Machinery Safety 101

Guarding Emergency Stop Devices

This article was originally published in 2010 and was updated in 2015. Additional updates are in the works and will be published during the month of June 2019.


Much confusion exists when it comes to Emergency Stop systems, and clients often ask me if it is ‘OK’ to guard emergency stop devices like e-stop buttons, foot pedals, pull-cords, etc. Without getting into a ton of regulatory details, this article will look at the requirements in for emergency stop devices in three key jurisdictions: Canada, the USA and the European Union.

PLEASE NOTE: Some jurisdictions, notably the USA under Federal OSHA regulations, do not permit any kind of structure around an emergency stop device, See RICHARD A RYDZA’s comment below this post. Take care to review your local requirements before adding any kind of protective mechanical structure around an emergency stop device.

If you need information on the functional aspects of emergency stop systems, see “Emergency Stop – What’s so confusing about that?

Why Guard an Emergency Stop?

Generally, emergency stop devices, or e-stop devices as they’re often called, need to be protected from unintentional activation. This problem occurs because e-stop devices have to be located close to where people work in order to be useful. An e-stop you can’t reach when you need it may as well not be there in the first place, so emergency stops are located at ‘normal operator stations’. This often means they are located under the edge of a machine table, or on an operator control bar like that used on power presses, putting the e-stop within reach, but also in the ‘line-of-fire’ when it comes to the operator’s normal movements.

To prevent unintended operation, people often want to put rings, collars, or worse – covers – on or around the e-stop device to keep people from bumping the device. Some of these can be done and should be done, and others are never permitted for good reason.

Regulatory Requirements

Let’s take a look at the key requirements from the regulations worldwide:


  1. Emergency Stop devices must be clearly identified. The technical standards require that emergency stop devices be coloured RED with a YELLOW background [1].
  2. They must be located within easy reach of the operator. This applies to all normal workstations where operators interact with the machine. For maintenance and service activities where workers may be in locations other than normal workstations, a pendant or other portable control must be used to cause machine motion. This device must include an emergency stop control along with other complementary safeguarding devices such as enabling devices and hold-to-run controls. Where access is only allowed under lockout conditions, this is not required [2], [3].
  3. Buttons must be palm or mushroom-shaped devices.
  4. Devices must require manual resetting. This means that the device must latch in the activated position and require deliberate action to reset the device. This includes actions such as: pulling put a pressed button, twisting a button to release the latched condition, pressing a reset button on a pull-cord to reset the tripped condition, etc [1].
  5. Unguarded. This means that easy access to the device may not be impeded, considering the personal protective equipment (PPE) that workers are required to wear.

    Devices that would be considered to be guards would include:
  • Close-fitting rings or collars that require a worker to insert a finger inside the ring or collar to reach the device and activate it,
  • covers that close over the device to prevent access,
  • a locking device that prevents access to the device, etc.

So, considering point 5 above, isn’t this the end of the discussion? Not at all! There are a few factors to consider first.

One important consideration is any potential for accidental operation. Depending on the machine or process, unintentional operation of emergency stop devices may result in significant lost production and/or damage to equipment, including damage to the emergency stop device itself. In cases like this, it is reasonable to protect the device from inadvertent operation as long as the measures taken to protect the device do not impede the operation of the device in emergency conditions.

ISO 13850 [4] supports this idea in Clause 4.4 Emergency stop device:

4.4.2 An emergency stop device shall be located at each operator control station, except where the risk assessment indicates that this is not necessary, as well as at other locations, as determined by the risk assessment. It shall be positioned such that it is readily accessible and capable of non-hazardous actuation by the operator and others who could need to actuate it. Measures against inadvertent actuation should not impair its accessibility.

[4]

Summing Up

The key difference between North American thinking and International/EU thinking is in the term “unguarded” as used in the North American standards, versus [4, § 4.2.2], where the designer is reminded, “Measures against inadvertent actuation should not impair its accessibility.”

In my opinion, it is reasonable to protect an emergency stop device from inadvertent operation by placing a ring or other similar structure around an emergency stop device as long as the structure does not impair easy access to the device by the operator.

I know this opinion appears initially to go against the established North American standards, however it can be logically argued, based on the definition of the word “guard”.

A guard is a device that prevents access to something, usually a hazard. Considering that we are talking about a control that is designed to reduce or limit harm, any structure that does not prevent access to the emergency stop device associated with the structure should be considered to be acceptable.

That said, devices like:

  • hinged covers;
  • doors;
  • locking devices;
  • narrow collars; and
  • any other device or structure

that unduly limits access to the emergency stop device cannot be considered acceptable.

Effects of PPE

The phrase ‘unduly limits access’ has specific meaning here. If workers are expected to be wearing PPE on the body part used to activate the emergency stop device, such as gloves or boots for example, then the design of the structure placed around the emergency stop device must take into account the added dimensions of the PPE, the reduction in tactile capability that may occur (e.g. heavy work gloves make it hard to feel things easily), and must compensate for the effects of the PPE. Big gloves/boots = Big opening in the structure.

Lighting and protective eyewear can also play a part. You may need to use reflective or luminescent paint, or illuminated e-stop devices, to highlight the location of the device in low light environments or where very dark eyewear is required, like that needed by welders or used by workers around some infrared lasers with open beam paths.

Effects of State-of-Mind

It’s also important to consider the likely state of mind of a worker needing to use an emergency stop device. They are urgently trying to stop the machine because either,

  1. another safeguard has failed and someone is involved with a hazard, including themselves, or
  2. the machine is damaging itself or the product and they need to limit the damage.

Both scenarios have a high level of urgency attached to them. The human mind tends to miss obvious things including training when placed under high levels of stress. Structures placed around emergency stop devices, such as covers, that completely block access, even though they may be easily opened, may be enough to prevent access in an emergency.

The answer you’ve been waiting for!

So in the end, can you put a structure around an emergency stop to reduce inadvertent operation of the device:

In many jurisdictions YES! If you are located in the USA, then no.

Make sure that you consider all the factors that may affect it’s use, document your analysis, and don’t unduly restrict access to the device.

Need more help? Feel free to email me!


References

IEC – International Electrotechnical Commission

ISO – International Organization for Standardization

[1]  Safety of machinery – Electrical equipment of machines – Part 1: General requirements, IEC 60204-1, 2005

[2]  Control of Hazardous Energy ­– Lockout and Other Methods, CSA Z460, 2005.

[3]  Control of Hazardous Energy – Lockout/Tagout and Alternative Methods, ANSI ASSE Z244.1, 2003.

[4]  Safety of machinery — Emergency stop — Principles for design, ISO 13850, 2006.

12 thoughts on “Guarding Emergency Stop Devices

  1. In the USA NFPA 79 governs US manufacturing machinery NO shrouds no guarding end of story. As a ny state certified building, fire and safety inspector, if you are in the US I will cite you under NFPA 79 to which OSHA has adopted and incorporated by reference. I have also been part and subpoenaed 17 times to testify against employers who intentionally violated NFPA 79. Every case the employer lost each bringing the complainants over 5 million dollar in liability damages each. 2 were awarded 8.9 and 9.2 million for loss of life and spouse third party.

    1. Hi Richard,
      Thanks for your comments. You are right – the US jurisdiction takes the strict view that you expressed so clearly. However, there are industry sectors, notably the semiconductor industry, where the equipment guidelines require the provision of a partial ring guard around e-stop buttons, see SEMI-S2. The difficulty is that there is no clear definition anywhere of what exactly constitutes a guard. In some cases, there are requirements that the button must be able to be operated with a flat palm. Other areas accept half-rings or split rings but not full circles. None allow covers that can be locked closed, although locking devices that will lock the button in the “stopped” position is generally allowed, as are buttons with a built-in key cylinder that locks in the pressed position and require an authorized person with a key to release and reset the device. So, while the US has taken the simplest approach to this by not accepting any kind of ring, this is not the case globally.

  2. oSHA has cited many US companies for impediment to ESTOP function Shrouds in USDA are a NO NO. You now added a product to the definition of E Stop. Mushroom red head Yellow background. Without legislation you are adding to the device definition by adding a shroud and if any one was to become injured due to their inability to engage the button when needed. (Ex Aurubis Buffalo NY 2011) cited when employee could not contact E Stop due to shroud. Third party lawsit awarded millions

    1. Hi Richard,
      You are correct. There are specific US OSHA requirements prohibiting the placement of obstacles around estop devices that might prevent a person from activating the device. This is particularly true if workers are required to wear protective gloves in the area where the estop device is located. Similar restrictions exist in other areas around the world too, as I mentioned in the post. Having said that, there are ways to protect an estop device to prevent inadvertent activation of the device which will not contravene the standards. The design of the device is critical and must be done correctly. I can’t comment on the lawsuit you cited, as I am not familiar with that case.

  3. There was a study done in conjunction with a Master’s Thesis by Patrica Zarate at Oregon State University in 1995-96 time frame that looked at a number of human factors regarding the effects of e-stop actuator guarding, orientation, and human response time to activate e-stops.

    I was the site Ergonomist at HP’s then largest and most complex site in Corvallis, OR with significant amounts of automated equipment producing inkjet cartridges. I presented the idea of a Master’s student to review some of the complexities and compliance issues around this topic to OSU’s Industrial Engineering professor Dr. Kenneth Funk, PhD. He had a student that was interested in this as a MS thesis and the rest was history. It was a real pleasure to work with Patricia on the study and to help navigate what would be helpful study outout to me as an Environmental Health and Safety professional, who was constantly battling the “you cannot guard” e-stops mindset.

    I have a PDF copy of the study and there were a number of interesting results but guarding did not in this study present significant degradation of e-stop actuation. By the way, the palm was very rarely used to actuate but thumb and first couple digits were primary modes to actuate. Reaction time was the biggest surprise, much longer reaction times than the car brake actuation of 3/4 of a second used in most driving schools, something you don’t develop muscle and mental memory for. The study, like all good studies, raised a number of other questions, translated to mean there is “free project/thesis material” for future human factors MS students! Also SEMI S2 standard makes allowances for protecting against accidental tripping of e-stop/EMO. I prefer the terms either shrouding or shielding and there are specific anthropometric attributes that a shield must have in my opinion to ensure safe and timely actuation of an e-stop device.

    1. Rob,

      Very interesting! I’m glad to hear that there is some more specific research that supports these ideas. There are certainly instances where the palm of the hand is used, but I think you are correct – thumb or the first couple of digits are more common. The other big issue comes in environments where bulky gloves can make access to small buttons or closely shrouded buttons challenging.

      If you would be willing to share the thesis PDF, I would be interested in having a look at the results.

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