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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:


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!


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 (95 Posts)

+DougNix is Managing Director and Principal Consultant at Compliance InSight Consulting, Inc. (http://​www​.com​pli​an​cein​sight​.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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