When it comes to emergency stop devices there is no doubt that the red mushroom-head push button is the most common – they seem to be everywhere. The second most common emergency stop device is the pull-cord, and like the light-curtain in safeguarding, the pull-cord is probably the most misapplied emergency stop device.
Emergency Stop pull-cords are most often used where there are long stretches of machinery between normal operator stations – think about conveyor systems as an example.
Bear in mind that emergency stop systems are complimentary protective measures, meaning that they are intended to complement the primary safeguards. In a situation like that shown below, the primary safeguards are the fixed guards over the rollers and along the edges of the belting. There is no guarding to protect a person from being hit or entangled and dragged by material on the belt. On the return roller, below the e-stop box and near the bottom of the picture, there is no guarding. This may not be a problem, as there probably is no entanglement hazard here. If clothing got wrapped around the idler roller, the roller would likely simply stop as there is unlikely to be enough friction between the belt and the roller to keep it turning.
Conveyors aren’t restricted to mining applications like that shown above – they can just as easily be used in shorter or smaller applications too. The basic idea is to provide a person with a way to stop the equipment motion from any place along its length. This same idea is sometimes used when a worker could be present at many points along a piece of machinery, with no explicit workstation defined.
The main standards references for conveyor applications are:
- ASME B20.1, 2015. Safety Standard for Conveyors and Related Equipment
- EN 617:2001+A1:2010, Continuous handling equipment and systems – Safety and EMC requirements for the equipment for the storage of bulk materials in silos, bunkers, bins and hoppers
- EN 618:2002+A1:2010, Continuous handling equipment and systems – Safety and EMC requirements for equipment for mechanical handling of bulk materials except fixed belt conveyors
- EN 619:2002+A1:2010, Continuous handling equipment and systems – Safety and EMC requirements for equipment for mechanical handling of unit loads
- EN 620:2002+A1:2010, Continuous handling equipment and systems – Safety and EMC requirements for fixed belt conveyors for bulk materials
Each of these standards includes requirements for the emergency stop systems associated with conveyor systems. It’s worth noting that there are other applications for pull-cord emergency stop devices that are not conveyor applications at all.
Pull-Cord System Components
Pull-cord systems are made up of the following components:
- Two anchor points
- Pull-cord switch
- Cable or rope
- Tensioning device
- Cable Guides
I will look at each of these system components in more detail in the following sections.
Anchors and Guides
The anchor points are not unique other than that they need to be robust enough to withstand the static and dynamic forces that can occur in the system. They need not be on the same plane. Since the cable is flexible, it can be made to turn corners so that it can follow the work area. There are specialized pulleys designed to guide the cable around corners that will also ensure that the cable can’t jam at the pulley. Usually, this will reduce the run length of the cable because the switch mechanism is only capable of supporting a certain amount of tension in the cable. Pulleys and eye-bolts are used to guide the cable along its length.
Guides and pullies may have to be installed at certain minimum or maximum distances along the length of the cable. Consult with the manufacturer’s installation instructions for these kinds of details.
Switches used in pull-cord systems are specialized, although it is possible to assemble a workable system without using them. There are many good reasons NOT to do this.
Pull-cord switches are available in two primary forms, single cable and double cable. The first two images in this article are both double-cable style switches. This style of switch is familiar in mining and other large-conveyor applications. Depending on the length of the cable run, and how readily the cable tension can be balanced on both sides of the switch, this may be a good solution for your application.
Single cable switches like that shown below are much more common in manufacturing applications and other relatively short applications.
The switches are designed so that the switch :
- will latch in the activated position
- will activate if the cable is pulled or the cable breaks or is cut
- has a tripping force of not less than 200 N (45 lbf) 
- has an indicator to show that the switch has been tripped
- has the means to reset the tripped condition
- conforms to the relevant electrical standard(s)
- is rated appropriately for the electrical circuit conditions
Cable or Rope Requirements
The exact material used for the pull cord is not that important, but there are a number of considerations that make using the OEM cable sold by the switch vendor a good idea.
- The cable needs to have sufficient strength to resist breaking under the required tension for a long time. At the same time, it must be light enough to permit the cable run length specified.
- The cable is customarily coloured RED so that it will stand out against the machine, and the jacket is designed to protect the cable from corrosion and damage from other environmental effects.
- Some standards specify a minimum breaking strength of 10x the activation force, i.e., not less than 2000 N (450 lbf) 
Since the switch has to activate if the cable is pulled OR if the cable breaks or is cut, the cable cannot be slack when the system is in the ready state. The tensioning device can be another switch as shown in the previous drawing, or it can be a spring-loaded tensioner or even a counterweight device. Turnbuckles and adjustable spring loaded devices are the most common type of tensioner. Turnbuckles rely on the spring tension supplied by the switch. Dual-cable switches require spring-loaded tensioners since the switch itself cannot provide the spring tension needed for this kind of application.
Emergency stop devices are required by the standards , ,  to have a RED operator device and a YELLOW background. With a pull-cord, this can be difficult, especially if there is no background that can be coloured yellow – think about cables that a strung through air with no immediate structure behind the cable. Cables can be fitted with flags or handles that are coloured appropriately as shown below.
The flags can be reflective for use outdoors at night or in low-light conditions. Where cables are located above a conveyor line, handles can be fitted to make it easy to reach the cable and pull it.
Problems with Pull-Cord Installations
There are likely as many ways a pull-cord installation can be messed up as there are applications, but I thought I might show you a few examples I’ve come across illustrating the ways this kind of application can go wrong.
Sludge presses are used in sewage treatment plants and similar processes. A filter belt moves at a certain speed, and the liquid being filtered is poured through the belt. The sludge remains on the surface of the belt, which then proceeds through a series of rollers that compress the sludge and squeeze the remaining liquid from it. At the end of the press the sludge is scraped off the belt into a hopper where it is collected for further treatment.
Diagram of a belt filter: sludge in the feed hopper is sandwiched between two filter cloths (shown green and purple). Fluid is extracted initially by gravity, then by squeezing the cloth through rollers. Filtrate exits through a drain, while solids are scraped off into a container.
As you might imagine, there are plenty of in-running nip points between the filter belts and the rollers, as well as between rollers.
The pull cord switch in the above photo is installed on a “sludge belt-press” built in the 1950’s and subsequently modified.
There are a number of things wrong here:
- The switch used is a simple roller cam limit switch with the cable hooked over the roller. This arrangement cannot conform with the 200 N minimum trip force requirement. The switch is not safety rated.
- The cable itself is slack, so breakage or cutting of the cable could not be detected
- The cable is dark in colour against a dark machine structure, making it difficult to identify in a panic situation
- There is no cable tensioner
- The cable turns a sharp corner through an eyebolt
This is an installation that needs immediate attention.
Lumber Sorting Line
The pull-cord shown in the photo below is installed on a lumber sorting machine in a facility that makes wooden industrial skids. The boards fall down the sloped ramp from centre top onto a flat belt conveyor where you can see the board in the centre of the picture. The pull cord runs along the face of the conveyor structure.
Problems with this installation include:
- No machine guarding (i.e., nothing to compliment, the e-stop is mistakenly believed to be the safeguard)
- The cable itself is dark in colour against a dark machine frame and is located at knee height. The cable does not have any flags or other markers to make it more visible.
- The cable is tensioned by a large spring, which is good, although it may be too heavy a spring to conform to the 200 N minimum trip force requirement
- The switch is not positively linked to the cable – the switch is a standard limit switch with a spring actuator. The switch is not safety rated.
This is an installation that needs immediate attention.
This example comes from a powered roller conveyor used in an insulated siding manufacturing line.
As you can see in the photos above, the cable has a hard anchor point on one end and is connected to a pull cord switch on the other end. Problems with this installation include:
- No tension adjustment on the pull-cord
- The cable is dark in colour on a dark machine frame making it hard to see. There are no flags or other measures taken to make the cord more visible.
- The reset device on the pull-cord switch is taped down
This installation is another one that needs immediate attention.
As you can see, installing a pull-cord emergency stop switch has some important details that need to be looked after in design and maintenance, and as with any other safety device, there are lots of ways to do the job poorly.
If you have any questions about e-stop pull cords or other machinery safety questions, feel free to get in touch!
Thanks to Howard Spencer for sharing the image of the sludge belt press with us, and for inspiring this article.
We do not have any commercial or promotional relationship with any of the product vendors whose products are shown in this article.
 CSA Z432-04, Safeguarding of Machinery.
 ISO 13850:2015, Safety of machinery – Emergency stop – Principles for design
 IEC 60204 – 1:2016, Safety of machinery – Electrical equipment of machines – Part 1: General requirements
 NFPA 79 – 2018, Electrical Standard for Industrial Machinery
 “Belt filter”, En.wikipedia.org, 2018. [Online]. Available: https://en.wikipedia.org/wiki/Belt_filter. [Accessed: 24- Aug- 2018].