I always look 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 failures result from 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:
- the button operator has failed, and
- 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 it is unnecessary as the colour combination is enough.
An ISO/IEC symbol for an emergency stop could also be used [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 in this way happens most often in high-vibration conditions. I have encountered this in real life while doing inspections on client sites.
Some 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 control 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 why the emergency stop for the machine he was operating was mounted on a different machine.
Use of covers
ISO 13850 allows for using 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 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,” 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.
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].
[2] Safeguarding of machinery, CSA Z432. Canadian Standards Association (CSA), Toronto. 2016.
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You forgot a really important and slightly obscure one – look up “Distance Control of AC Contactors” and you will see the role leakage current can have in AC circuits used for safety circuit (esp conveyors because of the cable lengths). The outcome is you press the E-Stop and the relay or contactor stays in because the leakage current is enough to provide continued sealing current.
Even worse, it works at time of install, but then water gets in the cable and changes the dielectric constant and what tested out OK yesterday, no longer works.
In short, first option is never use AC for control. If you must, understand in some depth the problems it can cause. I have met very few people (engineers) who are aware of even this issue alone.
Good point Gareth, and thanks for bringing this up. I appreciate your knowledge of these effects.
The impedance of the connections to the final control element can become quite significant as the distance increases, and especially if the wiring techniques are not good. Running the conductors in the same cable, and close to the earthing plane, i.e., structural members of the machine frame, or in a shielded cable, are necessary to minimize the impedance as much as possible. Better of course is to use DC for control, in which case these factors are minimized.