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 man­date the require­ment for emer­gency stop, such as CSA Z434-​​03, 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

This OLD but­ton is def­i­nitely non-​​compliant.

So what is an Emergency Stop, or e-​​stop, and when do you need to have one? Let’s look at a few def­i­n­i­tions taken from CSA Z432-​​04:

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.

and one more:

6.2.3.5.3 Complementary pro­tec­tive mea­sures
Following the risk assess­ment, the mea­sures in this clause either shall be applied to the machine or shall be dealt with in the infor­ma­tion for use.

Protective mea­sures that are nei­ther inher­ently safe design mea­sures, nor safe­guard­ing (imple­men­ta­tion of guards and/​or pro­tec­tive devices), nor infor­ma­tion for use may have to be imple­mented as required by the intended use and the rea­son­ably fore­see­able mis­use of the machine. Such mea­sures shall include, but not be lim­ited to,

a) emer­gency stop;

b) means of res­cue of trapped per­sons; and

c) means of energy iso­la­tion and dissipation.

This more mod­ern but­ton is non-​​compliant due to the RED back­ground and spring-​​return button.

So, an e-​​stop is a sys­tem 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. So far so, good.

Is an Emergency Stop Required?

Depending on the reg­u­la­tions and the stan­dards you choose to read, machin­ery is not required to have  an Emergency Stop. Quoting from CSA Z432-​​04:

6.2.5.2.1 Components and ele­ments to achieve the emer­gency stop func­tion
If, fol­low­ing a risk assess­ment, it is deter­mined that in order to achieve ade­quate risk reduc­tion under emer­gency cir­cum­stances a machine must be fit­ted with com­po­nents and ele­ments nec­es­sary to achieve an emer­gency stop func­tion so that actual or impend­ing emer­gency sit­u­a­tions can be con­trolled, the fol­low­ing require­ments shall apply:

a) The actu­a­tors shall be clearly iden­ti­fi­able, clearly vis­i­ble, and read­ily accessible.

b) The haz­ardous process shall be stopped as quickly as pos­si­ble with­out cre­at­ing addi­tional haz­ards.
If this is not pos­si­ble or the risk can­not be ade­quately reduced, this may indi­cate that an emer­gency stop func­tion may not be the best solu­tion (i.e., other solu­tions should be sought). (Bolding added for empha­sis — DN)

c) The emer­gency stop con­trol shall trig­ger or per­mit the trig­ger­ing of cer­tain safe­guard move­ments where necessary.

Note: For more detailed pro­vi­sions, see NFPA 79.

Download NFPA stan­dards through ANSI

This more mod­ern but­ton is non-​​compliant due to the RED background.

In fact, if you read Ontario’s Industrial Establishments reg­u­la­tion (Regulation 851), you will find that the only require­ment for an emer­gency stop is that it is prop­erly iden­ti­fied and located “within easy reach” of the oper­a­tor. 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 Annex I of the Machinery Directive:

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 har­mo­nizes 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 the same clause:

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. The direc­tive 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 have 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.

Finally, the last sen­tence har­mo­nizes with the idea of “Complementary Protective Measures” as described in CSA Z432.

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… 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). This trans­lates to 500–600 mm either side of the cen­ter line of most work stations.

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. This 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 Control Reliability 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 is the ref­er­ence stan­dard for Canada, although you can find very sim­i­lar require­ments in IEC 60204–1 if you are work­ing in an inter­na­tional market.

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

Functional Stop Categories

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

9.2.2 Stop Functions. 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 remove power 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. Note the Push-​​Pull-​​Twist oper­a­tor and the YELLOW background.

A bit later, the stan­dards says:

9.2.5.3 Stop.
9.2.5.3.1 Each machine shall be equipped with a Category 0 stop.

9.2.5.3.2 Category 0, Category 1, and/​or Category 2 stops shall be pro­vided where indi­cated by an analy­sis of 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. Stop func­tion shall oper­ate by de-​​energizing that rel­e­vant cir­cuit and shall over­ride related start functions.

Note that 9.2.5.3.1 does NOT mean that every machine must have an e-​​stop. It sim­ply says that every machine must have a way to stop the machine that is equiv­a­lent to “pulling the plug”. The main dis­con­nect on the con­trol panel can be used for this func­tion if sized and rated appro­pri­ately. The ques­tion of HOW to effect the Category 0 stop depends on WHEN it will be used — i.e. what risks must be reduced, or what haz­ards must be con­trolled by the e-​​stop.

You’ll also note that that pesky “risk assess­ment” pops up again in 9.2.5.3.2. You just can’t get away from it…

Control Reliability

Disconnect with E-​​Stop Colours indi­cates that this 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 degree of reli­a­bil­ity required. In Canada, CSA Z432 gives us these cat­e­gories: SIMPLE, SINGLE CHANNEL, SINGLE CHANNEL MONITORED and CONTROL RELIABLE. These cat­e­gories are being replaced slowly by Performance Levels (PL) as defined in ISO 13849–1 2007.

The short answer is that the greater the risk reduc­tion required, the higher the degree of reli­a­bil­ity required. In many cases, a SINGLE CHANNEL or SINGLE CHANNEL MONITORED solu­tion may be accept­able, par­tic­u­larly when there are more reli­able safe­guards in place. On the other hand, you may require CONTROL RELIABLE designs if the e-​​stop is the pri­mary risk reduc­tion for some risks or spe­cific tasks.

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

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

One last note: Emergency stop sys­tems (with the excep­tion of emer­gency switch­ing off devices, such as dis­con­nect switches used for e-​​stop) CANNOT be used for energy iso­la­tion in a Hazardous Energy Control Procedure (a.k.a. Lockout). Devices for this pur­pose must phys­i­cally sep­a­rate the energy source from the down-​​stream com­po­nents. See CSA Z460-​​05 for more on that subject.

Read our Article on Using E-​​Stops in HECP.

Pneumatic E-​​Stop/​Isolation device.

Standards Referenced in this post:

CSA Z432-​​04, Safeguarding of Machinery

NFPA 79–07, Electrical Standard for Industrial Machinery
Download NFPA stan­dards at ANSI

IEC 60204–1:09,  SAFETY OF MACHINERY — ELECTRICAL EQUIPMENT OF MACHINES — PART 1: GENERAL REQUIREMENTS

Download IEC stan­dards, International Electrotechnical Commission standards.

ISO 13849−1−2007, Safety of machin­ery — Safety-​​related parts of con­trol sys­tems — Part 1: General prin­ci­ples for design

See also

ISO 13850:06, SAFETY OF MACHINERY — EMERGENCY STOP — PRINCIPLES FOR DESIGN

Download IEC stan­dards, International Electrotechnical Commission stan­dards.
Download ISO Standards

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