Machinery Safety 101

More E‑Stop Questions

This entry is part 17 of 16 in the series Emer­gency Stop

Here are some more ques­tions I’ve been asked regard­ing emer­gency stop require­ments. These ones came to me through the IEEE PSES EMC-PSTC Product Com­pli­ance For­um mail­ing list.

Primary Sources

There are three primary sources for the require­ments for emer­gency stop devices:

[1] Safety of machinery — Emer­gency stop — Prin­ciples for design, 3rd Edi­tion. ISO 13850. 2015.

[2] Safety of machinery – Elec­tric­al equip­ment of machines – Part 1: Gen­er­al require­ments. IEC 60204 – 1. 2018.

[3] Low voltage switchgear and con­trol­gear — Part 5 – 5: Con­trol cir­cuit devices and switch­ing ele­ments — Elec­tric­al emer­gency stop device with mech­an­ic­al latch­ing func­tion. IEC 60947 – 5‑5/A1/A2/A11. 2013.

All of the oth­er Nation­al Stand­ards, like NFPA 79 [4], or CSA C22.2 No. 301 [5] and the type C stand­ards for par­tic­u­lar types of machinery have their emer­gency stop require­ments derived from these standards.

Question 1: The button shape

Q1. “Mush­room Shape” is spe­cific­ally called out in some stand­ards but the new­er style e‑stop switches are becom­ing less and less Mush­room Shaped.  Is this a concern?

A1. How mush­room-shaped is enough?
There’s no hard answer to this, but I can say this: there are three fun­da­ment­al shapes for push but­ton oper­at­ors, flush, exten­ded, and mush­room. The intent is that neither flush nor exten­ded head oper­at­ors be used for e‑stop func­tions. The images below show the vari­ous types of push but­ton oper­at­ors used industrially.

The IEC stand­ard that details the require­ments for the emer­gency stop device is [3]. This stand­ard details the elec­tric­al and mech­an­ic­al require­ments for these devices, includ­ing pull-cord switches and foot ped­al switches. No spe­cif­ic require­ments for the shape of the head are given.

Question 2: The background size

Q2. “Yel­low Back­ground” is get­ting smal­ler and smal­ler. Some small e‑stop switches has [sic] no yel­low back­ground but has [sic] a small area on the shaft of the switch that is yel­low. This is not tech­nic­ally the “back­ground”.  Is this accept­able or is a yel­low back­ground required in addi­tion?  The size of the yel­low area is not spe­cified as far as I have found. 

A2. There is no expli­cit guid­ance in any of the rel­ev­ant stand­ards on this spe­cif­ic detail. Yel­low back­grounds have to be big enough to provide a clear con­trast with the red but­ton oper­at­or. A tiny ring or just a yel­low stripe around the base of the but­ton is not enough in my opin­ion. Typ­ic­al rings are 2x the oper­at­or nom­in­al size, i.e., 30 mm but­tons get a 60 mm ring, 22 mm but­tons get a 44 mm ring. These are com­mon prac­tice recommendations. 

Question 3: The background colour

Q3. Must the back­ground be yel­low, or can it be any col­or as long as it provides strong con­trast with the red but­ton? Someone sug­ges­ted blue as a back­ground col­or because allegedly this is easi­er for col­or blind per­sons to see.

A3. No, it abso­lutely must be YELLOW as the recog­nized col­our com­bin­a­tion is RED and YELLOW. See IEC 60204 – 1:2018, 10.7, ISO 13850:2015, 4.3.6, and NFPA 79, 2015,
Here are the actu­al words with respect to the col­our and size [3, 4.3.6]:

4.3.6 The actu­at­or of the emer­gency stop device shall be col­oured RED. As far as a back­ground exists behind the actu­at­or and as far as it is prac­tic­able, the back­ground shall be col­oured YELLOW.
Emer­gency stop devices shall be designed and moun­ted in such a way that the actu­ation can­not be eas­ily blocked by simple means.

NOTE This can hap­pen when objects fall beneath the actu­at­ing sur­face or when there is an inten­tion of defeating.

Emer­gency stop device requir­ing a key on the actu­at­or to be dis­en­gaged (unlatched) should be avoided. When an emer­gency stop actu­at­or can only be dis­en­gaged by using a key, to avoid injur­ies to hands, instruc­tion for use of the machine shall describe the cor­rect use of the key and provide a warn­ing that the key should only be in the actu­at­or of the device to dis­en­gage the actuator.

IEC 60947 – 5‑5

Question 4: The legend text

Q4. Eng­lish text “Emer­gency Stop” prin­ted on the but­ton or on the yel­low back­ground ring:  Is this a prob­lem in non-eng­lish [sic] speak­ing coun­tries?  Do I have to trans­late (like in French for France and Canada) or are these words “glob­ally accept­able”?  I have been telling our engin­eers to choose e‑stop switches and yel­low rings without text but some man­u­fac­tur­ers only provide it with the text. 

A4. Print­ing “emer­gency stop” on the yel­low ring is not required in any jur­is­dic­tion any­more, and has been removed from IEC 60204 – 1 [2], as well as NFPA 79 [4]. ISO 13850 [1, 4.3.7] does not per­mit the use of text on the background. 

4.3.7 Neither the actu­at­or nor the back­ground should be labelled with text or sym­bols. Where a sym­bol is needed for cla­ri­fic­a­tion, the sym­bol from IEC 60417 – 5638 shall be used, see Fig­ure 2.

When it is neces­sary to identi­fy the dir­ec­tion of unlatch­ing of the actu­at­or (but­ton) then this iden­ti­fic­a­tion shall have the same or nearly the same col­our as the actu­at­or (see also IEC 60947 – 5‑5).

NOTE The iden­ti­fic­a­tion of unlatch­ing (i.e. arrows) could be mis­in­ter­preted as dir­ec­tion of actuation.

ISO 13850

Just leave it off, as the red/yellow com­bin­a­tion is inter­na­tion­ally recog­nized as the sign for e‑stop and emer­gency switch­ing off devices. The only mark­ings that are uni­ver­sally per­mit­ted are the arrows to show the dir­ec­tion to twist the oper­at­or for twist-to-release devices, and the emer­gency stop graph­ic shown below.

, More E‑Stop Questions, Machinery Safety 101
IEC 60947 – 5‑5


IEEE PSES Mem­bers and the EMC-PSTC list


[1] Safety of machinery — Emer­gency stop — Prin­ciples for design, 3rd Edi­tion. ISO 13850. 2015

[2] Safety of machinery – Elec­tric­al equip­ment of machines – Part 34: Require­ments for machine tools. IEC 60204 – 1. 2018.

[3] Low voltage switchgear and con­trol­gear — Part 5 – 5: Con­trol cir­cuit devices and switch­ing ele­ments — Elec­tric­al emer­gency stop device with mech­an­ic­al latch­ing func­tion. IEC 60947 – 5‑5/A1/A2/A11. 2013.

[4] Elec­tric­al Stand­ard for Indus­tri­al Machinery. NFPA 79. 2018.

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

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Acknow­ledge­ments: IEC, ISO, The Oth­er Bri­an, Mark Ortlieb
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Series Nav­ig­a­tionCan Emer­gency Stop be used as an “on/off” control?

10 thoughts on “More E‑Stop Questions

  1. Hi Doug
    Great thread, what’s your thoughts on the below.

    Are there recog­nized guidelines for illu­min­ated estops ?

    White LED back­ground ring – armed
    Red LEDES activated

    White armed
    Red flash­ing loc­al PB activated
    Red sol­id all oth­er PB’s

    C – mod­u­lar machines
    Con­figured Estops white LED armed
    Non illu­min­ated – not active
    Red as above

    1. Hi Bob,

      All of the options you’ve sug­ges­ted are non-com­pli­ant. ISO 13850 requires e‑stops to be red with a yel­low back­ground. End stop. This require­ment is sup­por­ted by IEC 60204 – 1 and NFPA 79, as well as CSA Z432 and the ANSI B11 fam­ily of machinery safety stand­ards. Sorry.

  2. hi con­rad
    our cranes have pendants that had a lock­ing red mush­room on the bot­tom . but these were taken off as they kept get­ting smashed off . and a flat e stop fit­ted .on the front of the pendant with all the oth­er but­tons,. and a reset but­ton once the e stop had been pressed .. is it law that a mush­room type e stop has to be fit­ted on cranes ? or can we leave it like it is ?
    thanks andy

    1. Hi Andy,

      Since I don’t have a copy of the type C stand­ard for your type of crane, I can only respond from the per­spect­ive of ISO 13850, the type B2 stand­ard cov­er­ing the emer­gency stop func­tion. With respect to the emer­gency stop device, ISo 13850 says:

      4.3.1 Emer­gency stop devices shall be designed to be eas­ily iden­ti­fied and actu­ated by the operator
      and oth­ers who could need to actu­ate them. The actu­at­or of the emer­gency stop device may be one of the
      fol­low­ing types:
      a) push­but­tons eas­ily activ­ated by the palm of a hand;
      b) wires, ropes, bars;
      c) handles;
      d) foot-ped­als without a pro­tect­ive cov­er, where oth­er solu­tions are not applicable.
      NOTE For a sup­ply dis­con­nect­ing device to effect emer­gency stop, see IEC 60204 – 1.
      4.3.2 An emer­gency stop device shall be located:
       — at each oper­at­or con­trol sta­tion, except where the risk assess­ment indic­ates that this is not necessary;
       — at oth­er loc­a­tions, as determ­ined by the risk assess­ment, e.g.:
       — at entrance and exit locations;
       — at loc­a­tions where inter­ven­tion to the machinery is needed, e.g. oper­a­tions with a hold-to-run con­trol function;
       — at all places where a man / machine inter­ac­tion is expec­ted by design (load­ing / unload­ing zone for example).

      Emer­gency stop devices shall be posi­tioned so that they are dir­ectly access­ible and cap­able of nonhazardous
      actu­ation by the oper­at­or and oth­ers who could need to actu­ate them.

      The actu­at­or of emer­gency stop device inten­ded to be actu­ated by hand should be moun­ted between
      0,6 m and 1,7 m above the access level (e.g. floor level, plat­form level).

      Foot-ped­als should be moun­ted in a fixed pos­i­tion dir­ectly at access level (e.g. floor level).

      4.3.3 An elec­tric­al emer­gency stop device shall apply the prin­ciple of dir­ect open­ing action with
      mech­an­ic­al latch­ing.
      Elec­tric­al emer­gency stop devices shall be in accord­ance with IEC 60947 – 5‑5.
      NOTE An example of the applic­a­tion of the dir­ect open­ing action prin­ciple is an emer­gency stop device
      employ­ing elec­tric­al con­tacts that are opened by means of a dir­ect rigid con­nec­tion with the pushbutton.
      Accord­ing to IEC 60947 – 5 – 1, dir­ect open­ing action (of a con­tact ele­ment) is the achieve­ment of con­tact separation
      as the dir­ect res­ult of a spe­cified move­ment of the switch actu­at­or through non-resi­li­ent mem­bers (for example,
      not depend­ent upon springs).

      I’ve high­lighted the key elec­tric­al require­ments for the emer­gency stop device – if your flat but­ton meets these require­ments and those in IEC 60204 – 1 for the emer­gency stop device, then it may be ok. Oth­er­wise your design may need some re-thinking.


  3. Hi Doug,

    I’ve got anoth­er col­or ques­tion. I can­’t find the source, but I remem­ber guid­ance that reserved Grey EStop Mush­rooms (red LED) to indic­ate an estop sta­tion which can be dis­con­nec­ted or bypassed, and Yel­low EStop Mush­rooms (red LED) indic­ate a zone or machine-level stop, while Red EStop Mush­rooms (red LED) are reserved for true “glob­al” EStops. Older Siemens port­able HMIs were equipped with grey mush­rooms for that reas­on I believe. Is that guid­ance still relavent today? I wish I remembered where I read it, I prom­ise this is not my imagination!

    1. Hi Mike,

      I’ve had this dis­cus­sion with cli­ents before too, and I’ve seen the Siemens pendants with the gray mush­room head but­ton on them first hand. 

      To my know­ledge, there is no stand­ard or reg­u­la­tion that jus­ti­fies this approach or the yel­low but­ton with the red LED you men­tioned, any­where in the world. ISO 13850, which for us machinery people is the source, does not per­mit these col­ors, nor does IEC 60204 – 1 nor NFPA 79. 

      Sorry I couldn’t be more help.


      1. That’s alright – ask­ing around, it seems like those pendants with the grey but­ton are quite old (+10 years ago) and were lis­ted as hav­ing “Stop-but­tons” in their spe­cific­a­tions. No men­tion of “Emer­gency Stop”

        I’m torn on how best to com­mu­nic­ate spe­cial Emer­gency Stops like machine-level, zone, glob­al, or dis­con­nect-able. As you’ve described, there isn’t wiggle room for altern­ate col­ors or labeling.

        If mul­tiple stop­ping “levels” are required, it seems like it would be clean­er to delin­eate the low­est “machine-level” stop as the true Emer­gency Stop func­tion, then eval­u­ate zone, glob­al, or dis­con­nect-able stops as sep­ar­ate com­pli­ment­ary stop func­tions. Since those oth­er but­tons would not be con­sidered true Emer­gency Stops at this point, there could be more labeling and col­or freedom?

        1. Hi Mike,

          ISO 13850 has some excel­lent guid­ance on zon­ing emer­gency stop func­tions in chapter 4.1.2. You might want to check that out. The basic premise is that you need good iden­ti­fic­a­tion if you have sep­ar­ate e‑stops for say a “mas­ter” line e‑stop and a zone or cell e‑stop. The way I’ve handled that in the past is to use a pull-cord switch down the length of the line, usu­ally run over­head with t‑handles or tri­an­gu­lar handles, and then e‑stop but­tons for the cell e‑stops. Pull the cord and the line dies, press the but­ton and the cell dies. An extra lamac­oid tag on the con­trol pan­el iden­ti­fies the but­ton as “Cell XXX e‑stop” and the pullcords have tags on the handles with “master/line e‑stop.”

  4. Why is it that I see so many Emer­gency Stop but­tons being sold by man­u­fac­tur­ers with text and logos on both actu­at­or and back­ground. Why have are these com­pan­ies still allowed to sell these devices in to the mar­ket if they do not meet the stand­ards and who is sup­posed to be poli­cing it.

    1. Hi Con­rad,

      Inter­est­ing ques­tion, with a rel­at­ively simple, if likely unsat­is­fy­ing answer. The tech­nic­al stand­ards are vol­un­tary unless they are spe­cific­ally called out in nation­al legis­la­tion. There are very few stand­ards like this that are spe­cific­ally called out this way, so there is often more than one way, leg­ally, that things can be done. Hav­ing said that, from a liab­il­ity stand­point machinery man­u­fac­tur­ers are put­ting them­selves at con­sid­er­able risk if they ignore the applic­able vol­un­tary stand­ards for their product since courts take the pub­lished stand­ards as evid­ence of the “state-of-the-art.”

      With respect to e‑stop devices, reput­able man­u­fac­tur­ers sup­ply con­form­ing devices, but they also sell what their cus­tom­ers want. It’s to you, the design engin­eer, to know the codes and stand­ards and to use them appropriately.

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