Machinery Safety 101

Emergency Stop Failures

I am always look­ing for inter­est­ing examples of machinery safety prob­lems to share on MS101. Recently I was scrolling Reddit/r/OSHA and found some real-world examples of emer­gency stop fail­ures, plus one from my own experience.

Broken Emergency Stop Buttons

The first and most obvi­ous kinds of fail­ures are those res­ult­ing from either wear out or dam­age to emer­gency stop devices like e‑stop but­tons or pull cords. Here’s a great example:

The oper­at­or device in this pic­ture has two problems:

  1. the but­ton oper­at­or has failed, and
  2. the e‑stop is incor­rectly marked.

The cor­rect mark­ing would be a yel­low back­ground in place of the red/silver legend plate, like the example below. The yel­low back­ground could have the words “emer­gency stop” on it, but this is not neces­sary as the col­our com­bin­a­tion is enough.

Yellow circular legend plate with the words "emergency stop" in black letters. Fits A-B 800T pushbutton operators.
Allen-Brad­ley 800T Emer­gency Stop legend plate

There is an ISO/IEC sym­bol for an emer­gency stop that could also be used [1].

Emergency stop symbol. A circle containing an equalateral triangle pointing downward, containing an exclamation mark.
Emer­gency Stop Sym­bol IEC 60417 – 5638 [1]

I won­der how the con­tact block(s) inside the enclos­ure are doing? Con­tact blocks have been known to fall off the back of emer­gency stop oper­at­or but­tons, leav­ing you with a but­ton that does noth­ing when pressed. Con­tact blocks secured with screws are most vul­ner­able to this kind of fail­ure. Los­ing a con­tact block like this hap­pens most often in high-vibra­tion con­di­tions. I have run across this in real life while doing inspec­tions on cli­ent sites.

There are con­tact blocks made to detect this kind of fail­ure, like Allen Brad­ley’s self-mon­it­or­ing con­tact block, 800TC-XD4S, or the sim­il­ar Siemens product, 3SB34. Most con­trols com­pon­ent man­u­fac­tur­ers will be likely to have sim­il­ar components.

Here’s anoth­er example from a machine inspec­tion I did a while ago on an indus­tri­al wash­ing machine. Note the wire “keep­er” that pre­vents the but­ton head from get­ting lost! The but­ton head had been snagged on a rope dangling from an over­head car­ri­er bag used to load the wash­er. Accord­ing to the main­ten­ance tech­ni­cian in the plant, these but­tons would typ­ic­ally get broken in this way with­in a few days of the machines being put into ser­vice. He had giv­en up repla­cing the oper­at­or device and instead came up with this cre­at­ive solu­tion. Of course, it’s not per­mit­ted either, since the device has to be ready for imme­di­ate use at all times.

Install­a­tion Failures

Here is an example of poor plan­ning when installing new bar­ri­er guards. The emer­gency stop but­ton is now out of reach. The ori­gin­al poster does not indic­ate a reas­on why the emer­gency stop for the machine he was oper­at­ing was moun­ted on a dif­fer­ent machine.

Use of covers

ISO 13850 allows for the use of a “shroud” as a last-ditch meth­od in cases where an emer­gency stop device must be loc­ated in an area where it might be inad­vert­ently activ­ated. Before the use of a shroud is con­tem­plated, mov­ing the but­ton to anoth­er loc­a­tion, slightly recess­ing the device into the face of the machine, or oth­er meas­ures must be investigated. 

Shrouds are not cov­ers. The oper­a­tion of the but­ton using the palm of the hand must still be eas­ily possible.

Here are some examples of ways this can­not be done:


In the USA, no struc­tures are per­mit­ted around the emer­gency stop device by the US OSHA reg­u­la­tions, so even the half-shrouds and U‑shaped col­lars that are often seen on European and Asi­an-built machines are allowed.

In Canada, CSA Z432-16 [2] requires that emer­gency stop devices are “unguarded,” which is effect­ively the same as the US OSHA requirements.

No Emergency Stop at all

Finally, and pos­sibly the worst example of all. Here is an impro­vised emer­gency stop using a set of wire cut­ters. No fur­ther com­ment is required.

If you have any examples you would like to share, feel free to add them in the com­ments below. Ref­er­ences to par­tic­u­lar employ­ers or man­u­fac­tur­ers will be deleted before posts are approved.


[1]     “IEC 60417 – 5638, Emer­gency Stop”,, 2017. [Online]. Avail­able: [Accessed: 27- Jun- 2017].

[2] Safe­guard­ing of machinery, CSA Z432. Cana­dian Stand­ards Asso­ci­ation (CSA), Toronto. 2016.

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