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

This entry is part 1 of 8 in the series Circuit Architectures Explored

In 1995 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. 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 stand­ard, ISO 13849 – 1?

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

EU changes direction on EN ISO 13849 – 1

Update on EN ISO 13849 – 1 man­dat­ory imple­ment­a­tion date.

In a post on 15-​Sep I repor­ted that the European Union had decided to delay the man­dat­ory imple­ment­a­tion date of  EN ISO 13849 – 1 for an addi­tion­al three years. This report was based on inform­a­tion obtained from an intern­al source at the European Commission and has since been reversed by that same source.

Mr. Glyn Garside provided the fol­low­ing update to this import­ant story:

It has been widely repor­ted, but nev­er con­firmed, that the EU com­mis­sion had accep­ted the CEN pro­pos­al to extend the date of ces­sa­tion of pre­sump­tion of con­form­ity of EN 954 – 1:1996 until the end of 2012. THESE REPORTS HAVE NOW BEEN AUTHORITATIVELY DENIED.

(By the way, this dis­cus­sion of dates of ces­sa­tion of pre­sump­tion of con­form­ity only affects the European stand­ards, EN 954 – 1 and EN ISO 13849 – 1. International stand­ard ISO 13849 – 1 is obvi­ously con­trolled by ISO and not by CEN or the EU. The cur­rent edi­tion of ISO 13849 – 1 is 2006, essen­tially identic­al to EN ISO 13849 – 1 : 2008.)

At this point the pos­sib­il­ity of an exten­sion of the trans­ition from EN 954 – 1 to EN ISO 13849 – 1 remains con­tro­ver­sial, con­fused and IMHO uncer­tain. (There’s been approx 3 years trans­ition peri­od already.) If I were still a man­u­fac­turer, I would not want to wait until Dec 29th to find out if I could still ship my product using EN 954 – 1!

The reports of an exten­sion were based on an email sent earli­er this month (3rd Sept) by a CEN employ­ee. However, the EU Commission nev­er con­firmed the report, and on September 24th the same CEN employ­ee, Marie Poidevin, has writ­ten,
— —  —  —  — –
> “We have been informed today by the European Commission […] that con­trary to what was expressed in
> my pre­vi­ous mes­sage sent on the 3rd of September, EN 954 – 1 will not give pre­sump­tion of con­form­ity
>  to the new MD 2006/​42/​EC until fur­ther notice.
> “Indeed, due to dis­cus­sions fol­low­ing the announce­ment made below, the EC wishes to gath­er experts’
> views and, there­fore, this issue will be dis­cussed at the next Machinery Working group to be held on
> the 7 – 8th December.”
— —  —  —  — –

A related email from Ian Fraser (“EC Policy Director for the Machinery Directive”), dated 2009-​09-​18 states,
— —  —  —  — –
“Following the dis­cus­sion at the meet­ing of the Machinery Working Group held on 7 and 8
July 2009, we have received a num­ber of ques­tions con­cern­ing the trans­ition from stand­ard
EN 954 – 1 to stand­ard EN ISO 13849 – 1 on safety-​related parts of con­trol sys­tems.
At the meet­ing of the Machinery Working Group, there was gen­er­al agree­ment on two
aspects:
1. Manufacturers who apply stand­ard EN ISO 13849 – 1 bene­fit from a pre­sump­tion of
con­form­ity, even if the har­mon­ised C-​type stand­ard relat­ing to the machinery con­cerned still
refers to the cat­egor­ies of EN 954 – 1;
2. Harmonised C-​type stand­ards that refer to the cat­egor­ies of EN 954 – 1 con­tin­ue to con­fer a
pre­sump­tion of con­form­ity until they are amended to refer to stand­ard EN ISO 13849 – 1.
These con­clu­sions will be recor­ded in the minutes of the meet­ing.

During the dis­cus­sion, sev­er­al par­ti­cipants indic­ated that more time was needed for the
industry, and in par­tic­u­lar for SMEs, to adapt to the new stand­ard. As Chairman of the
meet­ing, I asked wheth­er it might not be prefer­able to post­pone the date of ces­sa­tion of
pre­sump­tion of con­form­ity for EN 954 – 1.
In response to this sug­ges­tion, on 30 July 2009, Mr. Steiger wrote to the Commission, on
behalf of the CEN Machinery Sector, to request that the date of ces­sa­tion of pre­sump­tion of
con­form­ity for EN 954 – 1 be excep­tion­ally post­poned until 31 December 2012 […].
The Commission will reply to this request from CEN. However, giv­en the com­plex­ity of the
issues involved, the Commission intends to con­sult experts and to seek the opin­ion of the
Machinery Working Group to be held on 7 and 8 December 2009, before reach­ing a final
decision.
Kind regards,
Ian FRASER”
— —  —  — —

Thanks again to Glyn Garside and the EMC-​PSTC List Server!