func­tion, and
  • the sin­gle fault is detected at or before the next demand upon the safety func­tions, e.g. imme­di­ately, at switch on, or at end of a machine oper­at­ing cycle, but if this detec­tion is not pos­si­ble, then an accu­mu­la­tion of unde­tected faults shall not lead to the loss of the safety function.

This is the big one. This para­graph, and the two bul­lets that fol­low it, define the fun­da­men­tal per­for­mance require­ments for this cat­e­gory. No sin­gle fault can lead to the loss of the safety func­tion in Category 4, and test­ing is required that can detect fail­ures and pre­vent an accu­mu­la­tion of faults that could even­tu­ally lead to the loss of the safety func­tion. The sec­ond bul­let is the one that defines the multiple-​​fault-​​tolerance require­ment for this cat­e­gory. If you go back to the def­i­n­i­tion of Category 3, you will see that an accu­mu­la­tion of faults may lead to the loss of the safety func­tion in that Category. This is the key dif­fer­ence between the cat­e­gories in my opinion.

The diag­nos­tic cov­er­age (DCavg) of the total SRP/​CS shall be high, includ­ing the accu­mu­la­tion of faults. The MTTFd of each of the redun­dant chan­nels shall be high. Measures against CCF shall be applied (see
Annex F).

These three sen­tences give the designer the cri­te­ria for diag­nos­tic cov­er­age, chan­nel fail­ure rates and com­mon cause fail­ure pro­tec­tion. As you can see, the abil­ity to diag­nose fail­ures auto­mat­i­cally is a crit­i­cal part of the design, as is the use of highly reli­able com­po­nents, lead­ing to highly reli­able chan­nels. The strongest CCF pro­tec­tion you can include in the design is also needed, although the “pass­ing score” of 65 remains unchanged (see Annex F in ISO 13849–1 for more details on scor­ing your design).

NOTE 1 Category 4 sys­tem behav­iour allows that

  • when a sin­gle fault occurs the safety func­tion is always performed,
  • the faults will be detected in time to pre­vent the loss of the safety function,
  • accu­mu­la­tion of unde­tected faults is taken into account.

Note 2: …In prac­tice, the con­sid­er­a­tion of a fault com­bi­na­tion of two faults may be sufficient.

Note 1 expands on the first para­graph in the def­i­n­i­tion, fur­ther clar­i­fy­ing the per­for­mance require­ments by explicit state­ments. Notice that nowhere is there a require­ment that sin­gle faults or accu­mu­la­tion of sin­gle faults be pre­vented, only detected by the diag­nos­tic sys­tem. Prevention of sin­gle faults is nearly impos­si­ble, since com­po­nents do fail. It is impor­tant to first under­stand which com­po­nents are crit­i­cal to the safety func­tion, and sec­ond, what kinds of faults each com­po­nent is likely to have, is fun­da­men­tal to being able to design a diag­nos­tic sys­tem that can detect the faults.

The cat­e­gory relies on redun­dancy to ensure that the com­plete loss of one chan­nel will not cause the loss of the safety func­tion, but this is only use­ful if the com­mon cause fail­ures have been prop­erly dealt with. Otherwise, a sin­gle event could wipe out both chan­nels simul­ta­ne­ously, caus­ing the loss of the safety func­tion and pos­si­bly result in an injury or fatality.

Also notice that mul­ti­ple sin­gle faults are per­mit­ted, as long as the accu­mu­la­tion does not result in the loss of the safety func­tion. ISO 13849 allows for “fault exclu­sion”, a con­cept that is not used in the North American standards.

The final sen­tence from Note 2 sug­gests that con­sid­er­a­tion of two con­cur­rent faults may be enough, but be care­ful. You need to look closely at the fault lists to see if there are any groups of high prob­a­bil­ity faults that are likely to occur con­cur­rently. IF there are, you need to assess these com­bi­na­tions of faults, whether there are 5 or 50 to be evaluated.

Fault Exclusion

Fault exclu­sion involves assess­ing the types of faults that can occur in each com­po­nent in the crit­i­cal path of the sys­tem. The deci­sion to exclude cer­tain kinds of faults is always a tech­ni­cal com­pro­mise between the the­o­ret­i­cal improb­a­bil­ity of the fault, the exper­tise of the designer(s) and engi­neers involved and the spe­cific tech­ni­cal require­ments of the appli­ca­tion. Whenever the deci­sion is made to exclude a par­tic­u­lar type of fault, the deci­sion and the process used to make it must be doc­u­mented in the Reliability Report included in the design file. Section 7.3 of ISO 13849–1 pro­vides guid­ance on fault exclusion.

In the sec­tion dis­cussing Category 1, the stan­dard has this to say about fault exclu­sion, and the dif­fer­ence between “well-​​tried com­po­nents” and “fault exclusion”:

It is impor­tant that a clear dis­tinc­tion between “well-​​tried com­po­nent” and “fault exclu­sion” (see Clause 7) be made. The qual­i­fi­ca­tion of a com­po­nent as being well-​​tried depends on its appli­ca­tion. For exam­ple, a posi­tion switch with pos­i­tive open­ing con­tacts could be con­sid­ered as being well-​​tried for a machine tool, while at the same time as being inap­pro­pri­ate for appli­ca­tion in a food indus­try — in the milk indus­try, for instance, this switch would be destroyed by the milk acid after a few months. A fault exclu­sion can lead to a very high PL, but the appro­pri­ate mea­sures to allow this fault exclu­sion should be applied dur­ing the whole life­time of the device. In order to ensure this, addi­tional mea­sures out­side the con­trol sys­tem may be nec­es­sary. In the case of a posi­tion switch, some exam­ples of these kinds of mea­sures are

  • means to secure the fix­ing of the switch after its adjustment,
  • means to secure the fix­ing of the cam,
  • means to ensure the trans­verse sta­bil­ity of the cam,
  • means to avoid over-​​travel of the posi­tion switch, e.g. ade­quate mount­ing strength of the shock absorber and any align­ment devices, and
  • means to pro­tect it against dam­age from outside.

To assist the designer, ISO 13849–2 pro­vides lists of typ­i­cal faults and the allow­able exclu­sions in Annex D.5. As an exam­ple, let’s con­sider the typ­i­cal sit­u­a­tion where a robust guard inter­lock­ing device has been selected. The deci­sion has been made to use redun­dant elec­tri­cal cir­cuits to the switch­ing com­po­nents in the inter­lock, so elec­tri­cal faults can be detected. But what about mechan­i­cal fail­ures? A fault list is needed:

 Interlock Mechanical Fault List
#Fault DescriptionResultLikelihood
1Key breaks offControl sys­tem can­not deter­mine guard posi­tion. Complete fail­ure of sys­tem through a sin­gle fault.Unlikely
2Screws mount­ing key to guard failControl sys­tem can­not deter­mine guard posi­tion. Complete fail­ure of sys­tem through a sin­gle fault.Unlikely
3Screws mount­ing inter­lock device to guard failControl sys­tem can­not deter­mine guard posi­tion. Complete fail­ure of sys­tem through a sin­gle fault.Unlikely
4Key and inter­lock device misaligned.Guard can­not close, pre­vent­ing machine from operating.Very likely
5Key and inter­lock device misaligned.Key and /​ or inter­lock device dam­aged. Guard may not close, or the key may jam in the inter­lock device once closed. Machine is inop­er­a­ble if the inter­lock can­not be com­pleted, or the guard can­not be opened if the key jams in the device.Likely
6Screws mount­ing key to guard removed by user.Interlock can now be bypassed by fix­ing the key into the inter­lock­ing device. Control sys­tem can no longer sense the posi­tion of the guard.Likely
7Screws mount­ing inter­lock device to guard removed by userProbably com­bined with the pre­ced­ing con­di­tion. Control sys­tem can no longer sense the posi­tion of the guard.Unlikely, but could happen.

There may be more fail­ure modes, but for the pur­pose of this dis­cus­sion, lets limit them to this list.

Looking at Fault 1, there are a num­ber of things that could result in a bro­ken key. They include: mis­align­ment of the key and the inter­lock device, lack of main­te­nance on the guard and the inter­lock­ing hard­ware, or inten­tional dam­age by a user. Unless the hard­ware is excep­tion­ally robust, includ­ing the design of the guard and any align­ment fea­tures incor­po­rated in the guard­ing, devel­op­ing sound ratio­nale for exclud­ing this fault will be very difficult.

Fault 2 con­sid­ers mechan­i­cal fail­ure of the mount­ing screws for the inter­lock key. Screws are con­sid­ered to be well-​​tried com­po­nents (see Annex A.5), so you can con­sider them for fault exclu­sion. You can improve their reli­a­bil­ity by using thread lock­ing adhe­sives when installing the screws to pre­vent them from vibrat­ing loose, and “tamper-​​proof” style screw heads to deter unau­tho­rized removal. Inclusion of these meth­ods will sup­port any deci­sion to exclude these faults. This goes to address­ing faults 3, 6 and 7 as well.

Faults 4 & 5 occur fre­quently and are often caused by poor device selec­tion (i.e. an inter­lock device intended for straight-​​line sliding-​​gate appli­ca­tions is cho­sen for a hinged gate), or by poor guard design (i.e. the guard is poorly guided by the reten­tion mech­a­nism and can be closed in a mis­aligned con­di­tion). Rationale for pre­ven­tion of these faults will need to include dis­cus­sion of design fea­tures that will pre­vent these conditions.

Excluding any other kind of fault fol­lows the same process: Develop the fault list, assess each fault against the rel­e­vant Annex from ISO 13849–2, deter­mine if there are pre­ven­ta­tive mea­sures that can be designed into the prod­uct and whether these pro­vide suf­fi­cient risk reduc­tion to allow the exclu­sion of the fault from consideration.

DCavg and MTTFd requirements

NOTE 2 The dif­fer­ence between cat­e­gory 3 and cat­e­gory 4 is a higher DCavg in cat­e­gory 4 and a required MTTFd of each chan­nel of “high” only.

The first sen­tence in Note 2 clar­i­fies the two main dif­fer­ences from a design stand­point, aside from the addi­tional fault tol­er­ance require­ments: Better diag­nos­tics are required and much higher require­ments for indi­vid­ual com­po­nent, and there­fore chan­nel, MTTFd.

The Block Diagram

The block dia­gram for Category 4 is almost iden­ti­cal to Category 3, and was updated by Corrigendum 1 to the dia­gram shown below. The text from the cor­ri­gen­dum that accom­pa­nies the dia­gram has this to say about the change:

Replace the draw­ing show­ing the des­ig­nated archi­tec­ture for cat­e­gory 4 with the fol­low­ing draw­ing. This
cor­rects the arrowed lines labeled “m” between L1 and O1, and L2 and O2, by chang­ing them from dashed to solid lines, rep­re­sent­ing higher diag­nos­tic coverage.

I’ve high­lighted this area using red ovals on Figure 12 to make it eas­ier to see .

ISO 13849-1 Figure 12 - Category 4 Block Diagram

ISO 13849–1 Figure 12 — Category 4 Block Diagram

Here is Figure 11 for com­par­i­son. Notice that the “m” lines are solid in Figure 12 and dashed in Figure 11? Subtle, but sig­nif­i­cant! There are no other dif­fer­ences between the diagrams.

ISO 13849-1 Figure 11I went look­ing for a cir­cuit dia­gram to sup­port the block dia­gram, but wasn’t able to find one from a com­mer­cial source that I could share with you. Considering that the pri­mary dif­fer­ences are in the reli­a­bil­ity of the com­po­nents cho­sen and in the way the test­ing is done, this isn’t too sur­pris­ing. The basic phys­i­cal con­struc­tion of the two cat­e­gories can be vir­tu­ally identical.

Applications

The fol­low­ing is not from the stan­dards — this is my per­sonal opin­ion, based on 15 years of practice.

In the past, many man­u­fac­tur­ers decided that they were going to apply Category 4 archi­tec­ture with­out really under­stand­ing the design impli­ca­tions, because they believed that it was “the best”. With the change in the har­mo­niza­tion of EN 954–1 and ISO 13849–1 under the EU machin­ery direc­tive that comes into force on 29-​​Dec-​​2011, and con­sid­er­ing the great dif­fi­culty that many man­u­fac­tur­ers had in prop­erly imple­ment­ing EN 954–1, I can eas­ily imag­ine man­u­fac­tur­ers who have taken the approach that they already have Category 4 SRP/​CS on their sys­tems and mak­ing the state­ment that they now have PLe SRP/​CS sys­tem per­for­mance. This is a bad deci­sion for a lot of reasons:

  1. ISO 13849–1 PLe, Category 4 sys­tems should be reserved for very dan­ger­ous machin­ery where the tech­ni­cal effort and expense involved is war­ranted by the risk assess­ment. Attempting to apply this level of design to machin­ery where a PLb per­for­mance level is more suit­able based on a risk assess­ment, is a waste of design time and effort and a need­less expense. The prod­uct fam­ily stan­dards for these types of machines, such as EN 201 for plas­tic injec­tion mould­ing machines, or EN 692 for Mechanical Power Presses or EN 693 for Hydraulic Power Presses will explic­itly spec­ify the PL level required for these machines.
  2. Manufacturers have fre­quently claimed EN 954–1 Category 4 per­for­mance based on the rat­ing of the safety relay alone, with­out under­stand­ing that the rest of the SRP/​CS must be con­sid­ered, and clearly this is wrong. The SRP/​CS must be eval­u­ated as a com­plete system.

This lack of under­stand­ing endan­gers the users, the main­te­nance per­son­nel, the own­ers and the man­u­fac­tur­ers. If they con­tinue this approach and an injury occurs, it is my opin­ion that the courts will have more than enough evi­dence in the defendant’s pub­lished doc­u­ments to cause some seri­ous legal grief.

As design­ers involved with the safety of our company’s prod­ucts or with our co-worker’s safety, I believe that we owe it to every­one who uses our prod­ucts to be edu­cated and to cor­rectly apply these con­cepts. The fact that you have read all of the posts lead­ing up to this one is evi­dence that you are work­ing on get­ting educated.

Always con­duct a risk assess­ment and use the out­come from that work to guide your selec­tion of safe­guard­ing mea­sures, com­ple­men­tary pro­tec­tive mea­sures and the per­for­mance of the SRP/​CS that ties those sys­tems together. Choose per­for­mance lev­els that make sense based on the required risk reduc­tion and ensure that the design cri­te­ria is met by val­i­dat­ing the sys­tem once built.

As always, I wel­come your com­ments and ques­tions! Please feel free to com­ment below. I will respond to all your comments.

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Acknowledgements: ISO for excerpts from ISO 13849–1 and more…
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Interlock Architectures – Pt. 4: Category 3 — Control Reliable

Posted by on September 19, 2011 Comments Off
ISO 13849-1 Figure 11
This entry is part 4 of 8 in the series Circuit Architectures Explored

Category 3 sys­tem archi­tec­ture is the first cat­e­gory that could be con­sid­ered to have sim­i­lar­ity to “Control Reliable” cir­cuits or sys­tems as defined in the North American stan­dards. It is not the same as Control Reliable, but we’ll get to in a fol­low­ing post. If you haven’t read the first three posts in this series, you may want to go back and review them as the con­cepts in those arti­cles are the basis for the dis­cus­sion in this post.

So what is “Control Reliable” any­way? This term was coined by the ANSI RIA R15.06 tech­ni­cal com­mit­tee when they were devel­op­ing their def­i­n­i­tions for con­trol sys­tem reli­a­bil­ity, first pub­lished in the 1999 edi­tion of the stan­dard. No men­tion of the con­cept of con­trol reli­a­bil­ity appears in the 1994 edi­tion of CSA Z434 or the pre­ced­ing edi­tion of RIA R15.06.

Essentially, the term “Control Reliable” means that the con­trol sys­tem is designed with some degree of fault tol­er­ance. Depending on the def­i­n­i­tions that you read, this could be sin­gle– or multiple-​​fault-​​tolerance.

There are a num­ber of design tech­niques that can be used to increase the fault tol­er­ance of a con­trol sys­tem. The older approaches, such as those given in ANSI RIA R15.06–1999, CSA Z434-​​03 or EN 954–1:95, rely pri­mar­ily on the struc­ture or archi­tec­ture of the cir­cuit, and the char­ac­ter­is­tics of the com­po­nents selected for use. ISO 13849–1 uses the same basic archi­tec­tures defined by EN 954–1:95, and extends them to include diag­nos­tic cov­er­age, com­mon cause fail­ure resis­tance and an under­stand­ing of the fail­ure rate of the com­po­nents to deter­mine the degree of fault tol­er­ance and reli­a­bil­ity pro­vided by the design.

OK, enough back­ground for now! Let’s look at the def­i­n­i­tion for Category 3 sys­tems. Remember that “SRP/​CS” means “Safety Related Parts of the Control System”.

Definition

6.2.6 Category 3

For cat­e­gory 3, the same require­ments as those accord­ing to 6.2.3 for cat­e­gory B shall apply. “Well-​​tried safety prin­ci­ples” accord­ing to 6.2.4 shall also be fol­lowed. In addi­tion, the fol­low­ing applies. SRP/​CS of cat­e­gory 3 shall be designed so that a sin­gle fault in any of these parts does not lead to the loss of the safety func­tion. Whenever rea­son­ably prac­ti­ca­ble, the sin­gle fault shall be detected at or before the next demand upon the safety function.

The diag­nos­tic cov­er­age (DCavg) of the total SRP/​CS includ­ing fault-​​detection shall be low. The MTTFd of each of the redun­dant chan­nels shall be low-​​to-​​high, depend­ing on the PLr. Measures against CCF shall be applied (see Annex F).

NOTE 1 The require­ment of single-​​fault detec­tion does not mean that all faults will be detected. Consequently, the accu­mu­la­tion of unde­tected faults can lead to an unin­tended out­put and a haz­ardous sit­u­a­tion at the machine. Typical exam­ples of prac­ti­ca­ble mea­sures for fault detec­tion are use of the feed­back of mechan­i­cally guided relay con­tacts and mon­i­tor­ing of redun­dant elec­tri­cal outputs.

NOTE 2 If nec­es­sary because of tech­nol­ogy and appli­ca­tion, type-​​C stan­dard mak­ers need to give fur­ther details on the detec­tion of faults.

NOTE 3 Category 3 sys­tem behav­iour allows that

  • when the sin­gle fault occurs the safety func­tion is always performed,
  • some but not all faults will be detected,
  • accu­mu­la­tion of unde­tected faults can lead to the loss of the safety function.

NOTE 4 The tech­nol­ogy used will influ­ence the pos­si­bil­i­ties for the imple­men­ta­tion of fault detection.

5% Discount on ISO and IEC Standards with code: CC2011

Breaking it down

Let’s take the def­i­n­i­tion apart and look at the com­po­nents that make it up.

For cat­e­gory 3, the same require­ments as those accord­ing to 6.2.3 for cat­e­gory B shall apply. “Well-​​tried safety prin­ci­ples” accord­ing to 6.2.4 shall also be followed.

The first cou­ple of lines remind the designer of two key points:

  • The com­po­nents selected must be suit­able for the appli­ca­tion, i.e. cor­rectly spec­i­fied for volt­age, cur­rent, envi­ron­men­tal con­di­tions, etc.; and
  • “well-​​tried safety prin­ci­ples” must be used in the design.

It’s impor­tant to note here that we are talk­ing about “well tried safety prin­ci­ples” and NOT “well-​​tried com­po­nents”. The require­ment to use com­po­nents designed for safety appli­ca­tions comes from other stan­dards, like EN 1088 and ISO 13850. The require­ments from these stan­dards, such as the use of “direct-​​drive” con­tacts improves the fault tol­er­ance of the com­po­nent, and so ben­e­fits the design in the end. These improve­ments are gen­er­ally reflected in the B10d or MTTFd of the com­po­nent, and are points that inspec­tors will com­monly look for, since they are easy to spot in the field, since “safety-​​rated com­po­nents” often use red or yel­low caps to iden­tify them clearly in the con­trol panel.

In addi­tion, the fol­low­ing applies. SRP/​CS of cat­e­gory 3 shall be designed so that a sin­gle fault in any of these parts does not lead to the loss of the safety function.

This sen­tence makes the require­ment for single-​​fault tol­er­ance. This means that the fail­ure of any sin­gle com­po­nent in the func­tional chan­nel can­not result in the loss of the safety func­tion. To meet this require­ment, redun­dancy is needed. With redun­dant sys­tems, one com­plete chan­nel can fail with­out los­ing the abil­ity to stop the machin­ery. It is pos­si­ble to lose the func­tion of the mon­i­tor­ing sys­tem from a sin­gle com­po­nent fail­ure, but as long as the sys­tem con­tin­ues to pro­vide the safety func­tion this may be accept­able. The sys­tem should not per­mit itself to be reset if the mon­i­tor­ing sys­tem is not working.

One more “gotcha” from this sen­tence: In order to meet the require­ment that any sin­gle com­po­nent fail­ure can be detected, the design will require two sep­a­rate sen­sors to detect the posi­tion of a gate, for exam­ple. This per­mits the sys­tem to detect a fail­ure in either sen­sor, includ­ing mechan­i­cal fail­ures like bro­ken keys or attempts to defeat the safety sys­tem. You can clearly see this in both the block dia­gram, which does not show any mon­i­tor­ing con­nec­tion to the input devices, and in the cir­cuit dia­gram. Both of these dia­grams are shown later in this post. The only way out of the require­ment to have redun­dant sen­sors is to select a gate switch that is robust enough that mechan­i­cal faults can rea­son­ably be excepted. I’ll get into fault excep­tions later in this article.

Whenever rea­son­ably prac­ti­ca­ble, the sin­gle fault shall be detected at or before the next demand upon the safety function.

This sen­tence can be a bit sticky. The phrase “Whenever rea­son­ably prac­ti­ca­ble” means that your design needs to be able to detect sin­gle faults unless it would be “unrea­son­able” to do so. What con­sti­tutes an unrea­son­able degree of effort? This is for you to decide. I will say that if there is a com­mon, off the shelf com­po­nent (COTS) avail­able that will do the job, and you choose not to use it, you will have a dif­fi­cult time con­vinc­ing a court that you took every rea­son­ably prac­ti­ca­ble means to detect the fault.

Following the comma, the rest of the sen­tence pro­vides the designer with the basic require­ment for the test sys­tem: it must be able to detect a sin­gle com­po­nent fail­ure at the moment of demand (this is usu­ally how it’s done, since this is typ­i­cally the sim­plest way) or before it occurs, which can hap­pen if your test equip­ment has a means to detect a change in some crit­i­cal char­ac­ter­is­tic of the mon­i­tored component(s).

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

This sen­tence tells you that your design must meet the require­ments for LOW Diagnostic Coverage. To get to LOW DCavg, we need to look first at Table 6:

ISO 13849–1:06 Table 6

Diagnostic Coverage (DC)

Denotation Range
 None DC < 60%
 Low 60% ? DC < 90%
 Medium 90% ? DC < 99%
 High 99% ? DC
NOTE 1 For SRP/​CS con­sist­ing of sev­eral parts an aver­age value DCavg for DC is used in Figure 5, Clause 6 and E.2.

NOTE 2 The choice of the DC ranges is based on the key val­ues 60 %, 90 % and 99 % also estab­lished in other stan­dards (e.g. IEC 61508) deal­ing with diag­nos­tic cov­er­age of tests. Investigations show that (1 — DC) rather than DC itself is a char­ac­ter­is­tic mea­sure for the effec­tive­ness of the test. (1 — DC) for the key val­ues 60 %, 90 % and 99 % forms a kind of log­a­rith­mic scale fit­ting to the log­a­rith­mic PL-​​scale. A DC-​​value less than 60 % has only slight effect on the reli­a­bil­ity of the tested sys­tem and is there­fore called “none”. A DC-​​value greater than 99 % for com­plex sys­tems is very hard to achieve. To be prac­ti­ca­ble, the num­ber of ranges was restricted to four. The indi­cated bor­ders of this table are assumed within an accu­racy of 5 %.

Based on Table 6, the DCavg must be between 60% and 90%, all com­po­nents con­sid­ered. To score this, we must go to Annex E and look at Table E1. Using the fac­tors in Table E1, score the design. If you end up in the desired range between 60% and 90% DC cov­er­age, you can move on. If not, the design will require mod­i­fi­ca­tion to bring it into this range.

The MTTFd of each of the redun­dant chan­nels shall be low-​​to-​​high, depend­ing on the PLr.

This sen­tence reminds you that your com­po­nent selec­tions mat­ter. Depending on the PLr you are try­ing to achieve, you will need to choose com­po­nents with suit­able MTTFd rat­ings. Remember that just because you are using a Category 3 archi­tec­ture, you have not auto­mat­i­cally achieved the high­est lev­els of reli­a­bil­ity. If you refer to Figure 5 in the stan­dard, you can see that a Category 3 archi­tec­ture can meet a range of PL’s, all the way from PLa through PLe!

ISO 13849-1 Figure 5

ISO 13849–1 Figure 5

If you want, or need, to know the numeric bound­aries of each of the bands in the dia­gram above, look at Annex K of the stan­dard. The full numeric rep­re­sen­ta­tion of Figure 5 is pro­vided in that Annex.

Measures against CCF shall be applied (see Annex F).

In order for the archi­tec­ture of your design to meet Category 3 archi­tec­ture, CCF mea­sures are required. I’ve dis­cussed Common Cause Failures else­where on the blog, but as a reminder, a Common Cause Failure is one where a sin­gle event, like a light­ning strike on the power line, or a cable being cut, results in the fail­ure of the sys­tem. This is not the same as a Common Mode Failure, where sim­i­lar or dif­fer­ent com­po­nents fail in the same way. For instance, if both out­put con­tac­tors were to weld closed either simul­ta­ne­ously or at dif­fer­ent time due to over­load­ing because they were under­sized, this could be con­sid­ered to be a Common Mode Failure. If they both weld closed due to a light­ning strike, that is a Common Cause Failure.

Annex F pro­vides a check­list that is used to score the CCF of the design. The design must meet at least 65 points to be con­sid­ered to meet the min­i­mum level of CCF pro­tec­tion, and more is bet­ter of course! Score your design and see where you come out. Less than 65 and you need to do more. 65 or more and you are good to go.

The Notes

The notes given in the def­i­n­i­tion are also impor­tant. Note 1 reminds the designer that not all faults will be detected, and an accu­mu­la­tion of unde­tected faults can lead to the loss of the safety func­tion. Be aware that it is up to you as the designer to min­i­mize the kinds of fail­ures that can accu­mu­late undetected.

Note 2 speaks to the pos­si­bil­ity that a Type-​​C prod­uct stan­dard, like EN 201 for injec­tion mould­ing machines for exam­ple, may impose a min­i­mum PLr on the design. Make sure that you get a copy of any Type-​​C stan­dard that is rel­e­vant for your prod­uct and mar­ket. Note that the des­ig­na­tion “Type-​​C” comes from ISO. If you go look­ing for this ter­mi­nol­ogy in ANSI or CSA stan­dards, you won’t find it used because the con­cept doesn’t exist in the same way in these National standards.

Note 3 gives you the basic per­for­mance para­me­ters for the design. If your design can do these things, then you’re halfway there.

Finally, Note 4 is a reminder that dif­fer­ent kinds of tech­nol­ogy have greater or lesser capa­bil­ity to detect fail­ures. More sophis­ti­cated tech­nol­ogy may be required to achieve the PL level you need.

The Block Diagram

Let’s have a look at the func­tional block dia­gram for this Category.

ISO 13849-1 Figure 11By look­ing at the dia­gram you can see clearly the two inde­pen­dent chan­nels and the cross-​​monitoring con­nec­tion between the chan­nels. Input devices are not mon­i­tored, but out­put devices are mon­i­tored. This is another sig­nif­i­cant rea­son requir­ing the use of two phys­i­cally sep­a­rate input devices to sense the guard posi­tion or what­ever other safe­guard­ing device is inte­grated into the sys­tem. The only way that a fail­ure in the input devices can be detected is if one chan­nel changes state and one does not.

If you want to learn more about apply­ing the block dia­gram­ming method to you design, there is a good expla­na­tion of the method in the SISTEMA Cookbook 1, pub­lished by the IFA in Germany. You can down­load the English ver­sion from the link above, or get the doc­u­ment directly from the IFA web site.

Circuit Diagram

By now you prob­a­bly get the idea that there are as many ways to con­fig­ure a Category 3 cir­cuit as there are appli­ca­tions. Below is a typ­i­cal cir­cuit dia­gram bor­rowed from Rockwell Allen-​​Bradley, show­ing the appli­ca­tion of typ­i­cal safety relays in a com­plete sys­tem that includes the emer­gency stop sys­tem, a gate inter­lock and a safety mat. You can meet the require­ments for Category 3 archi­tec­ture in other ways, so don’t feel that you must use a COTS safety relay. It just may be the most straight­for­ward way in many cases.

This is not a plug for A-​​B prod­ucts. Neither Machinery Safety 101, nor I, have any rela­tion­ship with Rockwell Allen-​​Bradley.

From Rockwell Automation pub­li­ca­tion SAFETY-​​WD001A-​​EN-​​P – June 2011, p.6.

If you’re inter­ested in obtain­ing the source doc­u­ment con­tain­ing this dia­gram, you can down­load it directly from the Rockwell Automation web site.

Emergency Stop Subsystem

The emer­gency stop cir­cuit uses the 440R-​​512R2 relay on the left side of the dia­gram. This par­tic­u­lar sys­tem uses Category 3 archi­tec­ture in the e-​​stop sys­tem, which may be more than is required. A risk assess­ment and a start-​​stop analy­sis is required to deter­mine what per­for­mance level is needed for this sub­sys­tem. Get more infor­ma­tion on emer­gency stop.

 Gate Interlock Subsystem

The gate inter­lock cir­cuit is located in the cen­ter of the dia­gram, and uses the 440R-​​D22R2 relay. As you can see, there are two phys­i­cally sep­a­rate gate inter­lock switches. Only one con­tact from each switch is used, so one switch is con­nected to Channel 1, and the other to Channel 2. Notice that there is no other mon­i­tor­ing of these devices (i.e. no sec­ond con­nec­tion to either switch). The sec­ondary con­tacts on these switches could be con­nected to the PLC for annun­ci­a­tion pur­poses. This would allow the PLC to dis­play the open/​closed sta­tus of the gate on the machine HMI.

The out­put con­tac­tors, K3 and K4, are mon­i­tored by the reset loop con­nected to S34 and the +V rail.

One more inter­est­ing point — did you notice that there is a “zone e-​​stop” included in the gate inter­lock? If you look imme­di­ately below the cen­tral safety relay and a lit­tle to the left you will find an emer­gency stop device. This device is wired in series with the gate inter­lock, so acti­vat­ing it will drop out K3 and K4 but not dis­turb the oper­a­tion of the rest of the machine. The safety relay can’t dis­tin­guish between the e-​​stop but­ton and the gate inter­locks, so if annun­ci­a­tion is needed, you may want to use a third con­tact on the e-​​stop device to con­nect to a PLC input for this purpose.

Safety Mat Subsystem

The safety mat sub­sys­tem is located on the right side of the dia­gram and uses a sec­ond 440R-​​D22R2 relay. Safety mats can be either sin­gle or dual chan­nel in design. The mat show in this draw­ing is a dual-​​channel type. Stepping on the mat causes the con­duc­tive lay­ers in the mat to touch, short­ing Channel 1 to Channel 2. This cre­ates an input fault that will be detected by the 440R relay. The fault con­di­tion will cause the out­put of the relay to open, stop­ping the machine.

Safety mats can be dam­aged rea­son­ably eas­ily, and the cir­cuit design shown will detect shorts or opens within the mat and will pre­vent the haz­ardous motion from start­ing or continuing.

The out­put con­tac­tors, K5 and K6 are mon­i­tored by the relay reset loop con­nected to S34 and the +V rail.

This cir­cuit also includes a con­ven­tional start-​​stop cir­cuit that doesn’t rely on the safety relay.

One more thing — just like the gate inter­lock cir­cuit, this cir­cuit also includes a “zone e-​​stop”. Look below and to the left of the safety mat relay. As with the gate inter­lock, press­ing this but­ton will drop out K5 and K6, stop­ping the same motions pro­tected by the safety mat. Since the relay can’t tell the dif­fer­ence between the e-​​stop but­ton and the mat being acti­vated, you may want to use the same approach and add a third con­tact to the e-​​stop but­ton, con­nect­ing it to the PLC for annunciation.

Component Selection

The com­po­nents used in the cir­cuit are crit­i­cal to the final PL rat­ing of the design. The final PL of the design depends on the MTTFd of the com­po­nents used in each chan­nel. No knowl­edge of the inter­nal con­struc­tion of the safety relays is needed, because the relays come with a PL rat­ing from the man­u­fac­turer. They can be treated as a sub­sys­tem unto them­selves. The selec­tion of the input and out­put devices is then the sig­nif­i­cant fac­tor. Component data sheets can be down­loaded from the Rockwell site if you want to dig a bit deeper.

What did you think about this arti­cle? What ques­tions came to mind that weren’t answered for you? I look for­ward to hear­ing your thoughts and questions!

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Acknowledgements: ISO for excerpts from ISO 13849–1 and more…
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Understanding Risk Assessment

Posted by on January 31, 2011 10 comments

When peo­ple dis­cuss ‘Risk’ there are a lot of dif­fer­ent assump­tions made about what that means. For me, the study of risk and risk assess­ment tech­niques started in 1995. As a tech­nol­o­gist and con­trols designer, I had to some­how wrap my head around the whole con­cept in ways I’d never con­sid­ered. If you’re try­ing to fig­ure out risk and risk assess­ment this is a good place to get started!

What is risk?

From a machin­ery per­spec­tive, ISO 12100:2010 defines risk as:

“com­bi­na­tion of the prob­a­bil­ity of occur­rence of harm and the sever­ity of that harm”

Risk can have pos­i­tive or neg­a­tive out­comes, but when con­sid­er­ing safety, we only con­sider neg­a­tive risk, or events that result in neg­a­tive health effects for the peo­ple exposed.

The risk rela­tion­ship is illus­trated in ISO 12100:2010 Figure 3:


ISO 12100-2010 Figure 3

ISO 12100–2010 Figure 3


Where

R = Risk

S = Severity of Harm

P = Probability of Occurrence of Harm

The Probability of Occurrence of Harm fac­tor is often fur­ther bro­ken down into three sub-​​factors:

  • Probability of Exposure to the haz­ard
  • Probability of Occurrence of the Hazardous Event
  • Probability of Limiting or Avoiding the Harm

How is risk measured?

In order to esti­mate risk a scor­ing tool is needed. There is no one ‘cor­rect’ scor­ing tool, and there are flaws in most scales that can result in blind-​​spots where risks may be over or under-​​estimated.

At the sim­plest level are ‘screen­ing’ tools. These tools use very sim­ple scales like ‘High, Medium, Low’, or ‘A, B, C’. These tools are often used when doing a shop-​​floor inspec­tion and are intended to pro­vide a quick method of cap­tur­ing obser­va­tions and giv­ing a gut-​​feel assess­ment of the risk involved. These tools should be used as a way to iden­tify risks that need addi­tional, detailed assess­ment. To get an idea of what a good screen­ing tool can look like, have a look at the SOBANE Déparis sys­tem.

Every scor­ing tool requires a scale for each risk para­me­ter included in the tool. For instance, con­sider the CSA tool described in CSA Z434:

CSA Z434-03 Table 1As you can see, each para­me­ter (Severity, Exposure and Avoidance) has a scale, with two pos­si­ble selec­tions for each parameter.

When con­sid­er­ing selec­tion of a scor­ing tool, it’s impor­tant to take some time to really exam­ine the scales for each fac­tor. The scale shown above has a glar­ing hole in one scale. See if you can spot it and I’ll tell you what I think a bit later in this post.

There are more than 350 dif­fer­ent scales and method­olo­gies avail­able for assess­ing risk. You can find a good review of some of them in Bruce Main’s text­book “Risk Assessment: Basics and Benchmarks” avail­able from DSE online.

A sim­i­lar, although dif­fer­ent, tool is found in Annex 1 of ISO 13849–1. Note that this tool is pro­vided in an Informative Annex. This means that it is not part of the body of the stan­dard and is NOT manda­tory. In fact, this tool was pro­vided as an exam­ple of how a user could link the out­put of a risk assess­ment tool to the Performance Levels described in the nor­ma­tive text (the manda­tory part) of the standard.

Consider cre­at­ing your own scales. There is noth­ing wrong with deter­min­ing what char­ac­ter­is­tics (para­me­ters) you want to include in your risk assess­ment, and then assign­ing each para­me­ter a numeric scale that you think is suit­able; 1–10, 0–5, etc. Some scales may be inverted to oth­ers, for exam­ple: If the Severity scale runs from 0–10, the Avoidability scale might run from 10–0 (Unavoidable to Entirely Avoidable).

Once the scales in your tool have been defined, doc­u­ment the def­i­n­i­tions as part of your assessment.

Who should con­duct risk assessments?

Lake YogaIn many orga­ni­za­tions, I find that risk assess­ment has been del­e­gated to one per­son. This is a major mis­take for a num­ber of rea­sons. Risk assess­ment is not a solo activ­ity for a ‘guru’ in a lonely office somewhere!

Risk assess­ment is not a lot of fun to do, and since risk assess­ments can get to be quite involved, it rep­re­sents a sig­nif­i­cant amount of work to put on one per­son. Also, leav­ing it to one per­son means that the assess­ment will nec­es­sar­ily be biased to what that per­son knows, and may miss sig­nif­i­cant haz­ards because the asses­sor doesn’t know enough about that haz­ard to spot it and assess it properly.

Risk assess­ment requires mul­ti­ple view­points from par­tic­i­pants with var­ied exper­tise. This includes users, design­ers, engi­neers, lawyers and those who may have spe­cial­ized knowl­edge of a par­tic­u­lar haz­ard, like a Laser Safety Officer or a Radiation Safety Officer. The var­ied exper­tise of the peo­ple involved will allow the com­mit­tee to bal­ance the opin­ion of each haz­ard, and develop a more rea­soned assess­ment of the risk.

I rec­om­mend that risk assess­ment com­mit­tees never be less than three mem­bers. Five is fre­quently a good num­ber. Once you get beyond five, it becomes increas­ingly dif­fi­cult to obtain con­sen­sus on each haz­ard. Also, con­sider the cost. As each com­mit­tee mem­ber is added to the team, the cost of the assess­ment can esca­late exponentially.

Training in risk assess­ment is cru­cial to suc­cess. Ensure that the indi­vid­u­als involved are trained, and that at least one has some pre­vi­ous expe­ri­ence in the prac­tice so that they may guide the com­mit­tee as needed.

When should a risk assess­ment be conducted?


Risk Assessment Lifetime Flow Chart

Risk Assessment in the Lifetime of a Product


Risk assess­ment should begin at the begin­ning of a project, whether it’s the design of a prod­uct, the devel­op­ment of a process or ser­vice, or the design of a new build­ing. Understanding risk is crit­i­cal to the design process. Cost for changes made at the begin­ning of a project are min­i­mal com­pared to those that will be incurred to cor­rect prob­lems that might have been fore­seen at the start. Risk assess­ment should start at the con­cept stage and be included at each sub­se­quent stage in the devel­op­ment process. The accom­pa­ny­ing graphic illus­trates this idea.

Essentially, risk assess­ment is never fin­ished until the prod­uct, process or ser­vice ceases to exist.

What tools are available?

As men­tioned ear­lier in this post, the book ‘Risk Assessment: Basics and Benchmarks” pro­vides an overview of roughly 350 dif­fer­ent scor­ing tools. You can search the Internet and turn up quite a few as well. The key thing with all of these sys­tems is that you will need to develop any soft­ware based tools your­self. Depending on your com­fort with soft­ware, this might be a spread­sheet for­mat, a word pro­cess­ing doc­u­ment a data­base, or some other for­mat that works for your application.

There are a num­ber of risk assess­ment soft­ware tools avail­able as well, includ­ing ISI’s CIRSMA™ and DSE’s DesignSafe. As with the scor­ing tools, you need to be care­ful when eval­u­at­ing tools. Some have sig­nif­i­cant blind spots that may trip you up if you are not aware of their limitations.

Remember too that the out­put from the soft­ware can only be as good as the input data. The old saw “Garbage In, Garbage Out” holds true with risk assessment.

Where can you get training?

There are a few places to get train­ing. Compliance InSight Consulting pro­vides train­ing to cor­po­rate clients and will be launch­ing a series of web-​​based train­ing ser­vices in 2011 that will allow indi­vid­ual learn­ers to get train­ing too.

The IEEE PSES oper­ates a Risk Assessment Technical Committee that is open to the pub­lic as well. See the RATC web site.

The Answer to the Scale Question

The Exposure Scale in the CSA tool has a gap between E1 and E2. Looking at the def­i­n­i­tions for each choice, notice that E1 is less than once per day or shift, while E2 is more than once per hour. Exposures that occur once per hour or less, but more than once per day can­not be scored effec­tively using this scale.

Also, notice the Severity scale: S1 encom­passes injuries requir­ing not more than basic first aid. One com­mon ques­tion I get is “Does that include CPR*?”. This ques­tion comes up because most basic first aid courses taught in Canada include CPR as part of the course. There is no clear answer for this in the stan­dard. The S2 fac­tor extends from injuries requir­ing more than basic first aid, like a bro­ken fin­ger for instance, all the way to a fatal­ity. Does it make sense to group this broad range of injuries together? This def­i­n­i­tion doesn’t quite match with the Province of Ontario’s def­i­n­i­tion of a Critical Injury found in Regulation 834 either.

All of this points to the need to care­fully assess the scales that you choose before you start the process. Choosing the wrong tool can skew your results in ways that you may not be very happy about.

*Cardio-​​Pulmonary Resuscitation

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