Guarding Emergency Stop Devices

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, unin­ten­tion­al 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 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. (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 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 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.

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Author: Doug Nix

+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.

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  • Rob Hooper, CSP EHS Engineer

    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.

    • 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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