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.

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?

2 thoughts on “More E‑Stop Questions

  1. 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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