Emergency Stop Pull-Cords

This entry is part 14 of 14 in the series Emer­gency Stop

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.

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 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.

image: 4B Com­pon­ents Ltd.

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.

image: 4B Com­pon­ents Ltd.

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
  • Mark­ers

image: CSA Z432-04 [1]

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.

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.

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.

image: Ban­ner Engin­eer­ing
image: OMRON Sti
image: OMRON Sti

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) [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

Cable or Rope Requirements

image: Leer­dam Pty Ltd.

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) [1]

Tensioning Device

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.

Turn­buckle style ten­sion­er
Spring-loaded style ten­sion­er

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 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 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. 

image: Wiki­me­dia Com­mons [5]

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.

An example of poor install­a­tion of a pull-cord switch.

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.

Lum­ber sort­ing line

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.

Hard anchor point
Green cable on green back­gorund
Pull-cord switch with reset taped down

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, 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

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

Full Disclosure

We do not have any com­mer­cial or pro­mo­tion­al rela­tion­ship with any of the product vendors whose products are shown in this art­icle.

References

[1] CSA Z432-04, Safe­guard­ing of Machinery.

[2] ISO 13850:2015, Safety of machinery – Emer­gency stop – Prin­ciples for design

[3] IEC 60204 – 1:2016, Safety of machinery – Elec­tric­al equip­ment of machines – Part 1: Gen­er­al require­ments

[4] NFPA 79 – 2018, Elec­tric­al Stand­ard for Indus­tri­al Machinery

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

Series Nav­ig­a­tionEmer­gency Stop Fail­ures

Author: Doug Nix

Doug Nix is Managing Director and Principal Consultant at Compliance InSight Consulting, Inc. (http://www.complianceinsight.ca) in Kitchener, Ontario, and is Lead Author and Senior Editor of the Machinery Safety 101 blog. Doug's work includes teaching machinery risk assessment techniques privately and through Conestoga College Institute of Technology and Advanced Learning in Kitchener, Ontario, as well as providing technical services and training programs to clients related to risk assessment, industrial machinery safety, safety-related control system integration and reliability, laser safety and regulatory conformity. For more see Doug's LinkedIn profile.