More E-Stop Questions

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, ISO 13850. International Organization for Standardization (ISO). 2015.

[2] Safety of machinery — Electrical equipment of machines — Part 1: General requirements, IEC 60204-1. International Electrotechnical Commission (IEC). 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. International Electrotechnical Commission (IEC). 2013.

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 these devices’ electrical and mechanical requirements, including pull-cord and foot pedal switches. No specific requirements for the shape of the head are given.

Question 2: The size of the yellow background

Q2. The “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 relevant standards on this specific detail. Yellow backgrounds have to be big enough to provide a clear contrast with the red button operator. In my opinion, a tiny ring or just a yellow stripe around the base of the button is not enough. Typical rings are 2x the operator nominal size, i.e., 30 mm operators with a 60 mm diameter buttons get at least a 60 mm ring, and 22 mm operator with a 40 mm diameter button gets at least a 40 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 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 concerning 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 but does permit the emergency stop symbol,
IEC 60417-5638, see below.

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, ISO 13850. International Organization for Standardization (ISO). 2015.

[2] Safety of machinery — Electrical equipment of machines — Part 1: General requirements, IEC 60204-1. International Electrotechnical Commission (IEC). 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. International Electrotechnical Commission (IEC). 2013.

[4] Electrical Standard for Industrial Machinery, NFPA 79. National Fire Protection Association (NFPA). 2018.

[5] Industrial electrical machinery, CSA C22.2 No. 301. Canadian Standards Association (CSA). 2016.

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12 thoughts on “More E-Stop Questions

  1. Hi Sir. is it allowed for an emergency stop to convert as touch (digital) on screen?

    1. Hi Eman,

      The short answer: NO. ISO 13850 requires that emergency stop devices mechanically latch in the activated position. This requirement cannot be met by a graphical representation of an estop device on an HMI.

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

    Are there recognized guidelines for illuminated estops ?

    White LED background ring – armed
    Red LED – ES 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.

  3. 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.


  4. 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.”

  5. 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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