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 (DC_avg) of the total SRP/​CS includ­ing fault-​​detection shall be low. The MTTF_d of each of the redun­dant chan­nels shall be low-​​to-​​high, depend­ing on the PL_r. 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 B10_d or MTTF_d 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 (DC_avg) 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 DC_avg, we need to look first at Table 6:

Based on Table 6, the DC_avg 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 MTTF_d of each of the redun­dant chan­nels shall be low-​​to-​​high, depend­ing on the PL_r.

This sen­tence reminds you that your com­po­nent selec­tions mat­ter. Depending on the PL_r you are try­ing to achieve, you will need to choose com­po­nents with suit­able MTTF_d 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 PL_a through PL_e!

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 PL_r 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.

By 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 MTTF_d 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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