Emergency Stop Pull-Cords

Last updated on August 23rd, 2022 at 05:46 pm

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 devices, the pull-cord is probably the most misapplied emergency stop device.

Local regulations may require emergency stop pull-cords for conveyor applications, like Alberta’s OHS Code, however, not every jurisdiction makes this kind of mandatory requirement. Always check your local regulations early in the design cycle.

TL;DR

Pull cord emergency stop devices are commonly used on conveyors and other long machines. They can be challenging to install correctly as attention to cable tension and support is important. Most of the large industrial control companies sell these devices, including OMRON/STI, Pilz, Rockwell Allen-Bradley, Schmersal, Siemens, Telemecanque, etc. As a buyer, you can almost certainly find one from your preferred supplier. Just make sure it conforms to IEC 60947-5-5 [7].

---

Typical Applications

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 complementary protective measures. Complimentary protective measures are backup devices (they complement) the primary safeguards. The primary safeguards in the photo below 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 might not be any guarding (in this case, it looks like there are enclosing guards over the idler rollers.) This may not be a problem, as there is probably no entanglement hazard here. If clothing got wrapped around the idler roller, the roller would likely stop as it is unlikely that there would 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.

Technical Standards

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

Emergency stop devices (buttons, pull-cords and foot pedals) are covered by IEC 60947-5-5, Low voltage switchgear and controlgear, Part 5-5: Control circuit devices and switching elements ? Emergency stop device with mechanical latching function.

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
• Markers

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. Ring bolts are very commonly used, along with U-clamps for securing the cable to the anchor.

Anchors and switches 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 or guides 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.

For an example of manufacturers instructions, see: OMRON Guide to the Installation of Pull-Cord Switches

Pull-Cord Switches

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. I show a couple examples of do-it-yourself arrangements and explain why these do not meet safety requirements.

Single cable switches like that shown below are much more common in manufacturing applications and other relatively short-length 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 less than 200 N (45 lbf) [1]*
• 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

*Not?all?standards?have?the?200?N?requirement. This?requirement originates in IEC 60947-5-5, the globally prevalent switch standard.

There is no explicit minimum requirement in the standards, for good reason. Each pull cord switch manufacturer can decide on the cable tension necessary to balance the switch in the “operate” condition, where the normally closed contacts are closed. A cable break will result in the switch tripping into the off or open contact condition, as will a pull on the cord.

If you look at the OMRON Sti guidance document, Proper Installation of Rope or Wire Pull Emergency Stop Devices, you will see that there is no specific pull-cord tension specified. Instead, there is a tension indicator that is used when adjusting the cable tension. In the photos above, the indicator on the single-ended switch is adjacent to the reset button on the right. The indicators on the double-ended switch are on the cable entry housings on the left and right of the main switch body. Once the indicator is centred in the window, the cable tension is correct. Many other pull-cord switches have similar indicators for this purpose.

Cable or Rope Requirements

The cable or rope used for a pull cord does not have any stringent requirements, however, selecting a cable or rope that has a red jacket is generally a good idea. Alternatively, if the machine body is red, a yellow jacket on the able might be a good idea. Additional requirements include:

• 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) [1]*.

*this requirement originates in IEC 60947-5-5:1997, 6.4.2.

Tensioning Device

Since the switch has to activate if the cable is pulled, 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 Fig.1 above, 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.

Markers

Emergency stop devices are required by the standards [2], [3], [4] 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 the 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 like that shown above can be fitted to make it easy to reach the cable and pull it. They also serve to identify the cable’s function.

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 Press

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 1950s 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.

Gluing Line

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.

Conclusions

As you can see, there are many important details to the correct installation of an emergency stop pull-cord switch. These details 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. Cable tension, support and guidance are critical to correct operation.

With some care and attention to detail, you can install a system like this correctly without too much difficulty. Remember too, emergency stop functions are required to meet at least ISO 13849-1 PL_c according to ISO 13850 [2]. The risk assessment for the machine will tell you if a higher Performance Level is needed.

If you have any questions about e-stop pull cords or other machinery safety questions, feel free to get in touch! We offer a 30-minute free consultation online to help get you started.

Thanks

Thanks to Howard Spencer for sharing the image of the sludge belt press with us, and for inspiring this article.

Full Disclosure

Full disclosure: Compliance inSight Consulting Inc., the Machinery Safety 101 blog, and Doug Nix have no commercial relationship with any suppliers illustrated, mentioned or linked to in this article. No revenue is received for any clicks you make from this article.

Update notes

This article was updated 2021-07-22, clarifying the maximum force permitted by [1] for the force needed to trip the switch. Also added was an explanation of how the cord tension is set on OMRON Sti switches as an example of how this is done.

This article was updated 2020-06-13, adding links to the Rockwell Automation and Schmersal pull-cord data, and on 2019-04-26, adding specific details related to IEC 60947-5-5 [7]. Additional vendor links and some revisions, including the addition of the TL;DR section. Head down to the very bottom of the article for supplier component data links.

References

[1] Safeguarding of Machinery, CSA Z432-04. Canadian Standards Association (CSA), Toronto. 2004.

[2] Safety of machinery – Emergency stop – Principles for design, ISO 13850. International Organization for Standardization (ISO), Geneva. 2015.

[3] Safety of machinery – Electrical equipment of machines – Part 1: General requirements, IEC 60204-1. International Electrotechnical Commission (IEC), Geneva. 2016.

[4] Electrical Standard for Industrial Machinery, NFPA 79. National Fire Protection Association (NFPA), Quincy, MA. 2018.

[5] “Belt filter”, En.wikipedia.org, 2018. [Online]. Available: https://en.wikipedia.org/wiki/Belt_filter. [Accessed: 24- Aug- 2018].

[6] Occupational Health and Safety Code 2009 Part 25 Explanation Guide. Edmonton, AB: WorkSafe AB, 2009, p. 25.

[7] Low-voltage switchgear and controlgear – Part 5-5: Control circuit devices and switching elements – Electrical emergency stop device with mechanical latching function, IEC 60947-5-5. International Electrotechnical Commission (IEC). 1997+AMD1:2005+AMD2:2016.

Product links and data downloads

Euchner GmbH

Rope pull switch RPS

OMRON

OMRON Proper Installation of Rope or Wire Pull Emergency Stop Devices

PILZ Safety Automation

Safe rope pull switch PSENrope

Rockwell Allen-Bradley

Bulletin 440E Installation instructions

Bulletin 440E Catalogue (2020-06)

Schmersal

Schmersal Pull-Cord Switches

Siemens

SIRIUS 3SE7 cable-operated switches

Telemecanique

Telemecanique Simple and Emergency Stop Safety Cable Pull Switches

© 2018 – 2022, Compliance inSight Consulting Inc.
This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.