Emergency Stop — What’s so confusing about that?

Emergency Stop on machine console

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, 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.

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

 

 

Series Nav­i­ga­tionCheck­ing Emer­gency Stop Sys­tems

Author: Doug Nix

Doug Nix is Managing Director and Principal Consultant at Compliance InSight Consulting, Inc. (http://www.complianceinsight.ca) in Kitchener, Ontario, and is Lead Author and Senior Editor of the Machinery Safety 101 blog. Doug's work includes teaching machinery risk assessment techniques privately and through Conestoga College Institute of Technology and Advanced Learning in Kitchener, Ontario, as well as providing technical services and training programs to clients related to risk assessment, industrial machinery safety, safety-related control system integration and reliability, laser safety and regulatory conformity. For more see Doug's LinkedIn profile.