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.

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

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.

Reliability

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

Where

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]

References

[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!

Checking Emergency Stop Systems

This entry is part 2 of 13 in the series Emergency Stop

This short art­icle dis­cusses ways to test emer­gency stop sys­tems on machines.

This entry is part 2 of 13 in the series Emergency Stop

A while back I wrote about the basic design require­ments for Emergency Stop sys­tems. I’ve had sev­er­al people con­tact me want­ing to know about check­ing and test­ing emer­gency stops, so here are my thoughts on this pro­cess.

Figure 1 below, excerp­ted from the 1996 edi­tion of ISO 13850, Safety of machinery — Emergency stop — Principles for design, shows the emer­gency stop func­tion graph­ic­ally. As you can see, the ini­ti­at­ing factor in this func­tion is a per­son becom­ing aware of the need for an emer­gency stop. This is NOT an auto­mat­ic func­tion and is NOT a safety or safe­guard­ing func­tion.

Download ISO Standards 

ISO 13850 1996 Figure 1 - Emergency Stop Function
ISO 13850 1996 Figure 1 – Emergency Stop Function

Download ISO Standards 

I men­tion this because many people are con­fused about this point. Emergency stop sys­tems are con­sidered to be ‘com­pli­ment­ary pro­tect­ive meas­ures’, mean­ing that their func­tions com­ple­ment the safe­guard­ing sys­tems, but can­not be con­sidered to be safe­guards in and of them­selves. This is sig­ni­fic­ant. Safeguarding sys­tems are required to act auto­mat­ic­ally to pro­tect an exposed per­son. Think about how an inter­locked gate or a light cur­tain acts to stop haz­ard­ous motion BEFORE the per­son can reach it. Emergency stop is nor­mally used AFTER the per­son is already involved with the haz­ard, and the next step is nor­mally to call 911.

All of that is import­ant from the per­spect­ive of con­trol reli­ab­il­ity. The con­trol reli­ab­il­ity require­ments for emer­gency stop sys­tems are often dif­fer­ent from those for the safe­guard­ing sys­tems because they are a backup sys­tem. Determination of the reli­ab­il­ity require­ments is based on the risk assess­ment and on an ana­lys­is of the cir­cum­stances where you, as the design­er, anti­cip­ate that emer­gency stop may be help­ful in redu­cing or avoid­ing injury or machinery dam­age. Frequently, these sys­tems have lower con­trol reli­ab­il­ity require­ments than do safe­guard­ing sys­tems.

Before you begin any test­ing, under­stand what effects the test­ing will have on the machinery. Emergency stops can be par­tially tested with the machinery at rest. Depending on the func­tion of the machinery and the dif­fi­culty in recov­er­ing from an emer­gency stop con­di­tion, you may need to adjust your approach to these tests. Start by review­ing the emer­gency stop func­tion­al descrip­tion in the manu­al. Here’s an example taken from a real machine manu­al:

Emergency Stop (E-​Stop) Button

Emergency Stop Button
Figure 2.1 Emergency Stop (E-​Stop) Button

A red emer­gency stop (E-​Stop) but­ton is a safety device which allows the oper­at­or to stop the machine in an emer­gency. At any time dur­ing oper­a­tion, press the E-​Stop but­ton to dis­con­nect actu­at­or power and stop all con­nec­ted machines in the pro­duc­tion line. Figure 2.1 shows the emer­gency stop but­ton.

There is one E-​Stop but­ton on the pneu­mat­ic pan­el.

NOTE: After press­ing the E-​Stop but­ton, the entire pro­duc­tion line from spreader-​feeder to stack­er shuts down. When the E-​Stop but­ton is reset, all machines in the pro­duc­tion line will need to be restar­ted.

DANGER: These devices do not dis­con­nect main elec­tric­al power from the machine. See “Electrical Disconnect” on page 21.

As you can see, the gen­er­al func­tion of the but­ton is described, and some warn­ings are giv­en about what does and doesn’t hap­pen when the but­ton is pressed.

Now, if the emer­gency stop sys­tem has been designed prop­erly and the machine is oper­at­ing nor­mally, press­ing the emer­gency stop but­ton while the machine is in mid-​cycle should res­ult in the machinery com­ing to a fast and grace­ful stop. Here is what ISO 13850 has to say about this con­di­tion:

4.1.3 The emer­gency stop func­tion shall be so designed that, after actu­ation of the emer­gency stop actu­at­or, haz­ard­ous move­ments and oper­a­tions of the machine are stopped in an appro­pri­ate man­ner, without cre­at­ing addi­tion­al haz­ards and without any fur­ther inter­ven­tion by any per­son, accord­ing to the risk assess­ment.
An “appro­pri­ate man­ner” can include

  • choice of an optim­al decel­er­a­tion rate,
  • selec­tion of the stop cat­egory (see 4.1.4), and
  • employ­ment of a pre­de­ter­mined shut­down sequence.

The emer­gency stop func­tion shall be so designed that a decision to use the emer­gency stop device does not require the machine oper­at­or to con­sider the res­ult­ant effects.

The inten­tion of this func­tion is to bring the machinery to a halt as quickly as pos­sible without dam­aging the machine. However, if the brak­ing sys­tems fail, e.g. the servo drive fails to decel­er­ate the tool­ing as it should, then drop­ping power and poten­tially dam­aging the machinery is accept­able.

In many sys­tems, press­ing the e-​stop but­ton or oth­er­wise activ­at­ing the emer­gency stop sys­tem will res­ult in a fault or an error being dis­played on the machine’s oper­at­or dis­play. This can be used as an indic­a­tion that the con­trol sys­tem ‘knows’ that the sys­tem has been activ­ated.

ISO 13850 requires that emer­gency stop sys­tems exhib­it the fol­low­ing key beha­viours:

  • It must over­ride all oth­er con­trol func­tions, and no start func­tions are per­mit­ted (inten­ded, unin­ten­ded or unex­pec­ted) until the emer­gency stop has been reset;
  • Use of the emer­gency stop can­not impair the oper­a­tion of any func­tions of the machine inten­ded for the release of trapped per­sons;
  • It is not per­mit­ted to affect the func­tion of any oth­er safety crit­ic­al sys­tems or devices.

Tests

Once the emer­gency stop device has been activ­ated, con­trol power is nor­mally lost. Pressing any START func­tion on the con­trol pan­el, except POWER ON or RESET should have no effect. If any aspect of the machine starts, count this as a FAILED test.

If reset­ting the emer­gency stop device res­ults in con­trol power being re-​applied, count this as a FAILED test.

Pressing POWER ON or RESET before the activ­ated emer­gency stop device has been reset (i.e. the e-​stop but­ton has been pulled out to the ‘oper­ate’ pos­i­tion), should have no effect. If you can turn the power back on before you reset the emer­gency stop device, count this as a FAILED test.

Once the emer­gency stop device has been reset, press­ing POWER ON or RESET should res­ult in the con­trol power being restored. This is accept­able. The machine should not restart. If the machine restarts nor­mal oper­a­tion, count this as a FAILED test.

Once con­trol power is back on, you may have a num­ber of faults to clear. When all the faults have been cleared, press­ing the START but­ton should res­ult in the machine restart­ing. This is accept­able beha­viour.

If you break the machine while test­ing the emer­gency stop sys­tem, count this as a FAILED test.

Test all emer­gency stop devices. A wir­ing error or oth­er prob­lems may not be appar­ent until the emer­gency stop device is tested. Push all but­tons, pull all pull cords, activ­ate all emer­gency stop devices. If any fail to cre­ate the emer­gency stop con­di­tion, count this as a FAILED test.

If, hav­ing con­duc­ted all of these tests, no fail­ures have been detec­ted, con­sider the sys­tem to have passed basic func­tion­al test­ing. Depending on the com­plex­ity of the sys­tem and the crit­ic­al­ity of the emer­gency stop func­tion, addi­tion­al test­ing may be required. It may be neces­sary to devel­op some func­tion­al tests that are con­duc­ted while vari­ous EMI sig­nals are present, for example.

If you have any ques­tions regard­ing test­ing of emer­gency stop devices, please email me!

Download ISO Standards 

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

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

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-​July, 2017.

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

Old, non-compliant, E-Stop Button
Photo 1 – This OLD but­ton is def­in­itely non-​compliant.

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.

Modern, non-compliant e-stop button.
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 is may not be required to have an Emergency Stop. Quoting from [2, 6.3.5.2]:

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.

Later in [2, 7.15.1.2]:

Each oper­at­or con­trol sta­tion, includ­ing pendants, cap­able of ini­ti­at­ing machine motion and/​or auto­mat­ic motion shall have an emer­gency stop func­tion (see Clause 6.3.5.2), 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 bold text in the pre­ced­ing para­graph is mine. 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. In some cases, product fam­ily stand­ards often called “Type C” stand­ards, includ­ing 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

This more modern button is still wrong due to the RED background.
Photo 3 – This more mod­ern but­ton is non-​compliant due to the RED back­ground.

If you read Ontario’s Industrial Establishments Regulation (Regulation 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 behind it, is recog­nized as EMERGENCY STOP or EMERGENCY OFF, in the case of dis­con­nect­ing switches or con­trol switches. 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, 1.2.4.3]:

1.2.4.3. 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, 1.2.4.3]:

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:

9.2.5.3 Stop.

9.2.5.3.1* 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.

9.2.5.3.2 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 9.2.5.3.1. 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.

References

[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.
Download ISO Standards 

[11] Control of haz­ard­ous energy — Lockout and oth­er meth­ods. CSA Standard Z460. 2013.