Emergency Stop Failures

This entry is part 13 of 13 in the series Emer­gency Stop

I am always look­ing for inter­est­ing exam­ples of machin­ery safe­ty prob­lems to share on MS101. Recent­ly I was scrolling Reddit/r/OSHA and found these three real-world exam­ples.

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:

Won’t be stop­ping this ele­va­tor any­time soon. from OSHA

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. Note the wire “keep­er” that pre­vents the but­ton from get­ting lost!


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.

sure hope i nev­er need to hit that emer­gency stop but­ton. its for the machine on my side of the new fence. from OSHA

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 required.

Emer­gency stop but­ton. from OSHA

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

References

[1]     “IEC 60417–5638, Emer­gency Stop”, Iso.org, 2017. [Online]. Avail­able: https://www.iso.org/obp/ui/#iec:grs:60417:5638. [Accessed: 27- Jun- 2017].