Interlock Architectures – Pt. 2: Category 1

This entry is part 2 of 8 in the series Cir­cuit Archi­tec­tures Explored

This arti­cle expands on the first in the series “Inter­lock Archi­tec­tures – Pt. 1: What do those cat­e­gories real­ly mean?”. Learn about the basic cir­cuit archi­tec­tures that under­lie all safe­ty inter­lock sys­tems under ISO 13849–1, and CSA Z432 and ANSI RIA R15.06.

In Part 1 of this series we explored Cat­e­go­ry B, the Basic Cat­e­go­ry that under­pins all the oth­er Cat­e­gories. This post builds on Part 1 by tak­ing a look at Cat­e­go­ry 1. Let’s start by explor­ing the dif­fer­ence as defined in ISO 13849–1. When you are read­ing, remem­ber that “SRP/CS” stands for “Safe­ty Relat­ed Parts of Con­trol Sys­tems”.

SRP/CS of Cat­e­go­ry 1 shall be designed and con­struct­ed using well-tried com­po­nents and well-tried safe­ty prin­ci­ples (see ISO 13849–2).

Well-Tried Components

So what, exact­ly, is a “Well-Tried Com­po­nent”?? Let’s go back to the stan­dard for that:

A “well-tried com­po­nent” for a safe­ty-relat­ed appli­ca­tion is a com­po­nent which has been either

a) wide­ly used in the past with suc­cess­ful results in sim­i­lar appli­ca­tions, or
b) made and ver­i­fied using prin­ci­ples which demon­strate its suit­abil­i­ty and reli­a­bil­i­ty for safe­ty-relat­ed appli­ca­tions.

New­ly devel­oped com­po­nents and safe­ty prin­ci­ples may be con­sid­ered as equiv­a­lent to “well-tried” if they ful­fil the con­di­tions of b).

The deci­sion to accept a par­tic­u­lar com­po­nent as being “well-tried” depends on the appli­ca­tion.

NOTE 1 Com­plex elec­tron­ic com­po­nents (e.g. PLC, micro­proces­sor, appli­ca­tion-spe­cif­ic inte­grat­ed cir­cuit) can­not be con­sid­ered as equiv­a­lent to “well tried”.

[1, 6.2.4]

Lets look at what this all means by refer­ring to ISO 13849–2:

Table 1 — Well-Tried Com­po­nents [2]
Well-Tried Com­po­nents Con­di­tions for “well–tried” Stan­dard or spec­i­fi­ca­tion
Screw All fac­tors influ­enc­ing the screw con­nec­tion and the appli­ca­tion are to be con­sid­ered. See Table A.2 “List of well–tried safe­ty prin­ci­ples”. Mechan­i­cal joint­ing such as screws, nuts, wash­ers, riv­ets, pins, bolts etc. are stan­dard­ised.
Spring See Table A.2 “Use of a well–tried spring”. Tech­ni­cal spec­i­fi­ca­tions for spring steels and oth­er spe­cial appli­ca­tions are giv­en in ISO 4960.
Cam All fac­tors influ­enc­ing the cam arrange­ment (e. g. part of an inter­lock­ing device) are to be con­sid­ered. See Table A.2 “List of well–tried safe­ty prin­ci­ples”. See EN 1088 (ISO 14119) (Inter­lock­ing devices).
Break–pin All fac­tors influ­enc­ing the appli­ca­tion are to be con­sid­ered. See Table A.2 “List of well-tried safe­ty prin­ci­ples”.

Now we have a few ideas about what might con­sti­tute a ‘well-tried com­po­nent’. Unfor­tu­nate­ly, you will notice that ‘con­tac­tor’ or ‘relay’ or ‘lim­it switch’ appear nowhere on the list. This is a chal­lenge, but one that can be over­come. The key to deal­ing with this is to look at how the com­po­nents that you are choos­ing to use are con­struct­ed. If they use these com­po­nents and tech­niques, you are on your way to con­sid­er­ing them to be well-tried.

Anoth­er approach is to let the com­po­nent man­u­fac­tur­er wor­ry about the details of the con­struc­tion of the device, and sim­ply ensure that com­po­nents select­ed for use in the SRP/CS are ‘safe­ty rat­ed’ by the man­u­fac­tur­er. This can work in 80–90% of cas­es, with a small per­cent­age of com­po­nents, such as large motor starters, some ser­vo and step­per dri­ves and oth­er sim­i­lar com­po­nents unavail­able with a safe­ty rat­ing. It’s worth not­ing that many dri­ve man­u­fac­tur­ers are start­ing to pro­duce dri­ves with built-in safe­ty com­po­nents that are intend­ed to be inte­grat­ed into your SRP/CS.

Exclusion of Complex Electronics

Note 1 from the first part of the def­i­n­i­tion is very impor­tant. So impor­tant that I’m going to repeat it here:

NOTE 1 Com­plex elec­tron­ic com­po­nents (e.g. PLC, micro­proces­sor, appli­ca­tion-spe­cif­ic inte­grat­ed cir­cuit) can­not be con­sid­ered as equiv­a­lent to “well tried”.

I added the bold text to empha­size the impor­tance of this state­ment. While this is includ­ed in a Note and is there­fore con­sid­ered to be explana­to­ry text and not part of the nor­ma­tive body of the stan­dard, it illu­mi­nates a key con­cept. This lit­tle note is what pre­vents a stan­dard PLC from being used in Cat­e­go­ry 1 sys­tems. It’s also impor­tant to real­ize that this def­i­n­i­tion is only con­sid­er­ing the hard­ware — no men­tion of soft­ware is made here, and soft­ware is not dealt with until lat­er in the stan­dard.

Well-Tried Safety Principles

Let’s have a look at what ‘Well-Tried Safe­ty Prin­ci­ples’ might be.

Table 2 — Well-Tried Safe­ty Prin­ci­ples [2, A.2]
Well-tried Safe­ty Prin­ci­ples Remarks
Use of care­ful­ly select­ed mate­ri­als and man­u­fac­tur­ing Selec­tion of suit­able mate­r­i­al, ade­quate man­u­fac­tur­ing meth­ods and treat­ments relat­ed to the appli­ca­tion.
Use of com­po­nents with ori­ent­ed fail­ure mode The pre­dom­i­nant fail­ure mode of a com­po­nent is known in advance and always the same, see EN 292–2:1991, (ISO/TR 12100–2:1992), 3.7.4.
Over–dimensioning/safety fac­tor The safe­ty fac­tors are giv­en in stan­dards or by good expe­ri­ence in safe­ty-relat­ed appli­ca­tions.
Safe posi­tion The mov­ing part of the com­po­nent is held in one of the pos­si­ble posi­tions by mechan­i­cal means (fric­tion only is not enough). Force is need­ed for chang­ing the posi­tion.
Increased OFF force A safe position/state is obtained by an increased OFF force in rela­tion to ON force.
Care­ful selec­tion, com­bi­na­tion, arrange­ment, assem­bly and instal­la­tion of components/system relat­ed to the appli­ca­tion
Care­ful selec­tion of fas­ten­ing relat­ed to the appli­ca­tion Avoid rely­ing only on fric­tion.
Pos­i­tive mechan­i­cal action Depen­dent oper­a­tion (e. g. par­al­lel oper­a­tion) between parts is obtained by pos­i­tive mechan­i­cal link(s). Springs and sim­i­lar “flex­i­ble” ele­ments should not be part of the link(s) [see EN 292–2:1991 (ISO/TR 12100–2:1992), 3.5].
Mul­ti­ple parts Reduc­ing the effect of faults by mul­ti­ply­ing parts, e. g. where a fault of one spring (of many springs) does not lead to a dan­ger­ous con­di­tion.
Use of well–tried spring (see also Table A.3) A well–tried spring requires:
  • use of care­ful­ly select­ed mate­ri­als, man­u­fac­tur­ing meth­ods (e. g. pre­set­ting and cycling before use) and treat­ments (e. g. rolling and shot–peening),
  • suf­fi­cient guid­ance of the spring, and
  • suf­fi­cient safe­ty fac­tor for fatigue stress (i. e. with high prob­a­bil­i­ty a frac­ture will not occur).

Well–tried pres­sure coil springs may also be designed by:

  • use of care­ful­ly select­ed mate­ri­als, man­u­fac­tur­ing meth­ods (e. g. pre­set­ting and cycling before use) and treat­ments (e. g. rolling and shot-peen­ing),
  • suf­fi­cient guid­ance of the spring, and
  • clear­ance between the turns less than the wire diam­e­ter when unloaded, and
  • suf­fi­cient force after a fracture(s) is main­tained (i. e. a fracture(s) will not lead to a dan­ger­ous con­di­tion).
Lim­it­ed range of force and sim­i­lar para­me­ters Decide the nec­es­sary lim­i­ta­tion in rela­tion to the expe­ri­ence and appli­ca­tion. Exam­ples for lim­i­ta­tions are break pin, break plate, torque lim­it­ing clutch.
Lim­it­ed range of speed and sim­i­lar para­me­ters Decide the nec­es­sary lim­i­ta­tion in rela­tion to the expe­ri­ence and appli­ca­tion. Exam­ples for lim­i­ta­tions are cen­trifu­gal gov­er­nor; safe mon­i­tor­ing of speed or lim­it­ed dis­place­ment.
Lim­it­ed range of envi­ron­men­tal para­me­ters Decide the nec­es­sary lim­i­ta­tions. Exam­ples on para­me­ters are tem­per­a­ture, humid­i­ty, pol­lu­tion at the instal­la­tion. See clause 8 and con­sid­er manufacturer’s appli­ca­tion notes.
Lim­it­ed range of reac­tion time, lim­it­ed hys­tere­sis Decide the nec­es­sary lim­i­ta­tions.
Con­sid­er e. g. spring tired­ness, fric­tion, lubri­ca­tion, tem­per­a­ture, iner­tia dur­ing accel­er­a­tion and decel­er­a­tion,
com­bi­na­tion of tol­er­ances.

Use of Positive-Mode Operation

The use of these prin­ci­ples in the com­po­nents, as well as in the over­all design of the safe­guards is impor­tant. In devel­op­ing a sys­tem that uses ‘pos­i­tive mode oper­a­tion’, the mechan­i­cal link­age that oper­ates the elec­tri­cal con­tacts or the flu­id-pow­er valve that con­trols the prime-mover(s) (i.e. motors, cylin­ders, etc.), must act to direct­ly dri­ve the con­trol ele­ment (con­tacts or valve spool) to the safe state. Springs can be used to return the sys­tem to the run state or dan­ger­ous state, since a fail­ure of the spring will result in the inter­lock device stay­ing in the safe state (fail-safe or fail-to-safe­ty).

CSA Z432 [3] pro­vides us with a nice dia­gram that illus­trates the idea of “pos­i­tive-action” or “pos­i­tive-mode” oper­a­tion:

CSA Z432 Fig B.10 - Positive Mode Operation
Fig­ure 1 — Pos­i­tive Mode Oper­a­tion [3, B.10]

In Fig. 1, open­ing the guard door forces the roller to fol­low the cam attached to the door, dri­ving the switch con­tacts apart and open­ing the inter­lock. Even if the con­tacts were to weld, they would still be dri­ven apart since the mechan­i­cal advan­tage pro­vid­ed by the width of the door and the cam are more than enough to force the con­tacts apart.

Here’s an exam­ple of a ‘neg­a­tive mode’ oper­a­tion:

CSA Z432-04 Fig B.11 - Negative Mode operation
Fig­ure 2 — Neg­a­tive Mode oper­a­tion [3, B.11]

In Fig. 2, the inter­lock switch relies on a spring to enter the safe state when the door is opened. If the spring in the inter­lock device fails, the sys­tem fails-to-dan­ger. Also note that this design is very easy to defeat. A ‘zip-tie’ or some tape is all that would be required to keep the inter­lock in the ‘RUN’ con­di­tion.

You should have a bet­ter idea of what is meant when you read about pos­i­tive and neg­a­tive-modes of oper­a­tion now. We’ll talk about defeat resis­tance in anoth­er arti­cle.


Com­bin­ing what you’ve learned so far, you can see that cor­rect­ly spec­i­fied com­po­nents, com­bined with over-dimen­sion­ing and imple­men­ta­tion of design lim­its along with the use of well-tried safe­ty prin­ci­ples will go a long way to improv­ing the reli­a­bil­i­ty of the con­trol sys­tem. The next part of the def­i­n­i­tion of Cat­e­go­ry 1 speaks to some addi­tion­al require­ments:

The MTTFd of each chan­nel shall be high.

The max­i­mum PL achiev­able with cat­e­go­ry 1 is PL = c.

NOTE 2 There is no diag­nos­tic cov­er­age (DCavg = none) with­in cat­e­go­ry 1 sys­tems. In such struc­tures (sin­gle-chan­nel sys­tems) the con­sid­er­a­tion of CCF is not rel­e­vant.

NOTE 3 When a fault occurs it can lead to the loss of the safe­ty func­tion. How­ev­er, the MTTFd of each chan­nel in cat­e­go­ry 1 is high­er than in cat­e­go­ry B. Con­se­quent­ly, the loss of the safe­ty func­tion is less like­ly.

We now know that the integri­ty of a Cat­e­go­ry 1 sys­tem is greater than a Cat­e­go­ry B sys­tem, since the chan­nel MTTFd of the sys­tem has gone from “Low-to-Medi­um” in sys­tems exhibit­ing PLa or PLb per­for­mance to “High” in sys­tems exhibit­ing PLb or PLc per­for­mance. [1, Table 5] shows this dif­fer­ence in terms of pre­dict­ed years to fail­ure. As you can see, MTTFd “High” results in a pre­dict­ed fail­ure rate between 30 and 100 years. This is a pret­ty good result for sim­ply improv­ing the com­po­nents used in the sys­tem!

Table 3 – Mean time to dangerous failure  [1, Table 5]
Table 3 – Mean time to dan­ger­ous fail­ure

The oth­er ben­e­fit is the increase in the over­all PL. Where Cat­e­go­ry B archi­tec­ture can pro­vide PLb per­for­mance at best, Cat­e­go­ry 1 takes this up a notch to PLc. To get a han­dle on what PLc means, let’s look at our sin­gle and three shift exam­ples again. If we take a Cana­di­an oper­a­tion with a sin­gle shift per day, and a 50 week work­ing year we get:

7.5 h/shift x 5 d/w x 50 w/a = 1875 h/a


h = hours

d = days

w = weeks

a  = years

In this case, PLc is equiv­a­lent to one fail­ure in 533.3 years of oper­a­tion to 1600 years of oper­a­tion.

Look­ing at three shifts per day in the same oper­a­tion gives us:

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

In this case, PLc is equiv­a­lent to one fail­ure in 177.8 years of oper­a­tion to 533.3 years of oper­a­tion.

When com­plet­ing the analy­sis of a sys­tem, [1] lim­its the sys­tem MTTFd to 100 years regard­less of what the indi­vid­ual chan­nel MTTFd may be. Where the actu­al MTTFd is impor­tant relates to the need to replace com­po­nents dur­ing the life­time of the prod­uct. If a com­po­nent or a sub-sys­tem has an MTTFd that is less than the mis­sion time of the sys­tem, then the com­po­nent or sub­sys­tem must be replaced by the time the prod­uct reach­es it’s MTTFd. 20 years is the default mis­sion time, but you can choose a short­er or longer time span if it makes sense.

Remem­ber that these are prob­a­bil­i­ties, not guar­an­tees. A fail­ure could hap­pen in the first hour of oper­a­tion, the last hour of oper­a­tion or nev­er. These fig­ures sim­ply pro­vide a way for you as the design­er to gauge the rel­a­tive reli­a­bil­i­ty of the sys­tem.

Well-Tried Components versus Fault Exclusions

The stan­dard goes on to out­line some key dis­tinc­tions between ‘well-tried com­po­nent’ and ‘fault exclu­sion’. We’ll talk more about fault exclu­sions lat­er in the series.

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.

[1, 6.2.4]

System Block Diagram

Final­ly, let’s look at the block dia­gram for Cat­e­go­ry 1. You will notice that it looks the same as the Cat­e­go­ry B block dia­gram, since only the com­po­nents used in the sys­tem have changed, and not the archi­tec­ture.

ISO 13849-1 Figure 9
Fig­ure 3 — Cat­e­go­ry 1 Block Dia­gram [1, Fig. 9]


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

[2]       Safe­ty of machin­ery — Safe­ty-relat­ed parts of con­trol sys­tems — Part 2: Val­i­da­tion. ISO Stan­dard 13849–2, Ed. 2. 2012.

[3]       Safe­guard­ing of Machin­ery. CSA Stan­dard Z432. 2004.

Add to your Library

If you are work­ing on imple­ment­ing these design stan­dards in your prod­ucts, you need to buy copies of the stan­dards for your library.

  • ISO 13849–1:2006 Safe­ty of machin­ery — Safe­ty-relat­ed parts of con­trol sys­tems — Part 1: Gen­er­al prin­ci­ples for design
  • ISO 13849–2:2003 Safe­ty of machin­ery — Safe­ty-relat­ed parts of con­trol sys­tems — Part 2: Val­i­da­tion

Down­load IEC stan­dards, Inter­na­tion­al Elec­trotech­ni­cal Com­mis­sion stan­dards.

If you are work­ing in the EU, or are work­ing on CE Mark­ing your prod­uct, you should hold the har­mo­nized ver­sion of this stan­dard, avail­able through the CEN resellers:

  • EN ISO 13849–1:2008 Safe­ty of machin­ery — Safe­ty-relat­ed parts of con­trol sys­tems — Part 1: Gen­er­al prin­ci­ples for design
  • EN ISO 13849–2:2012 Safe­ty of machin­ery — Safe­ty-relat­ed parts of con­trol sys­tems — Part 2: Val­i­da­tion

Next Installment

Watch for the next part of this series, “Inter­lock Archi­tec­tures – Pt. 3: Cat­e­go­ry 2″ where we expand on the first two cat­e­gories by adding some diag­nos­tic cov­er­age to improve reli­a­bil­i­ty.

Have ques­tions? Email me!

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Acknowl­edge­ments: ISO, CSA. See ref­er­ences.
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Emergency Stop — What’s so confusing about that?

This entry is part 1 of 13 in the series Emer­gency Stop

I get a lot of calls and emails ask­ing about emer­gency stops. This is one of those decep­tive­ly sim­ple con­cepts that has man­aged to get very com­pli­cat­ed over time. Not every machine needs or can ben­e­fit from an emer­gency stop. In some cas­es, it may lead to an unrea­son­able expec­ta­tion of safe­ty from the user, which can lead to injury if they don’t under­stand the haz­ards involved. Some prod­uct-spe­cif­ic stan­dards

Editor’s Note: Since we first pub­lished this arti­cle on emer­gency-stop in March of 2009, it has become our most pop­u­lar post of all time! We decid­ed it was time for a lit­tle refresh. Enjoy, and please com­ment if you find the post help­ful, or if you have any ques­tions you’d like answered. DN Feb-2018.

The Emer­gency Stop func­tion is one of those decep­tive­ly sim­ple con­cepts that have man­aged to get very com­pli­cat­ed over time. Not every machine needs or can ben­e­fit from an emer­gency stop. In some cas­es, it may lead to an unrea­son­able expec­ta­tion of safe­ty from the user. Some prod­uct-spe­cif­ic stan­dards man­date the require­ment for an emer­gency stop, such as CSA Z434-14 [1], where robot con­trollers are required to pro­vide emer­gency stop func­tion­al­i­ty, and work cells inte­grat­ing robots are also required to have emer­gency stop capa­bil­i­ty.

Defining Emergency Stop

Pho­to 1 — This OLD but­ton is def­i­nite­ly non-com­pli­ant.

Before we look at the emer­gency-stop func­tion itself, we need to under­stand what the word “emer­gency” implies. This may seem obvi­ous but bear with me for a minute. The word “emer­gency” has the root “emer­gent”, mean­ing “in the process of com­ing into being or becom­ing promi­nent” accord­ing to the Oxford Dic­tio­nary of Eng­lish. An emer­gency con­di­tion is, there­fore, some con­di­tion that is aris­ing and becom­ing promi­nent at the moment. This con­di­tion implies that the sit­u­a­tion is not some­thing fore­seen by the machine design­er, and there­fore there are no design fea­tures present to con­trol the con­di­tion.

So what is the Emer­gency Stop func­tion, or E-stop func­tion, and when do you need to have one? Let’s look at a few def­i­n­i­tions tak­en from CSA Z432-14 [2]:

Emer­gency sit­u­a­tion
an imme­di­ate­ly haz­ardous sit­u­a­tion that needs to be end­ed or avert­ed quick­ly in order to pre­vent injury or dam­age.
Emer­gency stop
a func­tion that is intend­ed to avert harm or to reduce exist­ing haz­ards to per­sons, machin­ery, or work in progress.
Emer­gency stop but­ton
a red mush­room-head­ed but­ton that, when acti­vat­ed, will imme­di­ate­ly start the emer­gency stop sequence.

One more [2, 6.3.5]:

Com­ple­men­tary pro­tec­tive mea­sures
Pro­tec­tive mea­sures which are nei­ther inher­ent­ly safe design mea­sures, nor safe­guard­ing (imple­men­ta­tion of guards and/or pro­tec­tive devices), nor infor­ma­tion for use, could have to be imple­ment­ed as required by the intend­ed use and the rea­son­ably fore­see­able mis­use of the machine.

Old spring-return type of e-stop button with a plain red background legend plate.
Pho­to 2 — This more mod­ern but­ton is non-com­pli­ant due to the RED back­ground and spring-return but­ton.

An e-stop is a func­tion that is intend­ed for use in Emer­gency con­di­tions to try to lim­it or avert harm to some­one or some­thing. It isn’t a safe­guard but is con­sid­ered to be a Com­ple­men­tary Pro­tec­tive Mea­sure. Look­ing at emer­gency stop func­tions from the per­spec­tive of the Hier­ar­chy of Con­trols, emer­gency stop func­tions fall into the same lev­el as Per­son­al Pro­tec­tive Equip­ment like safe­ty glass­es, safe­ty boots, and hear­ing pro­tec­tion. 

So far so good.

Is an Emergency Stop Function Required?

Depend­ing on the reg­u­la­tions and the stan­dards you choose to read, machin­ery may not be required to have an Emer­gency Stop. Quot­ing from [2,]:

Com­po­nents and ele­ments to achieve the emer­gency stop func­tion

If fol­low­ing a risk assess­ment, a machine needs to be fit­ted with com­po­nents and ele­ments to achieve an emer­gency stop func­tion for enabling actu­al or impend­ing emer­gency sit­u­a­tions to be avert­ed, the fol­low­ing require­ments apply:

  • the actu­a­tors shall be clear­ly iden­ti­fi­able, clear­ly vis­i­ble and read­i­ly acces­si­ble;
  • the haz­ardous process shall be stopped as quick­ly as pos­si­ble with­out cre­at­ing addi­tion­al haz­ards, but if this is not pos­si­ble or the risk can­not be reduced, it should be ques­tioned whether imple­men­ta­tion of an emer­gency stop func­tion is the best solu­tion;
  • the emer­gency stop con­trol shall trig­ger or per­mit the trig­ger­ing of cer­tain safe­guard move­ments where nec­es­sary.

Note For more detailed pro­vi­sions, see ISO 13850.

I added the bold text in the pre­vi­ous quo­ta­tion, because that state­ment, “If after a risk assess­ment…” is very impor­tant. Lat­er in [2,]:

Each oper­a­tor con­trol sta­tion, includ­ing pen­dants, capa­ble of ini­ti­at­ing machine motion and/or auto­mat­ic motion shall have an emer­gency stop func­tion (see Clause, unless a risk assess­ment deter­mines that the emer­gency stop func­tion will not con­tribute to risk con­trol.

Note: There could be sit­u­a­tions where an e-stop does not con­tribute to risk con­trol and alter­na­tives could be con­sid­ered in con­junc­tion with a risk assess­ment.

The bold­ing in the text in the pre­ced­ing para­graph was added for empha­sis. I want­ed to be sure that you caught this impor­tant bit of text. Not every machine requires an E-stop func­tion. The func­tion is only required where there is a ben­e­fit to the user unless a prod­uct-spe­cif­ic stan­dard requires it. In some cas­es, prod­uct-spe­cif­ic stan­dards often called “Type C” stan­dards, include spe­cif­ic require­ments for the pro­vi­sion of an emer­gency stop func­tion. The require­ment may include a min­i­mum PLr or SILr, based on the opin­ion of the Tech­ni­cal Com­mit­tee respon­si­ble for the stan­dard and their knowl­edge of the par­tic­u­lar type of machin­ery cov­ered by their doc­u­ment.

Note: For more detailed pro­vi­sions on the elec­tri­cal design require­ments, see CSA C22.2 #301, NFPA 79 or IEC 60204–1.

Down­load NFPA stan­dards through ANSI

Pho­to 3 — This more mod­ern but­ton is non-com­pli­ant due to the RED back­ground.

If you read Ontario’s Indus­tri­al Estab­lish­ments Reg­u­la­tion (O. Reg. 851), you will find that prop­er iden­ti­fi­ca­tion of the emer­gency stop device(s) and loca­tion “with­in easy reach” of the oper­a­tor are the only require­ment. What does “prop­er­ly iden­ti­fied” mean? In Cana­da, the USA and Inter­na­tion­al­ly, a RED oper­a­tor device on a YELLOW back­ground, with or with­out any text on the back­ground, is rec­og­nized as EMERGENCY STOP or EMERGENCY OFF, in the case of dis­con­nect­ing switch­es or con­trol switch­es. You may also see the IEC sym­bol for emer­gency stop used to iden­ti­fy these devices.

IEC Symbol for emergency stop. Black and white figure showing a circle with an inverted equilateral triangle inside, with an exclamation point contained inside the triangle.
IEC 60417–5638 — Sym­bol for “emer­gency stop” ©IEC.

I’ve scat­tered some exam­ples of dif­fer­ent com­pli­ant and non-com­pli­ant e-stop devices through this arti­cle.

The EU Machinery Directive, 2006/42/EC, and Emergency Stop

Inter­est­ing­ly, the Euro­pean Union has tak­en what looks like an oppos­ing view of the need for emer­gency stop sys­tems. Quot­ing from the Machin­ery Direc­tive [3, Annex I,]: Emer­gency stop
Machin­ery must be fit­ted with one or more emer­gency stop devices to enable actu­al or impend­ing dan­ger to be avert­ed.

Notice the words “…actu­al or impend­ing dan­ger…” This har­monis­es with the def­i­n­i­tion of Com­ple­men­tary Pro­tec­tive Mea­sures, in that they are intend­ed to allow a user to “avert or lim­it harm” from a haz­ard. Clear­ly, the direc­tion from the Euro­pean per­spec­tive is that ALL machines need to have an emer­gency stop. Or do they? The same clause goes on to say:

The fol­low­ing excep­tions apply:

  • machin­ery in which an emer­gency stop device would not lessen the risk, either because it would not reduce the stop­ping time or because it would not enable the spe­cial mea­sures required to deal with the risk to be tak­en,
  • portable hand-held and/or hand-guid­ed machin­ery.

From these two bul­lets it becomes clear that, just as in the Cana­di­an and US reg­u­la­tions, machines only need emer­gency stops WHEN THEY CAN REDUCE THE RISK. This is huge­ly impor­tant and often over­looked. If the risks can­not be con­trolled effec­tive­ly with an emer­gency stop, or if the risk would be increased or new risks would be intro­duced by the action of an e-stop sys­tem, then it should not be includ­ed in the design.

Car­ry­ing on with [3,]:

The device must:

  • have clear­ly iden­ti­fi­able, clear­ly vis­i­ble and quick­ly acces­si­ble con­trol devices,
  • stop the haz­ardous process as quick­ly as pos­si­ble, with­out cre­at­ing addi­tion­al risks,
  • where nec­es­sary, trig­ger or per­mit the trig­ger­ing of cer­tain safe­guard move­ments.

Once again, this is con­sis­tent with the gen­er­al require­ments found in the Cana­di­an and US reg­u­la­tions. [3] goes on to define the func­tion­al­i­ty of the sys­tem in more detail:

Once active oper­a­tion of the emer­gency stop device has ceased fol­low­ing a stop com­mand, that com­mand must be sus­tained by engage­ment of the emer­gency stop device until that engage­ment is specif­i­cal­ly over­rid­den; it must not be pos­si­ble to engage the device with­out trig­ger­ing a stop com­mand; it must be pos­si­ble to dis­en­gage the device only by an appro­pri­ate oper­a­tion, and dis­en­gag­ing the device must not restart the machin­ery but only per­mit restart­ing.

The emer­gency stop func­tion must be avail­able and oper­a­tional at all times, regard­less of the oper­at­ing mode.

Emer­gency stop devices must be a back-up to oth­er safe­guard­ing mea­sures and not a sub­sti­tute for them.

The first sen­tence of the first para­graph above is the one that requires e-stop devices to latch in the acti­vat­ed posi­tion. The last part of that sen­tence is even more impor­tant: “…dis­en­gag­ing the device must not restart the machin­ery but only per­mit restart­ing.” That phrase requires that every emer­gency stop sys­tem has a sec­ond dis­crete action to reset the emer­gency stop sys­tem. Pulling out the e-stop but­ton and hav­ing pow­er come back imme­di­ate­ly is not OK. Once that but­ton has been reset, a sec­ond action, such as push­ing a “POWER ON” or “RESET” but­ton to restore con­trol pow­er is need­ed.

Point of Clar­i­fi­ca­tion: I had a ques­tion come from a read­er ask­ing if com­bin­ing the E-stop func­tion and the reset func­tion was accept­able. It can be, but only if:

  • The risk assess­ment for the machin­ery does not indi­cate any haz­ards that might pre­clude this approach; and
  • The device is designed with the fol­low­ing char­ac­ter­is­tics:
    • The device must latch in the acti­vat­ed posi­tion;
    • The device must have a “neu­tral” posi­tion where the machine’s emer­gency stop sys­tem can be reset, or where the machine can be enabled to run;
    • The reset posi­tion must be dis­tinct from the pre­vi­ous two posi­tions, and the device must spring-return to the neu­tral posi­tion.

The sec­ond sen­tence har­mo­nizes with the require­ments of the Cana­di­an and US stan­dards. The last sen­tence har­mo­nizes with the idea of “Com­ple­men­tary Pro­tec­tive Mea­sures” as described in [2].

How Many and Where?

Where? “With­in easy reach”. Con­sid­er the loca­tions where you EXPECT an oper­a­tor to be. Besides the main con­trol con­sole, these could include feed hop­pers, con­sum­ables feed­ers, fin­ished goods exit points, etc. You get the idea. Any­where you can rea­son­ably expect an oper­a­tor to be under nor­mal cir­cum­stances is a rea­son­able place to put an e-stop device. “Easy Reach” I inter­pret as with­in the arm-span of an adult (pre­sum­ing the equip­ment is not intend­ed for use by chil­dren). The “easy reach” require­ment trans­lates to 500–600 mm either side of the cen­tre line of most work­sta­tions.

How do you know if you need an emer­gency stop? Start with a stop/start analy­sis. Iden­ti­fy all the nor­mal start­ing and stop­ping modes that you antic­i­pate on the equip­ment. Con­sid­er all of the dif­fer­ent oper­at­ing modes that you are pro­vid­ing, such as Auto­mat­ic, Man­u­al, Teach, Set­ting, etc. Iden­ti­fy all of the match­ing stop con­di­tions in the same modes, and ensure that all start func­tions have a match­ing stop func­tion.

Do a risk assess­ment. Risk assess­ment is a basic require­ment in most juris­dic­tions today.

As you deter­mine your risk con­trol mea­sures (fol­low­ing the Hier­ar­chy of Con­trols), look at what risks you might con­trol with an Emer­gency Stop. Remem­ber that e-stops fall below safe­guards in the hier­ar­chy, so you must use a safe­guard­ing tech­nique if pos­si­ble, you can’t just default down to an emer­gency stop. IF the e-stop can pro­vide you with the addi­tion­al risk reduc­tion then use it, but first, reduce the risks in oth­er ways.

The Stop Function and Functional Safety Requirements

Final­ly, once you deter­mine the need for an emer­gency stop sys­tem, you need to con­sid­er the system’s func­tion­al­i­ty and con­trols archi­tec­ture. NFPA 79 [4] has been the ref­er­ence stan­dard for Cana­da and is the ref­er­ence for the USA. In 2016, CSA intro­duced a new elec­tri­cal stan­dard for machin­ery, CSA C22.2 #301 [5]. This stan­dard is intend­ed for cer­ti­fi­ca­tion of indus­tri­al machines. My opin­ion is that this stan­dard has some sig­nif­i­cant issues. You can find very sim­i­lar elec­tri­cal require­ments to this in [4] in IEC 60204–1 [6] if you are work­ing in an inter­na­tion­al mar­ket. EN 60204–1 applies to the EU mar­ket for indus­tri­al machines and is tech­ni­cal­ly iden­ti­cal to [6].

Down­load NFPA stan­dards through ANSI
Down­load IEC stan­dards, Inter­na­tion­al Elec­trotech­ni­cal Com­mis­sion stan­dards.

Functional Stop Categories

NFPA 79 calls out three basic cat­e­gories of stop func­tions. Note that these cat­e­gories are NOT func­tion­al safe­ty archi­tec­tur­al cat­e­gories, but are cat­e­gories describ­ing stop­ping func­tions. Reli­a­bil­i­ty is not addressed in these sec­tions. Quot­ing from the stan­dard:

9.2.2 Stop Func­tions

Stop func­tions shall over­ride relat­ed start func­tions. The reset of the stop func­tions shall not ini­ti­ate any haz­ardous con­di­tions. The three cat­e­gories of stop func­tions shall be as fol­lows:

(1) Cat­e­go­ry 0 is an uncon­trolled stop by imme­di­ate­ly remov­ing pow­er to the machine actu­a­tors.

(2) Cat­e­go­ry 1 is a con­trolled stop with pow­er to the machine actu­a­tors avail­able to achieve the stop then pow­er is removed when the stop is achieved.

(3) Cat­e­go­ry 2 is a con­trolled stop with pow­er left avail­able to the machine actu­a­tors.

This E-Stop Button is correct.
Pho­to 4 — This E-Stop but­ton is CORRECT. Note the Push-Pull-Twist oper­a­tor and the YELLOW back­ground.

A bit lat­er in the stan­dard, we find: Stop.* Cat­e­go­ry 0, Cat­e­go­ry 1, and/or Cat­e­go­ry 2 stops shall be pro­vid­ed as deter­mined by the risk assess­ment and the func­tion­al require­ments of the machine. Cat­e­go­ry 0 and Cat­e­go­ry 1 stops shall be oper­a­tional regard­less of oper­at­ing modes, and Cat­e­go­ry 0 shall take pri­or­i­ty. Where required, pro­vi­sions to con­nect pro­tec­tive devices and inter­locks shall be pro­vid­ed. Where applic­a­ble, the stop func­tion shall sig­nal the log­ic of the con­trol sys­tem that such a con­di­tion exists.

You’ll also note that that pesky “risk assess­ment” pops up again in You just can’t get away from it…

The func­tion­al stop cat­e­gories are aligned with sim­i­lar terms used with motor dri­ves. You may want to read this arti­cle if your machin­ery uses a motor dri­ve.

Functional Safety

Disconnect with E-Stop Colours indicates that this device is intended to be used for EMERGENCY SWITCHING OFF.
Pho­to 5 — Dis­con­nect with E-Stop Colours indi­cates that this dis­con­nect­ing device is intend­ed to be used for EMERGENCY SWITCHING OFF.

Once you know what func­tion­al cat­e­go­ry of stop you need, and what degree of risk reduc­tion you are expect­ing from the emer­gency stop sys­tem, you can deter­mine the func­tion­al safe­ty require­ments. In Cana­da, [2, 8.2.1] requires that all new equip­ment be designed to com­ply with ISO 13849 [7], [8], or IEC 62061 [9]. This is a new require­ment that was added to [2] to help bring Cana­di­an machin­ery into har­mo­niza­tion with the Inter­na­tion­al Stan­dards.

Emer­gency stop func­tions are required to pro­vide a min­i­mum of ISO 13849–1, PLc, or IEC 62061 SIL1. If the risk assess­ment shows that greater reli­a­bil­i­ty is required, the sys­tem can be designed to meet any high­er reli­a­bil­i­ty require­ment that is suit­able. Essen­tial­ly, the greater the risk reduc­tion required, the high­er the degree of reli­a­bil­i­ty required.

I’ve writ­ten exten­sive­ly about the appli­ca­tion of ISO 13849, so if you are not sure what any of that means, you may want to read the series on that top­ic.

Extra points go to any read­er who noticed that the ‘elec­tri­cal haz­ard’ warn­ing label imme­di­ate­ly above the dis­con­nect han­dle in Pho­to 5 above is

a) upside down, and

b) using a non-stan­dard light­ing flash.

Cheap haz­ard warn­ing labels, like this one, are often as good as none at all. I’ll be writ­ing more on haz­ard warn­ings in future posts. In case you are inter­est­ed, here is the cor­rect ISO elec­tri­cal haz­ard label:

Yellow triangular background with a black triangular border and a stylized black lighting-flash arrow travelling from top to bottom.
Pho­to 6 — Elec­tric Shock Haz­ard — IEC 60417–5036

You can find these labels at Clar­i­on Safe­ty Sys­tems.

Use of Emergency Stop as part of a Lockout Procedure or HECP

One last note: Emer­gency stop func­tions and the sys­tem that imple­ment the func­tions (with the excep­tion of emer­gency switch­ing off devices, such as dis­con­nect switch­es used for e-stop) CANNOT be used for ener­gy iso­la­tion in an HECP — Haz­ardous Ener­gy Con­trol Pro­ce­dure (which includes Lock­out). Devices for this pur­pose must phys­i­cal­ly sep­a­rate the ener­gy source from the down­stream com­po­nents. See CSA Z460 [10] for more on that sub­ject.

Read our Arti­cle on Using E-Stops in Haz­ardous Ener­gy Con­trol Pro­ce­dures (HECP) includ­ing lock­out.

Pneumatic E-Stop Device
Pho­to 7 — Pneu­mat­ic E-Stop/Iso­la­tion device.


[1]  Indus­tri­al robots and robot sys­tems (Adopt­ed ISO 10218–1:2011, sec­ond edi­tion, 2011-07-01, with Cana­di­an devi­a­tions and ISO 10218–2:2011, first edi­tion, 2011-07-01, with Cana­di­an devi­a­tions). Cana­di­an Nation­al Stan­dard CAN/CSA Z434. 2014. 

[2]  Safe­guard­ing of Machin­ery, CSA Stan­dard Z432. 2016

[3]  DIRECTIVE 2006/42/EC OF THE EUROPEAN PARLIAMENT AND OF THE COUNCIL  of 17 May 2006  on machin­ery, and amend­ing Direc­tive 95/16/EC (recast). Brus­sels: Euro­pean Com­mis­sion, 2006.

[4]  Elec­tri­cal Stan­dard for Indus­tri­al Machin­ery. ANSI/NFPA Stan­dard 79. 2015.

Down­load NFPA stan­dards at ANSI

[5] Indus­tri­al elec­tri­cal machin­ery. CSA Stan­dard C22.2 NO. 301. 2016. 

[6] Safe­ty of machin­ery — Elec­tri­cal Equip­ment of machines — Part 1: Gen­er­al require­ments. IEC Stan­dard 60204–1. 2016.  

Down­load IEC stan­dards, Inter­na­tion­al Elec­trotech­ni­cal Com­mis­sion stan­dards.

[7] Safe­ty of machin­ery — Safe­ty-relat­ed parts of con­trol sys­tems — Part 1: Gen­er­al prin­ci­ples for design. ISO Stan­dard 13849–1. 2015.

[8] Safe­ty of machin­ery — Safe­ty-relat­ed parts of con­trol sys­tems — Part 2: Val­i­da­tion. ISO Stan­dard 13849–2. 2012.

[9] Safe­ty of machin­ery — Func­tion­al safe­ty of safe­ty-relat­ed elec­tri­cal, elec­tron­ic and pro­gram­ma­ble elec­tron­ic con­trol sys­tems. IEC Stan­dard 62061+AMD1+AMD2. 2015.

[10] Safe­ty of machin­eryEmer­gency Stop—Principals for design. ISO Stan­dard 13850. 2015.

Down­load IEC stan­dards, Inter­na­tion­al Elec­trotech­ni­cal Com­mis­sion stan­dards.
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[11] Con­trol of haz­ardous energy—Lockout and oth­er meth­ods. CSA Stan­dard Z460. 2013.