Guarding Emergency Stop Devices
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Guarding Emergency Stop Devices

2010 September 3
by Doug Nix
This entry is part 3 of 10 in the series Emergency Stop

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.

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 use. 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 world wide:

  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 operated position and require a 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,
  • locking device that prevent 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.

An important consideration is the potential for accidental operation. Depending on the machine or process, accidental operation of emergency stop devices may result in significant lost production and/or damage to equipment. In cases like this, it is reasonable to protect the device from accidental 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. (Author’s Note: Bold text added for emphasis.)

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 structure placed around the emergency stop device must take the added dimensions of the PPE and 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 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 either urgently trying to stop the machine because,

  1. another safeguard has failed an 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 all been waiting for!

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

YES!

Just 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.

Post By Doug Nix (100 Posts)

+DougNix is Managing Director and Principal Consultant at Compliance InSight Consulting, Inc. (http://www.complianceinsight.ca) in Kitchener, Ontario, and is Lead Author and Managing Editor of the Machinery Safety 101 blog.

Doug's work includes teaching machinery risk assessment techniques privately and through Conestoga College Institute of Technology and Advanced Learning in Kitchener, Ontario, as well as providing technical services and training programs to clients related to risk assessment, industrial machinery safety, safety-related control system integration and reliability, laser safety and regulatory conformity.

Website: → Compliance inSight Consulting Inc.

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