Manufacturing Automation Round Table

On Sep­tem­ber 21st, I par­tic­i­pat­ed in a round table dis­cus­sion spon­sored by Man­u­fac­tur­ing Automa­tion mag­a­zine.

We cov­ered a lot of ground in the dis­cus­sions, touch­ing on har­mo­niza­tion of machin­ery safe­ty stan­dards, appli­ca­tion of OHS laws and the abil­i­ty of man­u­fac­tur­ers and employ­ers to com­ply with require­ments, and a lot more.

We had some great peo­ple around the table for these con­ver­sa­tions. At the table were:

Wayne De L’Orme – Ontario Min­istry of Labour;

Eliz­a­beth Rankin – CSA Stan­dards;

Dave Law­son – Advanced Motion & Con­trols;

Jeff Math­yssen – Elec­tro-Mag;

Rick Sauer – Fes­to; Dan Fournier – Omron;

Lisa Bolton – Sher­rard Kuzz LLP; and

myself.

The round table will be the fea­ture sto­ry in the Novem­ber / Decem­ber issue of Man­u­fac­tur­ing Automa­tion, and a video of the dis­cus­sions will be avail­able on their web­site. For more infor­ma­tion, con­tact Mary Del Cian­cio, edi­tor, Man­u­fac­tur­ing Automa­tion. Pick up a copy, or check it out on the Man­u­fac­tur­ing Automa­tion web site.

Emergency Stop Categories

This entry is part 5 of 13 in the series Emer­gency Stop

I’ve noticed a lot of peo­ple look­ing for infor­ma­tion on Emer­gency Stop cat­e­gories recent­ly; this arti­cle is aimed at those read­ers who want to under­stand this top­ic in more depth. First, a clar­i­fi­ca­tion: Emer­gency stop cat­e­gories DO NOT EXIST, but stop cat­e­gories do. A stop cat­e­go­ry is a descrip­tion of a con­trol func­tion — what the con­trol does — and not the archi­tec­ture of the sys­tem that pro­vides the func­tion. Stop cat­e­gories are often con­fused with cir­cuit or sys­tem archi­tec­ture cat­e­gories from EN 954–1[1] and ISO 13849–1 [2].  The con­fu­sion between these two sets of Cat­e­gories often leads to incor­rect assump­tions about the appli­ca­tion of these require­ments.

Emer­gency stop” is a descrip­tion of a con­trol func­tion, with the added “emer­gency” telling you WHEN this stop func­tion is intend­ed to be used — only dur­ing an emer­gency sit­u­a­tion. A “cycle stop” is also a func­tion­al descrip­tion that tells the user WHAT the stop func­tion does. Both the emer­gency stop func­tion and the cycle stop func­tion use the SAME stop cat­e­gories, with some lim­i­ta­tions on the emer­gency stop func­tion. More about that lat­er in this arti­cle.

Stop Categories

The stop cat­e­gories dis­cussed here are not exclu­sive to emer­gency stop func­tions. They are STOP func­tions and may be used for nor­mal stop­ping func­tions as well as the Emer­gency Stop func­tion.

Stop cat­e­gories and func­tion­al safe­ty sys­tem archi­tec­ture cat­e­gories are not the same, and there are sig­nif­i­cant dif­fer­ences that need to be under­stood by con­trol sys­tem design­ers. I’m going to sling a num­ber of stan­dards at you in this post, and I will pro­vide ref­er­ences at the end if you want to dig deep­er.

Func­tion­al safe­ty archi­tec­tur­al cat­e­gories are defined and described in ISO 13849–1, and I’ve writ­ten quite a bit on these in the past. If you want to know more about Cat­e­gories B, 1–4, check out this series of posts on ISO 13849–1 Cat­e­gories.

Originating Standards

There are three stan­dards that define the require­ments for stop cat­e­gories, and thank­ful­ly they are fair­ly close­ly har­monised, mean­ing that the def­i­n­i­tions for the cat­e­gories are essen­tial­ly the same in each doc­u­ment. They are:

  • ISO 13850, Safe­ty of machin­ery — Emer­gency stop func­tion — Prin­ci­ples for design [3]
  • IEC 60204–1, Safe­ty of machin­ery — Elec­tri­cal equip­ment of machines — Part 1: Gen­er­al require­ments (aka EN 60204–1) [4]
  • NFPA 79, Elec­tri­cal Stan­dard for Indus­tri­al Machin­ery [5]

A new Cana­di­an stan­dard was added in 2016, CSA C22.2 No. 301 [9]. This stan­dard draws heav­i­ly on a num­ber of stan­dards for core mate­r­i­al, includ­ing IEC 60204–1 and NFPA 79. No. 301 uses iden­ti­cal def­i­n­i­tions for stop func­tion cat­e­gories.

Down­load ANSI stan­dards

Down­load IEC stan­dards

Stop Category Definitions

Emergency Stop ButtonThe stop cat­e­gories are bro­ken down into three gen­er­al groups in [4], [5], and  [9]:

  • Cat­e­go­ry 0 — Equiv­a­lent to pulling the plug;
  • Cat­e­go­ry 1 — Bring things to a grace­ful stop, then pull the plug; and
  • Cat­e­go­ry 2 — Bring things to a stop and hold them there under pow­er.

Let’s look at the def­i­n­i­tions in more detail. For com­par­i­son, I’m going to show the def­i­n­i­tions from the stan­dards side-by-side.

Table 1
Com­par­i­son of Stop Cat­e­gories
Cat­e­go­ry IEC 60204–1 NFPA 79 CSA C22.2 No. 301
0 stop­ping by imme­di­ate removal of pow­er to the machine actu­a­tors (i.e. an uncon­trolled stop – see 3.56); is an uncon­trolled stop by imme­di­ate­ly remov­ing pow­er to the machine actu­a­tors.

stop­ping by imme­di­ate removal of pow­er to the machine actu­a­tors (i.e., an uncon­trolled stop;

1 a con­trolled stop (see 3.11) with pow­er avail­able to the machine actu­a­tors to achieve the stop and then removal of pow­er when the stop is achieved; is a con­trolled stop with pow­er to the machine actu­a­tors avail­able to achieve the stop then remove pow­er when the stop is achieved.

a con­trolled stop with pow­er avail­able to the machine actu­a­tors to achieve the stop and then removal of pow­er when the stop is achieved;

2 a con­trolled stop with pow­er left avail­able to the machine actu­a­tors. is a con­trolled stop with pow­er left avail­able to the machine actu­a­tors.

a con­trolled stop with pow­er left avail­able to the machine actu­a­tors.

Def­i­n­i­tions from IEC 60204–1:

3.11 con­trolled stop
>stop­ping of machine motion with elec­tri­cal pow­er to the machine actu­a­tors main­tained dur­ing the stop­ping process
3.56 uncon­trolled stop
stop­ping of machine motion by remov­ing elec­tri­cal pow­er to the machine actu­a­tors
NOTE This def­i­n­i­tion does not imply any par­tic­u­lar state of oth­er stop­ping devices, for exam­ple mechan­i­cal or hydraulic brakes.

As you can see, the Stop Cat­e­go­ry descrip­tions are vir­tu­al­ly iden­ti­cal, with the pri­ma­ry dif­fer­ence being the use of the def­i­n­i­tions in the IEC stan­dard instead of includ­ing that infor­ma­tion in the descrip­tion as in the NFPA stan­dard.

Down­load ANSI stan­dards

Down­load IEC stan­dards

Minimum Requirements

[4], [5], and [9] require that all machines have at least a Cat­e­go­ry 0 stop. This could be achieved by switch­ing off (i.e., by using the dis­con­nect­ing means to switch off pow­er for exam­ple), by phys­i­cal­ly “pulling the plug” from the pow­er sup­ply sock­et on the wall, or through a ‘mas­ter con­trol relay’ cir­cuit, or through an emer­gency stop cir­cuit. Note that this does not require that all machines have an e-stop!! The need for an emer­gency stop func­tion is deter­mined in two ways:

  1. Exis­tence of a Type-C (i.e., machine spe­cif­ic) tech­ni­cal stan­dard that requires that type of machin­ery to have an emer­gency stop func­tion, or
  2. through the risk assess­ment, based on the poten­tial to avoid or lim­it harm.

If these goals can­not be achieved through an emer­gency stop func­tion, there is no require­ment to have one. I have yet to read leg­is­la­tion (not stan­dards) in any juris­dic­tion that states that all machines must have an e-stop. Cer­tain class­es of machines may have this require­ment, nor­mal­ly defined in the rel­e­vant type-C machin­ery stan­dard, e.g., ISO 10218–1 [10] for indus­tri­al robots.

ISO 13850 lim­its the selec­tion of stop cat­e­go­ry to Cat­e­go­ry 0 or 1 and excludes Cat­e­go­ry 2. This exclu­sion can be found in NFPA 79, IEC 60204–1, and CSA C22.2 No. 301 as well. Cat­e­go­ry 2 may only be used for oper­a­tional or “nor­mal” stop­ping func­tions.

To learn more about how to deter­mine the need for an emer­gency stop, see, “Emer­gency Stop – What’s so con­fus­ing about that?”

Selecting a Stop Function

How do you decide on what stop cat­e­go­ry to use? First, a risk assess­ment is required. Sec­ond, a start/stop analy­sis should be con­duct­ed. More on this top­ic a bit lat­er.

Once the risk assess­ment is com­plete, ask these ques­tions:

1) Will the machin­ery stop safe­ly using an uncon­trolled stop?

If the machin­ery does not have a sig­nif­i­cant amount of iner­tia, mean­ing it won’t coast more than a very short time, then a Cat­e­go­ry 0 stop may be all that is required.

2) If the machin­ery can coast when pow­er is removed, or if the machin­ery can be stopped more quick­ly under con­trol than when pow­er is sim­ply removed, then a Cat­e­go­ry 1 stop is like­ly the best choice, even if the pow­er-off coast­ing time is fair­ly short.

Ver­ti­cal axes that may col­lapse when pow­er is removed will like­ly need addi­tion­al mechan­i­cal hard­ware to pre­vent the tool­ing from falling dur­ing an emer­gency stop con­di­tion. This could be a mechan­i­cal brake or oth­er means that will pre­vent the tool­ing from falling unex­pect­ed­ly.

3) If the machin­ery includes devices that require pow­er to keep them in a safe state, then a Cat­e­go­ry 2 stop is like­ly the best choice.

If you choose to use a Cat­e­go­ry 2 stop, be aware that leav­ing pow­er on the machin­ery leaves the user open to haz­ards relat­ed to hav­ing pow­er on the machin­ery. Care­ful risk assess­ment is required in these cas­es espe­cial­ly.

Cat­e­go­ry 2 stops are not per­mit­ted for emer­gency stop func­tions, although you may use them for nor­mal stop func­tions. ISO 13850, IEC 60204–1, and NFPA 79  explic­it­ly lim­it emer­gency stop func­tions to Cat­e­gories 0 and 1. CSA C22.2 No. 301 per­mits the use of Cat­e­go­ry 2 stop func­tions for emer­gency stop­ping.

Risk Assessment and Stop/Start Analysis

Risk assess­ment is crit­i­cal to the spec­i­fi­ca­tion of all safe­ty-relat­ed func­tions. While emer­gency stop is not a safe­guard, it is con­sid­ered to be a ‘com­ple­men­tary pro­tec­tive mea­sure’ [6, 6.2.3.5.3], [7, 3.19, 6.3]. Under­stand­ing the haz­ards that need to be con­trolled and the degree of risk relat­ed to the haz­ards is basic design infor­ma­tion that will pro­vide spe­cif­ic direc­tion on the stop cat­e­go­ry required and the degree of con­trol reli­a­bil­i­ty nec­es­sary to pro­vide the expect­ed risk reduc­tion.

Stop/Start Analy­sis is quite sim­ple, orig­i­nat­ing in ISO 12100. It amounts to con­sid­er­ing all of the intend­ed stop/start con­di­tions for the machin­ery and then includ­ing con­di­tions that may result from rea­son­ably fore­see­able fail­ure modes of the machin­ery and fore­see­able mis­us­es of the machin­ery. Cre­ate a table with three columns as a start­ing point, sim­i­lar to Table 2.

Table 2
Exam­ple Start/Stop Analy­sis

Descrip­tion Start Con­di­tion Stop Con­di­tion
Lubri­cant Pump Lubri­cant Pump Start But­ton Pressed Lubri­cant Pump Stop But­ton Pressed
Low Lubri­cant Lev­el in reser­voir
High-pres­sure drop across lubri­cant fil­ter
Main Spin­dle Motor Start enabled and Start But­ton Pressed Low Lubri­cant Pres­sure
Stop but­ton pressed
Feed Advance motor Feed Advance but­ton pressed Feed Stop but­ton pressed
Feed end of trav­el lim­it reached
Emer­gency Stop All motions stop, lubri­cant pump remains run­ning

The above table is sim­ply an exam­ple of what a start/stop analy­sis might look like. You can have as much detail as you like.

Control Reliability Requirements

Both ISO 13849–1 and IEC 62061 [8] base the ini­tial require­ments for reli­a­bil­i­ty on the out­come of the risk assess­ment (PLr or SILr). If the stop­ping con­di­tion is part of nor­mal oper­a­tion, then sim­ple cir­cuit require­ments (i.e. PLa, Cat­e­go­ry 1) are all that may be required. If the stop­ping con­di­tion is intend­ed to be an Emer­gency Stop, then addi­tion­al analy­sis is need­ed to deter­mine exact­ly what may be required.

More Information

How have you typ­i­cal­ly imple­ment­ed your stops and emer­gency stop sys­tems?

Have you ever used the START/STOP analy­sis method?

I care about what you think as a read­er, so please leave me com­ments and ques­tions! If you would pre­fer to dis­cuss your ques­tion pri­vate­ly,  con­tact me direct­ly.

Ed. Note: This arti­cle was updat­ed 15-Jan-2018.

References

5% Dis­count on All Stan­dards with code: CC2011 

[1]          Safe­ty of Machin­ery — Safe­ty Relat­ed Parts of Con­trol Sys­tems — Part 1: Gen­er­al Prin­ci­ples for Design. CEN Stan­dard EN 954–1.1996.

[2]          Safe­ty of Machin­ery — Safe­ty Relat­ed Parts of Con­trol Sys­tems — Part 1: Gen­er­al Prin­ci­ples for Design. ISO Stan­dard 13849–1. 2015. Down­load ISO Stan­dards 

[3]          Safe­ty of machin­ery — Emer­gency stop func­tion — Prin­ci­ples for design. ISO Stan­dard 13850. 2015

[4]          Elec­tri­cal Equip­ment of Indus­tri­al Machines. IEC Stan­dard 60204–1. 2009. Down­load IEC stan­dards

[5]          Elec­tri­cal Stan­dard for Indus­tri­al Machin­ery, ANSI/NFPA Stan­dard 79, 2015. Down­load stan­dards from ANSI

[6]          Safe­guard­ing of Machin­ery. CSA Stan­dard Z432, 2016.

[7]          Safe­ty of machin­ery — Gen­er­al prin­ci­ples for design — Risk assess­ment and risk reduc­tion. ISO Stan­dard 12100. 2010.

[8]          Safe­ty of machin­ery – Func­tion­al safe­ty of safe­ty-relat­ed elec­tri­cal, elec­tron­ic and pro­gram­ma­ble elec­tron­ic con­trol sys­tems. IEC Stan­dard 62061. 2005.

[9]         Indus­tri­al elec­tri­cal machin­ery. CSA Stan­dard C22.2 No. 301. 2016.

[10]       Robots and robot­ic devices — Safe­ty require­ments for indus­tri­al robots — Part 1: Robots. ISO Stan­dard 10218–1. 2011.

Busting Emergency Stop Myths

This entry is part 3 of 13 in the series Emer­gency Stop

There are a num­ber of myths that have grown up around emer­gency stops over the years. These myths can lead to injury or death, so it’s time for a lit­tle Myth Bust­ing here on the MS101 blog!

There are a num­ber of myths that have grown up around emer­gency stops over the years. These myths can lead to injury or death, so it’s time for a lit­tle Myth Bust­ing here on the MS101 blog!

What does ‘emergency’ mean?

Con­sid­er for a moment the roots of the word ‘emer­gency’. This word comes from the word ‘emer­gent’, mean­ing a sit­u­a­tion that is devel­op­ing or emerg­ing in the moment. Emer­gency stop sys­tems are intend­ed to help the user deal with poten­tial­ly haz­ardous con­di­tions that are emerg­ing in the moment. These con­di­tions have prob­a­bly arisen because the design­ers of the machin­ery failed to con­sid­er all the fore­see­able uses of the equip­ment, or because some­one has cho­sen to mis­use the equip­ment in a way that was not intend­ed by the design­ers. The key func­tion of an Emer­gency Stop sys­tem is to pro­vide the user with a back­up to the pri­ma­ry safe­guards. These sys­tems are referred to as “Com­ple­men­tary Pro­tec­tive Mea­sures” and are intend­ed to give the user a chance to “avert or lim­it harm” in a haz­ardous sit­u­a­tion. With that in mind, let’s look at three myths I hear about reg­u­lar­ly.

 

Myth #1 – The Emergency Stop Is A Safety Device

Waterwheel and belt. Credit: Harry Matthews & http://www.old-engine.com
A Fitz Water Wheel and Belt Dri­ve, Cred­it: Har­ry Matthews & http://www.old-engine.com

Ear­ly in the Indus­tri­al Rev­o­lu­tion machine builders real­ized that users of their machin­ery need­ed a way to quick­ly stop a machine when some­thing went wrong. At that time, over­head line-shafts were dri­ven by large cen­tral pow­er sources like water­wheels, steam engines or large elec­tric motors. Machin­ery was cou­pled to the cen­tral shafts with pul­leys, clutch­es and belts which trans­mit­ted the pow­er to the machin­ery.

See pic­tures of a line-shaft pow­ered machine shop or click the image below.

Line Shaft in the Mt. Wilson Observatory Machine Shop
Pho­to: Lar­ry Evans & www.oldengine.org

These cen­tral engines pow­ered an entire fac­to­ry, so they were much larg­er than an indi­vid­ual motor sized for a mod­ern machine. In addi­tion, they could not be eas­i­ly stopped, since stop­ping the cen­tral pow­er source would mean stop­ping the entire fac­to­ry – not a wel­come choice. Emer­gency stop devices were born in this envi­ron­ment.

Learn more about Line Shafts at Harry’s Old Engines.

See pho­tos and video of a work­ing line shaft machine shop. 

Due to their ear­ly use as a safe­ty device, some have incor­rect­ly con­sid­ered emer­gency stop sys­tems safe­guard­ing devices. Mod­ern stan­dards make the dif­fer­ence very clear. The eas­i­est way to under­stand the cur­rent mean­ing of the term “EMERGENCY STOP” is to begin by look­ing at the inter­na­tion­al stan­dards pub­lished by IEC1 and ISO2.

emer­gency stop3
emer­gency stop func­tion

func­tion that is intend­ed to

—   avert aris­ing, or reduce exist­ing, haz­ards to per­sons, dam­age to machin­ery or to work in progress,

—   be ini­ti­at­ed by a sin­gle human action

NOTE 1

Haz­ards, for the pur­pos­es of this Inter­na­tion­al Stan­dard, are those which can arise from

—   func­tion­al irreg­u­lar­i­ties (e.g. machin­ery mal­func­tion, unac­cept­able prop­er­ties of the mate­r­i­al processed, human error),

—   nor­mal oper­a­tion.

It is impor­tant to under­stand that an emer­gency stop func­tion is “ini­ti­at­ed by a sin­gle human action”. This means that it is not auto­mat­ic, and there­fore can­not be con­sid­ered to be a risk con­trol mea­sure for oper­a­tors or bystanders. Emer­gency stop may pro­vide the abil­i­ty to avoid or reduce harm, by pro­vid­ing a means to stop the equip­ment once some­thing has already gone wrong. Your next actions will usu­al­ly be to call 911 and admin­is­ter first aid.

Safe­guard­ing sys­tems act auto­mat­i­cal­ly to pre­vent a per­son from becom­ing involved with the haz­ard in the first place. This is a reduc­tion in the prob­a­bil­i­ty of a haz­ardous sit­u­a­tion aris­ing, and may also involve a reduc­tion in the sever­i­ty of injury by con­trol­ling the haz­ard (i.e., slow­ing or stop­ping rotat­ing machin­ery before it can be reached.) This con­sti­tutes a risk con­trol mea­sure and can be shown to reduce the risk of injury to an exposed per­son.

Emer­gency stop is reac­tive; safe­guard­ing sys­tems are proac­tive.

In Cana­da, CSA defines emer­gency stop as a ‘Com­ple­men­tary Pro­tec­tive Mea­sure’ in CSA Z432-046:

6.2.2.1.1
Safe­guards (guards, pro­tec­tive devices) shall be used to pro­tect per­sons from the haz­ards that can­not rea­son­ably be avoid­ed or suf­fi­cient­ly lim­it­ed by inher­ent­ly safe design. Com­ple­men­tary pro­tec­tive mea­sures involv­ing addi­tion­al equip­ment (e.g., emer­gency stop equip­ment) may have to be tak­en.

6.2.3.5.3 Com­ple­men­tary pro­tec­tive mea­sures
Fol­low­ing the risk assess­ment, the mea­sures in this clause either shall be applied to the machine or shall be dealt with in the infor­ma­tion for use.
Pro­tec­tive mea­sures that are nei­ther inher­ent­ly safe design mea­sures, nor safe­guard­ing (imple­men­ta­tion of guards and/or pro­tec­tive devices), nor infor­ma­tion for use may have to be imple­ment­ed as required by the intend­ed use and the rea­son­ably fore­see­able mis­use of the machine. Such mea­sures shall include, but not be lim­it­ed to,

(a) emer­gency stop;
(b) means of res­cue of trapped per­sons; and
© means of ener­gy iso­la­tion and dis­si­pa­tion.

In the USA, three stan­dards apply: ANSI B11ANSI B11.19–2003, and NFPA 79:

ANSI B11-2008

3.80 stop: Imme­di­ate or con­trolled ces­sa­tion of machine motion or oth­er haz­ardous sit­u­a­tions. There are many terms used to describe the dif­fer­ent kinds of stops, includ­ing user- or sup­pli­er-spe­cif­ic terms, the oper­a­tion and func­tion of which is deter­mined by the indi­vid­ual design. Def­i­n­i­tions of some of the more com­mon­ly used “stop” ter­mi­nol­o­gy include:

3.80.2 emer­gency stop: The stop­ping of a machine tool, man­u­al­ly ini­ti­at­ed, for emer­gency pur­pos­es;

7.6 Emergency stop

Elec­tri­cal, pneu­mat­ic and hydraulic emer­gency stops shall con­form to require­ments in the ANSI B11 machine-spe­cif­ic stan­dard or NFPA 79.
Infor­ma­tive Note 1: An emer­gency stop is not a safe­guard­ing device. See also, B11.19.
Infor­ma­tive Note 2: For addi­tion­al infor­ma­tion, see ISO 13850 and IEC 60204–1.

ANSI B11.19–2003

12.9 Stop and emergency stop devices

Stop and emer­gency stop devices are not safe­guard­ing devices. They are com­ple­men­tary to the guards, safe­guard­ing device, aware­ness bar­ri­ers, sig­nals and signs, safe­guard­ing meth­ods and safe­guard­ing pro­ce­dures in claus­es 7 through 11.

Stop and emer­gency stop devices shall meet the require­ments of ANSI / NFPA 79.

E12.9

Emer­gency stop devices include but are not lim­it­ed to, but­tons, rope-pulls, and cable-pulls.

A safe­guard­ing device detects or pre­vents inad­ver­tent access to a haz­ard, typ­i­cal­ly with­out overt action by the indi­vid­ual or oth­ers. Since an indi­vid­ual must actu­ate an emer­gency stop device to issue the stop com­mand, usu­al­ly in reac­tion to an event or haz­ardous sit­u­a­tion, it nei­ther detects nor pre­vents expo­sure to the haz­ard.

If an emer­gency stop device is to be inter­faced into the con­trol sys­tem, it should not reduce the lev­el of per­for­mance of the safe­ty func­tion (see sec­tion 6.1 and Annex C).

NFPA 79 deals with the elec­tri­cal func­tions of the emer­gency stop func­tion which is not direct­ly rel­e­vant to this arti­cle, so that is why I haven’t quot­ed direct­ly from that doc­u­ment here.

As you can clear­ly see, the essen­tial def­i­n­i­tions of these devices in the US and Cana­da match very close­ly, although the US does not specif­i­cal­ly use the term ‘com­ple­men­tary pro­tec­tive mea­sures’.

Myth #2 – Cycle Stop And Emergency Stop Are Equivalent

Emer­gency stop sys­tems act pri­mar­i­ly by remov­ing pow­er from the prime movers in a machine, ensur­ing that pow­er is removed and the equip­ment brought to a stand­still as quick­ly as pos­si­ble, regard­less of the por­tion of the oper­at­ing cycle that the machine is in. After an emer­gency stop, the machine is inop­er­a­ble until the emer­gency stop sys­tem is reset. In some cas­es, emer­gency stop­ping the machine may dam­age the equip­ment due to the forces involved in halt­ing the process quick­ly.

Cycle stop is a con­trol sys­tem com­mand func­tion that is used to bring the machine cycle to a grace­ful stop at the end of the cur­rent cycle. The machine is still ful­ly oper­a­ble and may still be in auto­mat­ic mode at the com­ple­tion of this stop.

Again, refer­ring to ANSI B11-2008:

3.80.1 con­trolled stop: The stop­ping of machine motion while retain­ing pow­er to the machine actu­a­tors dur­ing the stop­ping process. Also referred to as Cat­e­go­ry 1 or 2 stop (see also NFPA 79: 2007, 9.2.2);

3.80.2 emer­gency stop: The stop­ping of a machine tool, man­u­al­ly ini­ti­at­ed, for emer­gency pur­pos­es;

Myth #3 – Emergency Stop Systems Can Be Used For Energy Isolation

Disconnect Switch with Lock and TagFif­teen to twen­ty years ago it was not uncom­mon to see emer­gency stop but­tons fit­ted with lock­ing devices.  The lock­ing device allowed a per­son to pre­vent the reset­ting of the emer­gency stop device. This was done as part of a “lock­out pro­ce­dure”. Lock­out is one aspect of haz­ardous ener­gy con­trol pro­ce­dures (HECP).  HECPs rec­og­nize that live work needs to be done from time to time, and that nor­mal safe­guards may be bypassed or dis­con­nect­ed tem­porar­i­ly, to allow diag­nos­tics and test­ing to be car­ried out. This process is detailed in two cur­rent stan­dards, CSA Z460 and ANSI Z244.1. Note that these lock­ing devices are still avail­able for sale, and can be used as part of an HECP to pre­vent the emer­gency stop sys­tem or oth­er con­trols from being reset until the machine is ready for test­ing. They can­not be used to iso­late an ener­gy source.

No cur­rent stan­dard allows for the use of con­trol devices such as push but­tons or selec­tor switch­es to be used as ener­gy iso­la­tion devices.

CSA Z460-05 specif­i­cal­ly pro­hibits this use in their def­i­n­i­tion of ‘ener­gy iso­la­tion devices’:

Ener­gy-iso­lat­ing device — a mechan­i­cal device that phys­i­cal­ly pre­vents the trans­mis­sion or release of ener­gy, includ­ing but not lim­it­ed to the fol­low­ing: a man­u­al­ly oper­at­ed elec­tri­cal cir­cuit break­er; a dis­con­nect switch; a man­u­al­ly oper­at­ed switch by which the con­duc­tors of a cir­cuit can be dis­con­nect­ed from all unground­ed sup­ply con­duc­tors; a line valve; a block; and oth­er devices used to block or iso­late ener­gy (push-but­ton selec­tor switch­es and oth­er con­trol-type devices are not ener­gy-iso­lat­ing devices).4

Sim­i­lar require­ments are found in ANSI Z244.15 and in ISO 138503.

Myth #4 — All Machines are Required to have an Emergency Stop

Some machine design­ers believe that all machines are required to have an emer­gency stop. This is sim­ply not true. A read­er point­ed out to me that CSA Z432-04, clause 7.17.1.2, does make this require­ment. To my knowl­edge this is the only gen­er­al lev­el (i.e., not machine spe­cif­ic) stan­dard that makes this require­ment. I stand cor­rect­ed! Hav­ing said that, the rest of my com­ments on this top­ic still stand. Clause 7.17.1.2 lim­its the appli­ca­tion of this require­ment:

7.17.1.2

Each oper­a­tor con­trol sta­tion, includ­ing pen­dants, capa­ble of ini­ti­at­ing machine motion shall have a man­u­al­ly ini­ti­at­ed emer­gency stop device.

Emer­gency stop sys­tems may be use­ful where they can pro­vide a back-up to oth­er safe­guard­ing sys­tems. To under­stand where to use an emer­gency stop, a start-stop analy­sis must be car­ried out as part of the design process. This analy­sis will help the design­er devel­op a clear under­stand­ing of the nor­mal start and stop con­di­tions for the machine. The analy­sis also needs to include fail­ure modes for all of the stop func­tions. It is here that the emer­gency stop can be help­ful. If remov­ing pow­er will cause the haz­ard to cease in a short time, or if the haz­ard can be quick­ly con­tained in some way, then emer­gency stop is a valid choice. If the haz­ard will remain for a con­sid­er­able time fol­low­ing removal of pow­er, then emer­gency stop will have no effect and is use­less for avoid­ing or lim­it­ing harm.

For exam­ple, con­sid­er an oven. If the burn­er stop con­trol failed, and assum­ing that the only haz­ard we are con­cerned with is the hot sur­faces inside the oven, then using an emer­gency stop to turn the burn­ers off only results in the start of the nat­ur­al cool­ing cycle of the oven. In some cas­es that could take hours or days, so the emer­gency stop has no val­ue. It might be use­ful for con­trol­ling oth­er haz­ards, such as fire, that might be relat­ed to the same fail­ure. With­out a full analy­sis of the fail­ure modes of the con­trol sys­tem, a sound deci­sion can­not be made.

Sim­ple machines like drill press­es and table saws are sel­dom fit­ted with emer­gency stop sys­tems. These machines, which can be very dan­ger­ous, could def­i­nite­ly ben­e­fit from hav­ing an emer­gency stop. They are some­times fit­ted with a dis­con­nect­ing device with a red and yel­low han­dle that can be used for ‘emer­gency switch­ing off’. This dif­fers from emer­gency stop because the machine, and the haz­ard, will typ­i­cal­ly re-start imme­di­ate­ly when the emer­gency switch­ing off device is turned back on. This is not per­mit­ted with emer­gency stop, where reset­ting the emer­gency stop device only per­mits the restart­ing of the machine through oth­er con­trols. Reset of the emer­gency stop device is not per­mit­ted to reap­ply pow­er to the machine on its own.

These require­ments are detailed in ISO 138503, CSA Z4326 and oth­er stan­dards.

Design Considerations

Emer­gency Stop is a con­trol that is often designed in with lit­tle thought and used for a vari­ety of things that it was nev­er intend­ed to be used to accom­plish. The three myths dis­cussed in this arti­cle are the tip of the ice­berg.

Con­sid­er these ques­tions when think­ing about the design and use of emer­gency stop sys­tems:

  1. Have all the intend­ed uses and fore­see­able mis­us­es of the equip­ment been con­sid­ered?
  2. What do I expect the emer­gency stop sys­tem to do for the user of the machine? (The answer to this should be in the risk assess­ment.)
  3. How much risk reduc­tion am I expect­ing to achieve with the emer­gency stop?
  4. How reli­able does the emer­gency stop sys­tem need to be?
  5. Am I expect­ing the emer­gency stop to be used for oth­er pur­pos­es, like ‘Pow­er Off’, ener­gy iso­la­tion, or reg­u­lar stop­ping of the machine? (The answer to this should be ‘NO’.)

Tak­ing the time to assess the design require­ments before design­ing the sys­tem can help ensure that the machine con­trols are designed to pro­vide the func­tion­al­i­ty that the user needs, and the risk reduc­tion that is required. The answers lie in the five ques­tions above.

Have any of these myths affect­ed you?

Got any more myths about e-stops you’d like to share?

I real­ly appre­ci­ate hear­ing from my read­ers! Leave a com­ment or email it to us and we’ll con­sid­er adding it to this arti­cle, with cred­it of course!

References

5% Dis­count on All Stan­dards with code: CC2011

  1. IEC – Inter­na­tion­al Elec­trotech­ni­cal Com­mis­sion. Down­load IEC stan­dards, Inter­na­tion­al Elec­trotech­ni­cal Com­mis­sion stan­dards.
  2. ISO – Inter­na­tion­al Orga­ni­za­tion for Stan­dard­iza­tion Down­load ISO Stan­dards
  3. Safe­ty of machin­ery — Emer­gency stop — Prin­ci­ples for design, ISO 13850, 2006, ISO, Gene­va, Switzer­land.
  4. Con­trol of Haz­ardous Ener­gy ­– Lock­out and Oth­er Meth­ods, CSA Z460, 2005, Cana­di­an Stan­dards Asso­ci­a­tion, Toron­to, Cana­da.
    Buy CSA Stan­dards online at CSA.ca
  5. Safe­guard­ing of Machin­ery, CSA Z432-04, Cana­di­an Stan­dards Asso­ci­a­tion, Toron­to, Cana­da.
  6. Con­trol of Haz­ardous Ener­gy – Lockout/Tagout and Alter­na­tive Meth­ods, ANSI/ASSE Z244.1, 2003, Amer­i­can Nation­al Stan­dards Insti­tute / Amer­i­can Soci­ety of Safe­ty Engi­neers, Des Plaines, IL, USA.
    Down­load ANSI stan­dards
  7. Amer­i­can Nation­al Stan­dard for Machine Tools – Per­for­mance Cri­te­ria for Safe­guard­ing, ANSI B11.19–2003, Amer­i­can Nation­al Stan­dards Insti­tute, Des Plaines, IL, USA.
  8. Gen­er­al Safe­ty Require­ments Com­mon to ANSI B11 Machines, ANSI B11-2008, Amer­i­can Nation­al Stan­dards Insti­tute, Des Plaines, IL, USA.
  9. Elec­tri­cal Stan­dard for Indus­tri­al Machin­ery, NFPA 79–2007, NFPA, 1 Bat­tery­march Park, Quin­cy, MA 02169–7471, USA.
    Buy NFPA Stan­dards online.

5% Dis­count on All Stan­dards with code: CC2011

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