Machinery Safety 101

Emergency Stop – What’s so confusing about that?

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

Red "stop" button on a grey box.
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, including 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 is 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 on 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 the 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] and 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

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.

Resetting the emergency stop function

To restart the machine after an emergency stop there are three steps that must occur:

  1. Mechanically reset the emergency stop device, e.g., pull the button back out to the “operate” position,
  2. Reset the emergency stop function,
  3. Restart the machine, e.g., press the “start” button.

The manual reset function cannot be done via an HMI, so a separate manual push-button must be used. The standard colour for the reset button is BLUE according to [4], [5] and [6]. The push-button must be located where the complete area inside the safeguarding can be seen.

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.

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

32 thoughts on “Emergency Stop – What’s so confusing about that?

  1. Hi,

    Thanks for a good read.

    I’m curious as to when we can use a disconnect switch as emergency stop? I’ve seen it be done but can’t seem to find relevant standard that support it.

    In 60204-1 it’s described what it must fullfil to be used but not when it can be used
    In 13850 it says that all emergency stops must be PLc

    Can a disconnect switch that fulfill 60204-1 be used as the only emergency stop on a machine?

    /Tim

    1. Hi Tim,

      Great question!

      I can see that you’ve spent some time reading EN 60204-1, Chapter 10.8. The first point is that a disconnecting device that will be used for emergency switching off must be rated to break the full running current of the machine, including the locked rotor current of the largest motor in the system. Most disconnecting devices are designed for isolation only, meaning that they are not designed to be operated under load. The machine is expected to be stopped and normally switched off before the disconnecting device is operated to isolate the machine. Next, the handle of the device must be correctly coloured – RED on a YELLOW background. Finally, if there is any possibility that an emergency switching off device could be confused with an emergency stop device, the ESO device should be clearly marked as such. I can even be placed behind glass.

      The questions for you as a designer are: 1) Did you do a risk assessment? If not, that’s the first step. You need to decide based on the risk assessment and any applicable type C standard whether an ESO or E-Stop is needed. 2) If ESO is to be used, can you get a suitable device that meets both the on-load switching requirements and the isolation requirements. If the answers to both of those are YES, then the rest is location and colour.

      I hope that helps. 🙂

  2. Hi Doug,

    Would you consider laser cutting and engraving machines (40-watt CO2 lasers) designed for small office / home office / hobby use would need an emergency stop. The biggest risk is fire (when working in class 1 mode) and having an emergency stop makes it easy to switch off the laser source and laser movement, yet I see a number of manufacturers supplying equipment without emergency stops.

    1. Gareth,

      That depends. The premise for needing a risk assessment is two-fold:
      1) There needs to be the potential for unforeseen (emergent) conditions to occur, and
      2) The e-stop should be able to permit avoiding or limiting harm.

      With laser engravers/markers/cutting equipment you basically have three main classes of hazards related to the laser:
      a) Laser light – specular or diffuse reflections from the workpiece;
      b) LGACS – Laser Generated Air Contaminants, i.e., fumes, smoke, dust, particulates; and
      c) Hot-work hazards – spatter, sparks, hot material which can then create fire hazards.

      An e-stop can definitely deal with a runaway laser beam, but that will really only be effective for specular reflections. The NHZ for diffuse reflections from CO2 laser markers is usually quite small.

      An e-stop will do nothing for LGACS. You need a good ventilation/fume extraction/dust collector for that, and you don’t want that to shut down unexpectedly, unless the dust collector is on fire.

      An estop will do nothing for the hot-work hazards.

      There may be other hazards related to the operation of the equipment where the e-stop can be useful for risk reduction. A risk assessment is required to determine the need for an e-stop.

      In my experience, most laser systems that are Class 3 or 4 come with e-stops and safety interlock inputs as standard equipment. The interlocks are there so you can interface interlocking devices on the laser guarding enclosure to the laser to prevent or stop laser beam emissions when the guards are open. If you are using home-built or very inexpensive units, you may have bigger problems. I don’t know where you are geographically located, but if you are located in the EU, EN 60825-1 is applicable as well as the appropriate electrical safety standard, probably EN 61010-1, although there are a couple others that could be used. If you’re in the USA, the laser has to be registered with the CDRH by the laser manufacturer or distributor. The laser will need to conform to 29 CFR 1040 or ANSI Z136.1 or IEC 60825-1. It will also require an NRTL mark for electrical safety. If you’re in Canada, the laser may need to conform to the Radiation Emitting Devices Act, and can be certified under CAN/CSA E60825-1. The application may also need to be evaluated under ANSI Z136.1, as E60825-1 does not cover application requirements. Equipment used in Canada requires an electrical safety mark or a field evaluation label to show compliance with electrical safety requirements.

      Note that IEC 60825-1 (and the national variants) does not require an emergency stop, nor does ANSI Z136.1.

      So, there are the “it depends” criteria. I hope that helps.

      1. Hi Doug,

        Thank you for the very detailed response. Time to get checking the specific standards.

  3. Hi. I’m after some information on E-Stop. I am currently looking at having the wiring done in my garage/workshop. I would like to have 2 ring mains fitted. One as normal from the board for general power, batter charger, radio etc, the other I would like to run through a contactor set up as an E-Stop power ring that power tools will be run from. The reason behind this is my daughter is getting older and likes to join me/help on projects. The concept it I will have a key switch to lock off the start button, this must be on and all the ‘mushroom’ buttons released so that when I press the green ‘on’ button the contactor pulls in and turns on the ring to the power tools, bench grinder, pillar drill etc. I know this this is overkill for a home/garage set up BUT I would rather do it now and have the ability to lock off the machine power when required (not in there etc) but also to just have the buttons there if needed and I’m on the other side of the room. I am used to this set up from work (D&T teacher in a school) and have put a lot of thought into it in my planning (I would hate myself for not doing it and something going wrong later that it could have stopped). As for the wiring, now I could do the installation myself but obviously it wouldn’t be ‘signed off’ and the normal run of the mill house sparks are clueless on this. I was an electronics engineer for many years prior to my teaching work and my father worked on industrial machine control. I would like to be able to get all the information so I can get it all together for a willing spark to carry out.
    The issues I have currently are the spec of the contactor as it is going ring with several machines that could be running from it I am working on the principle that not all machines will be running at any one time, the largest motor is 1hp lathe (all single phase) and I would say there would be the possibility of that running and a pillar drill or band saw each 3/4hp.
    The contactor will not be switching on load, just disconnecting a load in an emergency. the rest of the time all it will switch would be a handful of LED lights, one on each machine.
    The other area I want to know about is cable colours, an where to find out what to use to/from the start switch and to the 4 stop buttons in series round the room.
    Basic principle, Live is on a NO pushbutton, pulls contactor in, which then feeds a live through all the NC E-Stops in a ring back to the contactor keeping the coil activated once the start button is released. Other contact on supply the live feed to the machine ring. Hit any stop button, power to coil in contactor is lost, contactor drops out, power disconnected from machine ring.
    Thanks

    1. Pete,
      Thanks for your comment and all the details you provided!
      First, kudos for considering putting in a system like this in a home shop. I am definitely supportive of this idea.
      The “easy” way to do this is to select an emergency stop module from a company like Pilz, Rockwell, Schmersal, Telemecanique, or your favourite supplier. These companies will provide you with suggested wiring diagrams.

      Sizing the contactors (yes, contactors, multiple) is critical. They need to be overdimensioned for the application to reduce the stresses on the contactors in operation. For example: If your single run from the panel is sized at 32 A, select 64 A contactors to provide a 2x over dimensioning of the contactors. Next, carefully consider how many e-stop devices you really need. Daisy-chaining the e-stop devices into a single e-stop module will result in fault masking and may eventually lead to a fail-to-function condition. ISO 13849-1 addresses this idea in Table 11. It’s also discussed in ISO/TR 24119 (https://www.iso.org/standard/63160.html). If you daisy chain more than 2 devices, you will be decreasing the reliability of the control system. You can segment the system, using 2-3 devices into an estop module, and then subsequently daisy-chaining the output contacts of the e-stop modules and subsequently into the contactor coils.
      You will need redundant contactors for high-reliability, or if you feel you can tolerate a somewhat reduced level of reliability, you could drop down to one. Just be aware that if that contactor fails in a welded condition, the system will fail.

      Hopefully that helps. If you need more help, I’d be happy to offer you some coaching. You can book a 1-hour session with me at this link: https://dougnix.acuityscheduling.com/schedule.php?appointmentType=3948927

      1. On more thing: Connect the ring mains to the load side of the contactor(s). Depending on the requirements in the local BS code for this type of wiring, you may need additional overcurrent protection downstream of the contactors. 🙂

  4. Hello Mister Nix,

    In CSA Z432-04 we find clause 7.17.1.2:
    Each operator control station, including pendants, capable of initiating machine motion shall have a manually initiated emergency stop device.
    Can we find the same rule somewhere in the EU regulation?

    1. Herman,

      1) That version of the standard is withdrawn and has been replaced by CSA Z432-16.
      2) Have a look at EN 60204-1, 9.2.4, 9.2.5.4, and 10.7.1. This standard is harmonised under both the Machinery Directive 2006/42/EC and the Low Voltage Directive, 2014/35/EU.

  5. Hello. I have one question regarding the emergency stop push buttons for the machines. Can we use Normal Open Contact push button with lead break detection in the cicuit. Is there any reference standard which pemits use of NO contact estop button with lead break detection

    1. None of the standards make explicit requirements for the contact functionality for any estop device, including buttons, however, ISO 13849-1 requires that all categories of architecture except Category B use “well-tried safety principles” which include opening a circuit in order to turn something off. The lists covering the requirements for well-tried safety principles can be found in ISO 13849-2, Annexes A-D, Tables A.2, B.2, C.2 and D.2.

      Based on this, use of a normally open contact for initiation of emergency stop would not meet the criteria for any architecture Category except B. Since ISO 13850 requires that emergency stop systems provide at least ISO 13849-1 PLc, and since PLc requires Category 1, 2 or 3 architecture, the use of a normally open contact would not be acceptable.

  6. Hello Mr. Nix. Can you tell me if the EU directive shows any requirements for E-Stop devices on Engine Driven machines such as Pressure Washers? I know on most industrial electric industrial equipment it is required but I am having a hard time believing that it may be required for an engine driven pressure washer. The pressure washer uses a key switch to start the engine and enables other devices to operate. When the key switch is off it disables the entire system.

    1. Adam, great question!

      To understand the requirements, the first stop is the Machinery Directive, 2006/42/EC, Annex I. In Annex I, you will find clause 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.

      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.

      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.

      Since a pressure washer is covered by the second bullet in the third paragraph, “portable hand-held and/or hand-guided machinery”, there is no requirement for an e-stop system on any hand-guided pressure washing system, regardless of energy source.

  7. Hello Mr. Nix,

    is it legally possible to install an emergency stop push button in a machine for a different purpose than emergency stopping it? Obviously, without the ’emergency stop’ marking.

    Thank you.

    1. I’m confused as to why you would want to do what you are suggesting, and here’s why: There is a fundamental functional difference between the way an emergency stop function works, and how a normal stop function works. Let me explain a bit more.

      In a normal stopping condition, there is no urgency as to how quickly the stop occurs. The stop may have constraints placed on it for repeatability, i.e., you always want a power press ram to stop at top-dead-centre in normal operation, but with regard to the stopping time involved, normally the time it takes is the time it takes. Also, you don’t want to inadvertently damage the machinery by forcing an unduly quick stop. Power remains on the system and no recovery mode is required because the machine has never gone outside the normal control envelope. Normal stopping is usually done via the process PLC or controller, and no functional safety requirements apply because normal stopping is not usually considered to be a safety function. There are exceptions of course, like the service brake on mobile machinery which is both a normal process function and a safety function.

      In an emergency stopping condition, the primary goal is to bring the machinery to a stop as quickly as possible, and damaging the machinery to do this is permitted. To do this the function may include high-friction mechanical braking systems, and may use the maximum available deceleration possible with variable frequency drives, servo or stepper systems. Complete removal of power at the end of the stopping cycle is the final step. The machine will normally need some kind of recovery mode because the system may be partially or completely out of control during the emergency stopping time. In fact, this may be WHY an emergency stop was invoked. Emergency stop is classified as a Complementary Protective Measure (see ISO 12100:2010), and is always considered to be a safety function. ISO 13850 requires PLc / SIL1 as a minimum performance level for emergency stopping functions.

      As you can see, the two functions are completely different. From a legality standpoint, to my knowledge there are no laws or regulations in any jurisdiction that regulate which type of stop function you choose – that is strictly a design decision. Once taken, that decision then drives the rest of the requirements regarding the details of the way the function is realized.

      1. Hello, Mr Nix.

        Thank you for your extensive answer. It must be I didn’t explain my question very well. I meant if I could install an emergency stop button, which for example has a particular mechanism for rearm it, for any other purpose than emergency stopping or stopping at all a machine. I was just wondering if an engineer or technician thinks of a function for what the hardware of an emergency button is just right, it would be acceptable or not to use it for it.

        1. If you are wondering if you can use an e-stop device, like a latching pushbutton for example, for other purposes, the answer is technically YES, and practically NO. The relevant standards (IEC 60204-1, ISO 13850, NFPA 79, CSA C22.2 #301, etc.) limit the use of the colour RED for emergency stop device actuators – that is, the head of the pushbutton. Also, mushroom head operators on pushbuttons are normally only used for e-stop devices. To my knowledge, none of the component manufacturers make latching pushbuttons with anything other than a red, mushroom head operator. So, based on that I cannot see how you could use a device intended for e-stop in anything other than an e-stop system without violating the requirements of one or more standards. IF you can find a latching pushbutton with a BLACK, WHITE, GRAY, or BLUE operator device, you could certainly use it for other purposes, consistent with the coding requirements given in the standards.

          1. Ok, that was exactly what I was looking for. Thank you.

  8. Is it possible to connect several emergency stops for different motors located in the same area, connect them to a junction box, wire a multicore cable towards the substation to another junction box, and then segregate towards the MCC drive for each of the motors? or is it mandatory a single cable for each of the emergency stops. Motors are not related to each other.

    1. Enrique,

      From a purely functional perspective, this would work, however, you are creating a single point of failure for multiple emergency stop systems (I’m assuming that each e-stop affects different machinery).

      If you read ISO 13850, you will find that the minimum Performance Level is ISO 13849-1 PLc. PLc can be achieved using Category 1, 2 or 3 architecture. If you do this using Category 1 or 2, no channel separation is possible, since these are both single-channel architectures. If you use Category 3, then channel separation is one of the basic Common Cause Failure mitigation methods, so grouping the channels in a single cable would eliminate the possibility of separating the channels.

      So the short answer is: It depends on the architecture of the control system, but no matter what, it creates a single point of failure for all the systems grouped in that cable.

      1. I just read this article. Do I understand correctly – so for cat 3 architecture – each estop station should be wired back to safety relay with two cables? (one per channel) I must admit – I have never seen anyone doing it before.

        1. Radoslaw,
          Not necessarily, BUT, if you look at the CCF table in the Annexes you will see that you can gain CCF points if you provide channel separation, which is done by running separate cables to separate sensing devices. For conventional estop button installations, this usually not done because both channels are run to a single physical device. By the way, you can achieve the minimum PLc required by ISO 13850 using a Cat. 1, 2, or 3 architecture. Nothing says Cat 3 is required.

        2. Radoslaw,

          Sorry for the delay in replying to your question – I tried to reply another way a few days ago and didn’t realize it hadn’t posted properly.

          This entire discussion starts with a risk assessment AND with ISO 13850, the estop standard. If you read this standard, you will find that the minimum requirement for an emergency stop is PLc, unless the risk assessment says you need a higher level of reliability. PLc can be achieved using Cat. 1, 2, or 3 architectures and appropriate component selection. If you review ISO 13849-1, Annex F, you will find Table F.1, the Common Cause Estimation scoring questionnaire. One of the ways to reduce the likelihood of CCFs is to provide channel separation, and it’s important enough that it is given a score of 15, making it one of the more highly weighted measures in the table. You need a minimum score of 65 to claim the PL value that your architecture and component selections may be able to yield for you. So, must you separate the channels? No. Is there a benefit in high reliability conditions? Yes. Is it commonly done? No. You need to carefully evaluate your application and make the right decision. Don’t forget Table F.1. Without scoring your CCF properly, you cannot claim your PL, regardless of what your calculations may tell you.

        3. Radoslaw,

          It is ONE way to achieve the CCF score you need to claim your PL. See Annex F, Table F.1. If you can’t hit 65 on the CCF scale, you can’t claim your PL, regardless of what the rest of your calculations may tell you.

  9. Hi All, Just wanted to query is it possible to put a clear plastic cover over an E-stop for accidental activations, This cover would just be a lift panel with the estop as normal under it.

  10. educational information posted about emergency stop (e-stop) are very interesting keep your good work. By the way i understand most of the new generations of engines, machinery and equipment are manufactured with built in safety and protection devices. In any case when they are in operation any problems may arise such as pressure, temperature and mechanical problems it will give and alarm if not rectified subsequently the system will stop it to avert further damage. Please bare with me its my only own opinion..

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