Guarding Emergency Stop Devices

Emergency Stop on machine console

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.

Series Nav­i­ga­tionBust­ing Emer­gency Stop MythsEmer­gency Stop Cat­e­gories

Author: Doug Nix

Doug Nix is Managing Director and Principal Consultant at Compliance InSight Consulting, Inc. (http://www.complianceinsight.ca) in Kitchener, Ontario, and is Lead Author and Senior 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. For more see Doug's LinkedIn profile.