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-​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.