Interlock Architectures – Pt. 1: What do those categories really mean?

Basic Stop/Start Circuit
This entry is part 1 of 8 in the series Circuit Architectures Explored

It all started with EN 954 – 1

In 1996 CEN pub­lished an import­ant stand­ard for machine build­ers – EN 954 – 1, “Safety of Machinery – Safety Related Parts of Control Systems – Part 1: General Principles for Design” [1]. This stand­ard set the stage for defin­ing con­trol reli­ab­il­ity in machinery safe­guard­ing sys­tems, intro­du­cing the Reliability cat­egor­ies that have become ubi­quit­ous. So what do these cat­egor­ies mean, and how are they applied under the latest machinery func­tion­al safety stand­ard, ISO 13849 – 1 [2]?

Download ISO Standards 

Circuit Categories

The cat­egor­ies are used to describe sys­tem archi­tec­tures for safety related con­trol sys­tems. Each archi­tec­ture car­ries with it a range of reli­ab­il­ity per­form­ance that can be related to the degree of risk reduc­tion you are expect­ing to achieve with the sys­tem. These archi­tec­tures can be applied equally to elec­tric­al, elec­tron­ic, pneu­mat­ic, hydraul­ic or mech­an­ic­al con­trol systems.

Historical Circuits

Early elec­tric­al ‘master-​control-​relay’ cir­cuits used a simple archi­tec­ture with a single con­tact­or, or some­times two, and a single chan­nel style of archi­tec­ture to main­tain the con­tact­or coil cir­cuit once the START or POWER ON but­ton (PB2 in Fig. 1) had been pressed. Power to the out­put ele­ments of the machine con­trols was sup­plied via con­tacts on the con­tact­or, which is why it was called the Master Control Relay or ‘MCR’. The POWER OFF but­ton (PB1 in Fig. 1) could be labeled that way, or you could make the same cir­cuit into an Emergency Stop by simply repla­cing the oper­at­or with a red mushroom-​head push but­ton. These devices were usu­ally spring-​return, so to restore power, all that was needed was to push the POWER ON but­ton again (Fig.1).

Basic Stop/Start Circuit
Figure 1 – Basic Stop/​Start Circuit

Typically, the com­pon­ents used in these cir­cuits were spe­cified to meet the cir­cuit con­di­tions, but not more. Controls man­u­fac­tur­ers brought out over-​dimensioned ver­sions, such as Allen-Bradley’s Bulletin 700-​PK con­tact­or which had 20 A rated con­tacts instead of the stand­ard Bulletin 700’s 10 A contacts.

When inter­locked guards began to show up, they were integ­rated into the ori­gin­al MCR cir­cuit by adding a basic con­trol relay (CR1 in Fig. 2) whose coil was con­trolled by the inter­lock switch(es) (LS1 in Fig. 2), and whose out­put con­tacts were in series with the coil cir­cuit of the MCR con­tact­or. Opening the guard inter­lock would open the MCR coil cir­cuit and drop power to the machine con­trols. Very simple.

Start/Stop Circuit with Guard Relay
Figure 2 – Old-​School Start/​Stop Circuit with Guard Relay

Ice-​cube’ style plug-​in relays were often chosen for CR1. These devices did not have ‘force-​guided’ con­tacts in them, so it was pos­sible to have one con­tact in the relay fail while the oth­er con­tin­ued to oper­ate properly.

LS1 could be any kind of switch. Frequently a ‘micro-​switch’ style of lim­it switch was chosen. These snap-​action switches could fail shor­ted intern­ally, or weld closed and the actu­at­or would con­tin­ue to work nor­mally even though the switch itself had failed. These switches are also ridicu­lously easy to bypass. All that is required is a piece of tape or an elast­ic band and the switch is no longer doing it’s job.

Micro-Switch style limit switch used as an interlock switch
Photo 1 – Micro-​Switch style lim­it switch used as a cov­er inter­lock switch in a piece of indus­tri­al laun­dry equipment

The prob­lem with these cir­cuits is that they can fail in a num­ber of ways that aren’t obvi­ous to the user, with the res­ult being that the inter­lock might not work as expec­ted, or the Emergency Stop might fail just when you need it most.

Modern Circuits

Category B

These ori­gin­al cir­cuits are the basis for what became known as ‘Category B’ (‘B’ for ‘Basic’) cir­cuits. Here’s the defin­i­tion from the stand­ard. Note that I am tak­ing this excerpt from ISO 13849 – 1: 2007 (Edition 2). “SRP/​CS” stands for “Safety Related Parts of Control Systems”:

6.2.3 Category B
The SRP/​CS shall, as a min­im­um, be designed, con­struc­ted, selec­ted, assembled and com­bined in accord­ance with the rel­ev­ant stand­ards and using basic safety prin­ciples for the spe­cif­ic applic­a­tion to withstand

  • the expec­ted oper­at­ing stresses, e.g. the reli­ab­il­ity with respect to break­ing capa­city and frequency,
  • the influ­ence of the pro­cessed mater­i­al, e.g. deter­gents in a wash­ing machine, and
  • oth­er rel­ev­ant extern­al influ­ences, e.g. mech­an­ic­al vibra­tion, elec­tro­mag­net­ic inter­fer­ence, power sup­ply inter­rup­tions or disturbances.

There is no dia­gnost­ic cov­er­age (DCavg = none) with­in cat­egory B sys­tems and the MTTFd of each chan­nel can be low to medi­um. In such struc­tures (nor­mally single-​channel sys­tems), the con­sid­er­a­tion of CCF is not relevant.

The max­im­um PL achiev­able with cat­egory B is PL = b.

NOTE When a fault occurs it can lead to the loss of the safety function.

Specific require­ments for elec­tro­mag­net­ic com­pat­ib­il­ity are found in the rel­ev­ant product stand­ards, e.g. IEC 61800 – 3 for power drive sys­tems. For func­tion­al safety of SRP/​CS in par­tic­u­lar, the immunity require­ments are rel­ev­ant. If no product stand­ard exists, at least the immunity require­ments of IEC 61000 – 6-​2 should be followed.

The stand­ard also provides us with a nice block dia­gram of what a single-​channel sys­tem might look like:

Category B Designated Architecture
ISO 13849 – 1 Category B Designated Architecture

If you look at this block dia­gram and the Start/​Stop Circuit with Guard Relay above, you can see how this basic cir­cuit trans­lates into a single chan­nel archi­tec­ture, since from the con­trol inputs to the con­trolled load you have a single chan­nel. Even the guard loop is a single chan­nel. A fail­ure in any com­pon­ent in the chan­nel can res­ult in loss of con­trol of the load.

Lets look at each part of this require­ment in more detail, since each of the sub­sequent Categories builds upon these BASIC requirements.

The SRP/​CS shall, as a min­im­um, be designed, con­struc­ted, selec­ted, assembled and com­bined in accord­ance with the rel­ev­ant stand­ards and using basic safety prin­ciples for the spe­cif­ic application…

Basic Safety Principles

We have to go to ISO 13849 – 2 to get a defin­i­tion of what Basic Safety Principles might include. Looking at Annex A.2 of the stand­ard we find:

Table A.1 — Basic Safety Principles

Basic Safety Principles Remarks
Use of suit­able mater­i­als and adequate manufacturing Selection of mater­i­al, man­u­fac­tur­ing meth­ods and treat­ment in rela­tion to, e. g. stress, dur­ab­il­ity, elasti­city, fric­tion, wear,
cor­ro­sion, temperature.
Correct dimen­sion­ing and shaping Consider e. g. stress, strain, fatigue, sur­face rough­ness, tol­er­ances, stick­ing, manufacturing.
Proper selec­tion, com­bin­a­tion, arrange­ments, assembly and install­a­tion of components/​systems. Apply manufacturer’s applic­a­tion notes, e. g. cata­logue sheets, install­a­tion instruc­tions, spe­cific­a­tions, and use of good engin­eer­ing prac­tice in sim­il­ar components/​systems.
Use of de – ener­gisa­tion principle The safe state is obtained by release of energy. See primary action for stop­ping in EN 292 – 2:1991 (ISO/​TR 12100 – 2:1992), 3.7.1. Energy is sup­plied for start­ing the move­ment of a mech­an­ism. See primary action for start­ing in EN 292 – 2:1991 (ISO/​TR 12100 – 2:1992), 3.7.1.Consider dif­fer­ent modes, e. g. oper­a­tion mode, main­ten­ance mode.

This prin­ciple shall not be used in spe­cial applic­a­tions, e. g. to keep energy for clamp­ing devices.

Proper fasten­ing For the applic­a­tion of screw lock­ing con­sider manufacturer’s applic­a­tion notes.Overloading can be avoided by apply­ing adequate torque load­ing technology.
Limitation of the gen­er­a­tion and/​or trans­mis­sion of force and sim­il­ar parameters Examples are break pin, break plate, torque lim­it­ing clutch.
Limitation of range of envir­on­ment­al parameters Examples of para­met­ers are tem­per­at­ure, humid­ity, pol­lu­tion at the install­a­tion place. See clause 8 and consider
manufacturer’s applic­a­tion notes.
Limitation of speed and sim­il­ar parameters Consider e. g. the speed, accel­er­a­tion, decel­er­a­tion required by the application
Proper reac­tion time Consider e. g. spring tired­ness, fric­tion, lub­ric­a­tion, tem­per­at­ure, iner­tia dur­ing accel­er­a­tion and deceleration,
com­bin­a­tion of tolerances.
Protection against unex­pec­ted start – up Consider unex­pec­ted start-​up caused by stored energy and after power “sup­ply” res­tor­a­tion for dif­fer­ent modes as
oper­a­tion mode, main­ten­ance mode etc.
Special equip­ment for release of stored energy may be necessary.
Special applic­a­tions, e. g. to keep energy for clamp­ing devices or ensure a pos­i­tion, need to be considered
separately.
Simplification Reduce the num­ber of com­pon­ents in the safety-​related system.
Separation Separation of safety-​related func­tions from oth­er functions.
Proper lub­ric­a­tion
Proper pre­ven­tion of the ingress of flu­ids and dust Consider IP rat­ing [see EN 60529 (IEC 60529)]

Download ISO Standards 
As you can see, the basic safety prin­ciples are pretty basic – select com­pon­ents appro­pri­ately for the applic­a­tion, con­sider the oper­at­ing con­di­tions for the com­pon­ents, fol­low manufacturer’s data, and use de-​energization to cre­ate the stop func­tion. That way, a loss of power res­ults in the sys­tem fail­ing into a safe state, as does an open relay coil or set of burnt contacts.

…the expec­ted oper­at­ing stresses, e.g. the reli­ab­il­ity with respect to break­ing capa­city and frequency,”

Specify your com­pon­ents cor­rectly with regard to voltage, cur­rent, break­ing capa­city, tem­per­at­ure, humid­ity, dust,…

…oth­er rel­ev­ant extern­al influ­ences, e.g. mech­an­ic­al vibra­tion, elec­tro­mag­net­ic inter­fer­ence, power sup­ply inter­rup­tions or disturbances.”

Specific require­ments for elec­tro­mag­net­ic com­pat­ib­il­ity are found in the rel­ev­ant product stand­ards, e.g. IEC 61800 – 3 for power drive sys­tems. For func­tion­al safety of SRP/​CS in par­tic­u­lar, the immunity require­ments are rel­ev­ant. If no product stand­ard exists, at least the immunity require­ments of IEC 61000 – 6-​2 should be followed.”

Probably the biggest ‘gotcha’ in this point is “elec­tro­mag­net­ic inter­fer­ence”. This is import­ant enough that the stand­ard devotes a para­graph to it spe­cific­ally. I added the bold text to high­light the idea of ‘func­tion­al safety’. You can find oth­er inform­a­tion in oth­er posts on this blog on that top­ic. If your product is destined for the European Union (EU), then you will almost cer­tainly be doing some EMC test­ing, unless your product is a ‘fixed install­a­tion’. If it’s going to almost any oth­er mar­ket, you prob­ably are not under­tak­ing this test­ing. So how do you know if your design meets this cri­ter­ia? Unless you test, you don’t. You can make some edu­cated guesses based on using sound engin­eer­ing prac­tices , but after that you can only hope.

Diagnostic Coverage

…There is no dia­gnost­ic cov­er­age (DCavg = none) with­in cat­egory B systems…”

Category B sys­tems are fun­da­ment­ally single chan­nel. A single fault in the sys­tem will lead to the loss of the safety func­tion. This sen­tence refers to the concept of “dia­gnost­ic cov­er­age” that was intro­duced in ISO 13849 – 1:2007, but what this means in prac­tice is that there is no mon­it­or­ing or feed­back from any crit­ic­al ele­ments. Remember our basic MCR cir­cuit? If the MCR con­tact­or wel­ded closed, the only dia­gnost­ic was the fail­ure of the machine to stop when the emer­gency stop but­ton was pressed.

Component Failure Rates

…the MTTFd of each chan­nel can be low to medium.”

This part of the state­ment is refer­ring to anoth­er new concept from ISO 13849 – 1:2007, “MTTFd”. Standing for “Mean Time to Failure Dangerous”, this concept looks at the expec­ted fail­ure rates of the com­pon­ent in hours. Calculating MTTFd is a sig­ni­fic­ant part of imple­ment­ing the new stand­ard. From the per­spect­ive of under­stand­ing Category B, what this means is that you do not need to use high-​reliability com­pon­ents in these systems.

Common Cause Failures

In such struc­tures (nor­mally single-​channel sys­tems), the con­sid­er­a­tion of CCF is not relevant.”

CCF is anoth­er new concept from ISO 13849 – 1:2007, and stands for “Common Cause Failure”. I’m not going to get into this in any detail here, but suf­fice to say that design tech­niques, as well as chan­nel sep­ar­a­tion (impossible in a single chan­nel archi­tec­ture) and oth­er tech­niques are used to reduce the like­li­hood of CCF in high­er reli­ab­il­ity systems.

Performance Levels

The max­im­um PL achiev­able with cat­egory B is PL = b.”

PL stands for “Performance Level”, divided into five degrees from ‘a’ to ‘e’. PLa is equal to an aver­age prob­ab­il­ity of dan­ger­ous fail­ure per hour of >= 10-5 to < 10-4 fail­ures per hour. PLb is equal to >= 3 × 10-6 to < 10-5 fail­ures per hour or once in 10,000 to 100,000 hours, to once in 3,000,000 hours of oper­a­tion. This sounds like a lot, but when deal­ing with prob­ab­il­it­ies, these num­bers are actu­ally pretty low.

If you con­sider an oper­a­tion run­ning a single shift in Canada where the nor­mal work­ing year is 50 weeks and the nor­mal work­day is 7.5 hours, a work­ing year is

7.5 h/​d x 5 d/​w x 50 w/​a = 1875 hours/​a

Taking the fail­ure rates per hour above, yields:

PLa = one fail­ure in 5.3 years of oper­a­tion to one fail­ure in 53.3 years

PLb = one fail­ure in 1600 years of operation

If we go to an oper­a­tion run­ning three shifts in Canada, a work­ing year is:

7.5 h/​shift x 3 shifts x 5 d/​w x 50 w/​a = 5625 hours/​a

Taking the fail­ure rates per hour above, yields:

PLa = one fail­ure in 1.8 years of oper­a­tion to one fail­ure in 17 years

PLb = one fail­ure in 533 years of operation

Now you should be start­ing to get an idea about where this is going. It’s import­ant to remem­ber that prob­ab­il­it­ies are just that – the fail­ure could hap­pen in the first hour of oper­a­tion or at any time after that, or nev­er. These fig­ures give you some way to gauge the rel­at­ive reli­ab­il­ity of the design, and ARE NOT any sort of guarantee.

Watch for the next post in this series where I will look at Category 1 requirements!

References

[1] Safety of Machinery – Safety Related Parts of Control Systems – Part 1: General Principles for Design. CEN Standard EN 954 – 1. 1996.

[2] Safety of machinery — Safety-​related parts of con­trol sys­tems — Part 1: General prin­ciples for design. ISO Standard 13849 – 1. 2006.

[3] Safety of machinery — Safety-​related parts of con­trol sys­tems — Part 2: Validation, ISO Standard 13849 – 2. 2003.

[4] Safety of machinery — Safety-​related parts of con­trol sys­tems — Part 100: Guidelines for the use and applic­a­tion of ISO 13849 – 1. ISO Technical Report TR 100. 2000.

[5] Safety of machinery — Safety-​related parts of con­trol sys­tems — Part 1: General prin­ciples for design. CEN Standard EN ISO 13849 – 1. 2008.

Download ISO Standards 

Series NavigationInterlock Architectures – Pt. 2: Category 1

Author: Doug Nix

+DougNix is Managing Director and Principal Consultant at Compliance InSight Consulting, Inc. (http://www.complianceinsight.ca) in Kitchener, Ontario, and is Lead Author and Managing Editor of the Machinery Safety 101 blog. Doug's work includes teaching machinery risk assessment techniques privately and through Conestoga College Institute of Technology and Advanced Learning in Kitchener, Ontario, as well as providing technical services and training programs to clients related to risk assessment, industrial machinery safety, safety-related control system integration and reliability, laser safety and regulatory conformity. Follow me on Academia.edu//a.academia-assets.com/javascripts/social.js

  • con­trols­girl

    Great explan­a­tion and trans­la­tion into how these stand­ards are applied in the real world. One thing that I think is often con­fus­ing is the defin­i­tion of fail­ure. I often have found myself won­der­ing if the stand­ard means fail­ure of a device to work as expec­ted or fail­ure in the sense that someone had to press an e-stop..etc. Could you help cla­ri­fy that in the dif­fer­ent places that the word is men­tioned? I know that some­times it is more obvi­ous than others.

    • Hey con­trols­girl! Thanks for post­ing this ques­tion – it’s a good one. 

      When we’re talk­ing about safety related con­trols there are a num­ber of dif­fer­ent types of fail­ures we could be talk­ing about. From the per­spect­ive of ISO 13849 – 1, what we care about are dan­ger­ous fail­ures, mean­ing that the safety-​related con­trol func­tion has failed in a way that imme­di­ately increases the risk to the oper­at­or. If a con­trol func­tion doesn’t work as expec­ted, but no increase in risk occurs, it’s not a dan­ger­ous fail­ure. If a dan­ger­ous fail­ure occurs in a guard inter­lock, the res­ult could be a situ­ation where the oper­at­or opens the guard and the machine fails to stop. That is a dan­ger­ous failure.

      To sum up, fail­ures as dis­cussed in ISO 13849 – 1 are always faults in the safety-​related parts of the con­trol sys­tem that res­ult in an increase in risk to the oper­at­or. They may be dangerous-​detected fail­ures, or dangerous-​undetected fail­ures. The stand­ard doesn’t pay any atten­tion to safe fail­ures, detect­able or not.

      Emergency stop is there to deal with ‘emer­gent’ con­di­tions, i.e., fail­ures that weren’t fore­seen by the design­er, and so aren’t dealt with by the auto­mat­ic safety func­tions designed into the machine. For example, a ‘silent’ fail­ure occurs in the guard inter­lock we were talk­ing about. ‘Silent’ means the con­trol sys­tem dia­gnostics don’t detect it for whatever reas­on. The oper­at­or opens the guard and is imme­di­ately and unex­pec­tedly exposed to the machine haz­ard, res­ult­ing in an injury. A co-​worker presses the emer­gency stop to try to lim­it any addi­tion­al harm that might occur, and then dials 911 (or 112, or whatever your loc­al emer­gency phone num­ber is). E-​stops are con­sidered ‘com­ple­ment­ary pro­tect­ive meas­ures’ because they com­ple­ment the primary safe­guards, like the guard interlocks.

      I think that cov­ers it. Let me know if you have any more questions!

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