Safe Drive Control including Safe Torque Off (STO)

This entry is part 12 of 13 in the series Emer­gency Stop

Ed. Note: This arti­cle was revised 25-Jul-17 to include infor­ma­tion on safe stand­still.

Safe Drive Control including STO

Variable Frequency Drive for conveyor speed control
Vari­able Fre­quen­cy Dri­ve for con­vey­or speed con­trol [1]
Motor dri­ves are every­where. From DC vari­able speed dri­ves and index­ing dri­ves, through AC Vari­able Fre­quen­cy dri­ves, ser­vo dri­ves and step­per motor dri­ves, the capa­bil­i­ties and the flex­i­bil­i­ty of these elec­tron­ic sys­tems has giv­en machine design­ers unprece­dent­ed capa­bil­i­ties when com­pared to basic relay or con­tac­tor-based motor starters. We now have the capa­bil­i­ty to con­trol mech­a­nisms using motors in ways that would have been hard to imag­ine at the begin­ning of the indus­tri­al rev­o­lu­tion. Along with these con­trol capa­bil­i­ties come safe­ty-relat­ed func­tions like Safe Torque Off (STO).

Since we are con­trol­ling machin­ery, safe­ty is always a con­cern. In the 1990’s when I start­ed design­ing machin­ery with motor dri­ves, deal­ing with safe­ty con­cerns usu­al­ly meant adding a suit­ably rat­ed con­tac­tor upstream of the dri­ve so that you could inter­rupt pow­er to the dri­ve in case some­thing went wrong. With ear­ly ser­vo dri­ves, inter­rupt­ing the sup­ply pow­er often meant los­ing posi­tion data or worse. Plac­ing con­tac­tors between the dri­ve and the motor solved this prob­lem, but inter­rupt­ing the sup­ply pow­er would some­times cause the dri­ve stage of the ser­vo con­troller to blow up if the switch-off hap­pened with the motor run­ning and under high load. Motor dri­ve man­u­fac­tur­ers respond­ed by pro­vid­ing con­tac­tors and oth­er com­po­nents built into their dri­ves, cre­at­ing a fea­ture called Safe Torque Off (STO).

STO describes a state where “The dri­ve is reli­ably torque-free” [2]. The func­tions dis­cussed in this arti­cle are described in detail in IEC 61800–5-2 [3]. The func­tions are also list­ed in [10, Table 5.2]. Note that only Safe Torque Off and Safe Stop 1 can be used for emer­gency stop func­tions. Safe Torque Off, Safe Stop 1 and Safe Stop 2 can be used for safe­ty-relat­ed stop func­tions ini­ti­at­ed by a safe­guard­ing device. This dis­tinc­tion, between emer­gency stop func­tions and safe­guard­ing func­tions, is an impor­tant one.

If you have been a read­er of this blog for a while, you may recall that I have dis­cussed stop cat­e­gories before. This arti­cle expands on those con­cepts with the focus on motor dri­ves and their stop­ping func­tions specif­i­cal­ly. I’ve also talked about Emer­gency Stop exten­sive­ly. You might be inter­est­ed in read­ing more about the e-stop func­tion, start­ing with the post “Emer­gency Stop – What’s so con­fus­ing about that?”

Safe Torque Off (STO)

Accord­ing to Siemens, “The STO func­tion is the most com­mon and basic dri­ve-inte­grat­ed safe­ty func­tion. It ensures that no torque-gen­er­at­ing ener­gy can con­tin­ue to act upon a motor and pre­vents unin­ten­tion­al start­ing.” Risk assess­ment of the machin­ery can iden­ti­fy the need for an STO func­tion. The devices used for this appli­ca­tion are described in IEC 60204–1 in clause 5.4 [4]. The design fea­tures for pre­ven­tion of unex­pect­ed start­ing are cov­ered in more detail in EN 1037 [5] or ISO 14118 [6]. If you are inter­est­ed in these stan­dards, ISO 14118 is in the process of being revised. A new ver­sion should be avail­able with­in 12–18 months.

The STO func­tion oper­ates as shown in Fig.1. The blue line rep­re­sents the dri­ve speed/velocity, V, on the y-axis, with time, t, on the x-axis. The orange arrow and the dot­ted line show the ini­ti­a­tion of the stop­ping func­tion.

Graph showing motor drive output over time when the STO function is activated.
Fig­ure 1 — Safe Torque Off func­tion [1]
At the begin­ning of the stop­ping process (orange arrow and dot­ted line), the dri­ve gate puls­es are imme­di­ate­ly shut off, remov­ing torque from the motor (i.e., zero torque). The speed of the dri­ven equip­ment will drop at a rate deter­mined by the sys­tem fric­tion and iner­tia until stand­still is achieved. The zero torque con­di­tion is main­tained until the safe­ty func­tion per­mits restart­ing (area out­lined with yellow/black zebra stripe). Note that dri­ve stand­still may occur if the fric­tion and iner­tia of the sys­tem per­mit, but it is pos­si­ble that the dri­ven equip­ment may coast for some time. You may be able to move the dri­ven equip­ment by hand or grav­i­ty with the dri­ve in the STO mode.

STO is an uncon­trolled stop­ping mode [4, 3.56]:

uncon­trolled stop
stop­ping of machine motion by remov­ing elec­tri­cal pow­er to the machine actu­a­tors
NOTE This def­i­n­i­tion does not imply any oth­er state of oth­er (for exam­ple, non-elec­tri­cal) stop­ping devices, for exam­ple, mechan­i­cal or hydraulic brakes that are out­side the scope of this stan­dard.

The def­i­n­i­tion above is impor­tant. Uncon­trolled stops are the most com­mon form of stop­ping used in machines of all types and is required as a basic func­tion for all machines. There are var­i­ous ways of achiev­ing STO, includ­ing the use of a dis­con­nect­ing device, emer­gency stop sys­tems, and gate inter­lock­ing sys­tems that remove pow­er from machine actu­a­tors.

The embod­i­ment of the uncon­trolled stop con­cept is Stop Cat­e­go­ry 0 [4, 9.2.2]:

stop cat­e­go­ry 0 — stop­ping by imme­di­ate removal of pow­er to the machine actu­a­tors (i.e., and uncon­trolled stop, see 3.56)

Stop cat­e­go­ry 0 is only appro­pri­ate where the machin­ery has lit­tle iner­tia, or where mechan­i­cal fric­tion is high enough that the stop­ping time is short. It may also be used in cas­es where the machin­ery has very high iner­tia, but only for nor­mal stop­ping when coast­ing time is not a fac­tor, not for safe­ty stop­ping func­tions where the time to a no-motion state is crit­i­cal.

There are a few oth­er stop­ping modes that are often con­fused with STO:

  • Safe Stop 1
  • Safe Stop 2
  • Safe Oper­at­ing Stop
  • Safe Stand­still

Let’s explore the dif­fer­ences.

Safe Stop 1 (SS1)

If a defined stop­ping time is need­ed, a con­trolled stop­ping func­tion will be required fol­lowed by entry into STO. This stop­ping func­tion is called “Safe Stop 1” (SS1).

SS1 is direct­ly relat­ed to Stop Cat­e­go­ry 1 [4, 9.2.2]. As described in [4], Stop Cat­e­go­ry 1 func­tions as fol­lows:

stop cat­e­go­ry 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;

A “con­trolled stop” is defined in [4, 3.11]:

con­trolled stop
stop­ping of machine motion with elec­tri­cal pow­er to the machine actu­a­tor main­tained dur­ing the stop­ping process

Once the con­trolled stop is com­plet­ed, i.e., machine motion has stopped, the dri­ve may then be placed into STO (or cat­e­go­ry 0 stop). The stop­ping process is shown in Fig. 2 [7].

Graph showing the reduction of drive speed over time following the beginning of a controlled stopping process.
Fig­ure 2 — Safe Stop 1

The stop­ping process starts where the orange arrow and dot­ted line are shown. As com­pared to Fig. 1 where the decel­er­a­tion curve is gen­tle and expo­nen­tial, the active stop­ping peri­od in Fig. 2 is a lin­ear curve from oper­at­ing speed to zero speed. At the blue dot­ted line, the dri­ve enters and stays in STO. The yellow/black zebra striped area of the curve out­lines the com­plete stop­ping func­tion. This stop­ping method is typ­i­cal of many types of machin­ery, par­tic­u­lar­ly those with ser­vo-dri­ven mech­a­nisms.

Safe Stop 2 (SS2)

In some cas­es, the risk assess­ment may show that remov­ing pow­er com­plete­ly from a mech­a­nism will increase the risk. An exam­ple might be a ver­ti­cal axis where the motor dri­ve is used to main­tain the posi­tion of the tool­ing. Remov­ing pow­er from the dri­ve with the tool raised would result in the tool­ing crash­ing to the bot­tom of the axis in an uncon­trolled way. Not the desired way to achieve any type of stop!

There are var­i­ous to pre­vent this kind of occur­rence, but I’m going to lim­it the dis­cus­sion here to the Safe Stop 2 func­tion.

Let’s start with the def­i­n­i­tion [4, 3.11]:

con­trolled stop
stop­ping of machine motion with elec­tri­cal pow­er to the machine actu­a­tor main­tained dur­ing the stop­ping process

Wait! The def­i­n­i­tion of a con­trolled stop is exact­ly the same as a stop cat­e­go­ry 1, so what is the dif­fer­ence? For that we need to look to [4, 9.2.2]:

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

Emer­gency Stop func­tions can­not use Stop Cat­e­go­ry 2 [4,]. If you have tool­ing where Stop Cat­e­go­ry 2 is the most appro­pri­ate stop­ping func­tion under nor­mal con­di­tions, you will have to add an anoth­er means to pre­vent the axis from falling dur­ing the emer­gency stop. The addi­tion­al means could be a spring-set brake that is held released by the emer­gency stop sys­tem and is applied when the e-stop sys­tem removes pow­er from the tool­ing. There are many ways to achieve auto­mat­ic load-hold­ing besides brakes, but remem­ber, what­ev­er you choose it must be effec­tive in pow­er loss con­di­tions.

As shown in Fig. 3, the oper­a­tion of Safe Stop 2 dif­fers from Safe Stop 1 in that, instead of enter­ing into STO when motion stops, the sys­tem enters Safe Oper­at­ing Stop (SOS) [8], not STO. SOS is a Stop Cat­e­go­ry 2 func­tion. Full torque remains avail­able from the motor to hold the tool­ing in posi­tion. Safe stand­still is mon­i­tored by the dri­ve or oth­er means.

Graph showing speed reduction to zero, followed by entry into stop category 2.
Fig­ure 3 — Safe Stop 2

Depend­ing on the ISO 13849–1 PLr, or the IEC 62061 SILr need­ed for the appli­ca­tion, the dri­ve may not have high enough reli­a­bil­i­ty on its own. In this case, a sec­ond chan­nel may be required to ensure that safe stand­still mon­i­tor­ing is ade­quate­ly reli­able. This can be achieved by adding anoth­er means of stand­still detec­tion, like a sec­ond encoder, or a stand­still mon­i­tor­ing device. An exam­ple cir­cuit dia­gram show­ing this type of mon­i­tor­ing can be found in Fig. 4 [10, Fig. 8.37], show­ing a safe­ty PLC and dri­ve used to pro­vide an “inch­ing” or “jog” func­tion.

Circuit diagram for a safe inching mode using a motor drive. Taken from Fig 8.37 in BGIA Report 2/2008e
Fig­ure 4 — Safe­ly lim­it­ed speed for inch­ing mode — PLd, Cat. 3 [10]
In Fig. 4, the encoders are labelled G1 and G2. Both encoders are con­nect­ed to the safe­ty PLC to pro­vide two-chan­nel feed­back required for Cat­e­go­ry 3 archi­tec­ture. G1 is also con­nect­ed to the motor dri­ve for posi­tion and veloc­i­ty feed­back as need­ed for the appli­ca­tion. Note that this par­tic­u­lar dri­ve also has a con­tac­tor upstream, Q1, to pro­vide one chan­nel of the two required for Cat­e­go­ry 3. The sec­ond chan­nel would be pro­vid­ed by the pulse block­ing input on the dri­ve. For more on how this cir­cuit func­tions and how the func­tion­al safe­ty analy­sis is com­plet­ed, see [10].

Safe Operating Stop (SOS)

Dur­ing a safe oper­at­ing stop (SOS), the motor is brought to a spe­cif­ic posi­tion and held there by the dri­ve. Full torque is avail­able to keep the tool­ing in posi­tion. The stop is mon­i­tored safe­ly by the dri­ve. The func­tion is shown in Fig­ure 4 [9].

A graph showing a drive maintaining position following a stop
Fig­ure 5 — Safe Oper­at­ing Stop

In Fig. 5, the y-axis, s, rep­re­sents the posi­tion of the tool­ing, NOT the veloc­i­ty, while the x-axis rep­re­sents time, t. The start of the posi­tion hold­ing func­tion is shown by the orange arrow and dashed line. The peri­od fol­low­ing the green dashed line is the SOS peri­od.

SOS can­not be used for the emer­gency stop func­tion. Under cer­tain con­di­tions it may be used when guard inter­locks are opened, i.e., the guard door on a CNC lathe is opened so that the oper­a­tor can place a new work­piece.

There a quite a few addi­tion­al “safe” dri­ve func­tions. For more on these func­tions and how to imple­ment them, see [2] and appli­ca­tion data from your favourite dri­ve man­u­fac­tur­er. Ref­er­ence is also pro­vid­ed in [9, Table 5.2].

Safe Standstill

Safe stand­still is a con­di­tion where motion has stopped and is being mon­i­tored by a safe­ty-rat­ed device whose out­put sig­nals are used to con­trol the release of guard lock­ing devices. Safe stand­still is not the same as zero-speed because zero-speed can be achieved with­out the use of safe­ty-rat­ed con­trol com­po­nents and design, while safe stand­still requires both suit­able com­po­nents and design.

There are var­i­ous ways to achieve safe stand­still. Here are three approach­es [12]:

  1. Rota­tion sen­sors
    Sen­sors includ­ing prox­im­i­ty sen­sors, resolvers, and encoders can be used to mon­i­tor the motion of the dri­ve com­po­nents. A safe stand­still mon­i­tor­ing device is used to when stand­still has occurred.  When a machine has an unsta­ble rest posi­tion, a prox­im­i­ty sen­sor should be used to ensure the machine is in a safe con­di­tion before the guard lock­ing devices are released.
  2. Back EMF mon­i­tor­ing
    Back elec­tro­mo­tive force or Back EMF is the volt­age cre­at­ed in an elec­tric motor due to the rota­tion of the arma­ture in the mag­net­ic field in the motor. This volt­age oppos­es the applied volt­age and is approx­i­mate­ly pro­por­tion­al to the rota­tion­al speed of the motor. Back EMF remains after the sup­ply volt­age has been removed, allow­ing mon­i­tor­ing devices to indi­rect­ly mea­sure motor speed and stand­still.
  3. Fail­safe timer
    Fail­safe timers are time delay relays designed for use in safe­ty func­tions. Fail­safe timers can be used when the stop­ping per­for­mance of the machin­ery is con­sis­tent and known.
    Fol­low­ing removal of pow­er from the dri­ve motor, the time delay starts. At the end of the time delay, the relay releas­es the guard lock­ing devices.
    Reg­u­lar time delay relays can­not be used for this pur­pose, only fail-safe relays designed to be used in safe­ty func­tions can be used, along with suit­able safe­ty sys­tems design tech­niques like ISO 13849 or IEC 62061.


As you can see, there are sig­nif­i­cant dif­fer­ences between STO, SS1, SS2, SOS and Safe Stand­still. While these func­tions may be used togeth­er to achieve a par­tic­u­lar safe­ty func­tion, some are func­tions of the imple­men­ta­tion of the motor dri­ve, e.g., STO. Some are a func­tion of the design of the motor dri­ve itself, e.g., STO, SS1, SS2, and SOS, or the design of con­trols exter­nal to the motor dri­ve, e.g., safe stand­still. The sim­i­lar­i­ties between these var­i­ous func­tions can make it easy to con­fuse them. Care needs to be tak­en to ensure that the cor­rect tech­ni­cal approach is used when real­is­ing the safe­ty func­tion required by the risk assess­ment.


[1]    “Vari­able Fre­quen­cy Dri­ves — Indus­tri­al Wiki — ode­sie by Tech Trans­fer”,, 2017. [Online]. Avail­able: [Accessed: 19- Jun- 2017].

[2] “Safe Torque Off (STO) — Safe­ty Inte­grat­ed — Siemens”,, 2017. [Online]. Avail­able: [Accessed: 19- Jun- 2017].

[3]      Adjustable speed elec­tri­cal pow­er dri­ve sys­tems — Part 5–2: Safe­ty require­ments — Func­tion­al. IEC Stan­dard 61800–5-2. 2nd Ed. 2016.

[4]     Safe­ty of machin­ery — Elec­tri­cal equip­ment of machines — Part 1: Gen­er­al require­ments. IEC Stan­dard 60204–1. 2006.

[5]     Safe­ty of machin­ery — Pre­ven­tion of unex­pect­ed start-up. EN Stan­dard 1037+A1. 2008.

[6]     Safe­ty of machin­ery — Pre­ven­tion of unex­pect­ed start-up. ISO Stan­dard 14118. 2000.

[7]     “Safe Stop 1 (SS1) — Safe­ty Inte­grat­ed — Siemens”,, 2017. [Online]. Avail­able: [Accessed: 19- Jun- 2017].

[8]     “Safe Stop 2 (SS2) — Safe­ty Inte­grat­ed — Siemens”,, 2017. [Online]. Avail­able: [Accessed: 19- Jun- 2017].

[9]     “Safe Oper­at­ing Stop (SOS) — Safe­ty Inte­grat­ed — Siemens”,, 2017. [Online]. Avail­able: [Accessed: 19- Jun- 2017].

[10]     M. Hauke, M. Schae­fer, R. Apfeld, T. Boe­mer, M. Huelke, T. Borows­ki, K. Bülles­bach, M. Dor­ra, H. Foer­mer-Schae­fer, W. Grigule­witsch, K. Heimann, B. Köh­ler, M. Krauß, W. Küh­lem, O. Lohmaier, K. Mef­fert, J. Pil­ger, G. Reuß, U. Schus­ter, T. Seifen and H. Zil­li­gen, “Func­tion­al safe­ty of machine controls–Application of EN ISO 13849–Report 2/2008e”, BGIA – Insti­tute for Occu­pa­tion­al Safe­ty and Health of the Ger­man Social Acci­dent Insur­ance, Sankt Augustin, 2017.

[11]     “Glos­sary”,, 2017. [Online]. Avail­able: [Accessed: 10- Jan-2018].

[12]     Schm­er­sal Tech Briefs: Safe Speed & Stand­still Mon­i­tor­ing. Schm­er­sal USA, 2017.


Spe­cial thanks go out to two of my reg­u­lar read­ers for sug­gest­ing this post: Matt Ernst and con­trols­girl, who com­ments fre­quent­ly. Thanks for the ideas and the ques­tions that sparked this post!

Industrial Exception” Becomes Permanent in Ontario

The “Industrial Exception”

The “Indus­tri­al Excep­tion” is a clause in the Ontario Pro­fes­sion­al Engineer’s Act that per­mits unli­censed peo­ple to do work nor­mal­ly reserved for licenced engi­neers. Ontario is the only Cana­di­an Province or Ter­ri­to­ry to have this kind of excep­tion.

Ontario Fall Economic Outlook

The Ontario Gov­ern­ment com­mit­ted to main­tain­ing the “indus­tri­al excep­tion” in the Pro­fes­sion­al Engi­neers Act in their Fall Eco­nom­ic Out­look and Fis­cal Review (p.17), released on the 27th of Novem­ber, 2015. This state­ment put an end, at least for the moment, to the dis­cus­sions that start­ed in Sep­tem­ber of 2010 when Pro­fes­sion­al Engi­neers Ontario (PEO) formed the “Repeal of Indus­tri­al Excep­tion Task Force (RIETF)” and result­ed in the announce­ment of the repeal in Jan­u­ary of 2013. The tran­si­tion peri­od giv­en to busi­ness was 90 days, in which time busi­ness­es were expect­ed to con­duct an inter­nal audit and vol­un­tar­i­ly report any vio­la­tions to PEO. They would then be giv­en anoth­er 12 months to rec­ti­fy their sit­u­a­tion, after which time they would be sub­ject to penal­ties under the Pro­fes­sion­al Engi­neers Act.

Pro­fes­sion­al Engi­neers Ontario claimed to have involved more than 100 indus­try groups in the con­sul­ta­tion process pri­or to propos­ing the repeal, and yet 25 groups, among them Canada’s largest indus­tri­al asso­ci­a­tion the Cana­di­an Man­u­fac­tur­ers & Exporters, seem­ing­ly had no knowl­edge of the pro­pos­al until the repeal was qui­et­ly announced.

The back­lash from indus­try groups and indi­vid­ual busi­ness­es result­ed in the gov­ern­ment decid­ing to aban­don the repeal, which result­ed in a press release from PEO express­ing their shock and indig­na­tion in June of 2013. Noth­ing more was pub­licly announced until Novem­ber of 2015, a two-year silence.

Engineers Canada Press Release

On the 30th of Novem­ber 2015, Engi­neers Cana­da put out a press release regard­ing the “indus­tri­al excep­tion” [1], spin­ning this deci­sion as one that will neg­a­tive­ly influ­ence the safe­ty of work­ers in Ontario, and some­how neg­a­tive­ly impact licensed engi­neers in the Province. This is sim­ply spin by Pro­fes­sion­al Engi­neers Ontario and Engi­neers Cana­da. Both orga­ni­za­tions are com­plete­ly ignor­ing the huge poten­tial impact revok­ing the excep­tion could have on Ontario’s man­u­fac­tur­ing sec­tor while mak­ing overblown claims about the poten­tial neg­a­tive effects of mak­ing the excep­tion per­ma­nent. I want to explore this a bit in this arti­cle, as it has a direct bear­ing on machin­ery safe­ty in the Province.

Professional Engineers

In the Province of Ontario, Cana­da, where I live and where my prac­tice is based, the engi­neer­ing pro­fes­sion is reg­u­lat­ed by the “Pro­fes­sion­al Engi­neers Act” (PEA) [2]. This act aims to reg­u­late the pro­fes­sion and pro­vides the author­i­ty need­ed for Pro­fes­sion­al Engi­neers Ontario to license prac­ti­tion­ers. Only licenced engi­neers are autho­rized to prac­tice pro­fes­sion­al engi­neer­ing as defined in the PEA.

Certified Engineering Technologists and Technicians

If you are a tech­nol­o­gist or a tech­ni­cian, you may choose to cer­ti­fy through the Ontario Asso­ci­a­tion of Cer­ti­fied Engi­neer­ing Tech­ni­cians and Tech­nol­o­gists (OACETT), how­ev­er, this does not per­mit you to do any work defined as the “prac­tice of pro­fes­sion­al engi­neer­ing”. Cer­ti­fied Engi­neer­ing Tech­nol­o­gists and Tech­ni­cians gain no legal ben­e­fit from cer­ti­fi­ca­tion, beyond the use of titles pro­tect­ed under the OACETT Act, 1998 [3].

Who is an “engineer”?

The PEA defines the “prac­tice of pro­fes­sion­al engi­neer­ing” as fol­lows:

prac­tice of pro­fes­sion­al engi­neer­ing” means any act of plan­ning, design­ing, com­pos­ing, eval­u­at­ing, advis­ing, report­ing, direct­ing or super­vis­ing that requires the appli­ca­tion of engi­neer­ing prin­ci­ples and con­cerns the safe­guard­ing of life, health, prop­er­ty, eco­nom­ic inter­ests, the pub­lic wel­fare or the envi­ron­ment, or the man­ag­ing of any such act; (“exer­ci­ce de la pro­fes­sion d’ingénieur”) [2]

This is a tremen­dous­ly broad def­i­n­i­tion, espe­cial­ly in that it includes the man­age­ment of the prac­tice, as well as the oth­er activ­i­ties involved, and the safe­guard­ing of “eco­nom­ic inter­ests”, health, prop­er­ty, the pub­lic wel­fare, and the envi­ron­ment. It’s also worth­while not­ing that there is no def­i­n­i­tion of “engi­neer­ing prin­ci­ples”, so the lynch­pin for the def­i­n­i­tion is itself unde­fined. If strict­ly applied, this def­i­n­i­tion would result in vir­tu­al­ly every busi­ness in the province that designs or man­u­fac­tures a prod­uct being legal­ly required to employ a licensed pro­fes­sion­al engi­neer and hold a Cer­tifi­cate of Autho­riza­tion!

It’s also impor­tant to know that the title “engi­neer” is pro­tect­ed in the Province [2]:

pro­fes­sion­al engi­neer” means a per­son who holds a licence or a tem­po­rary licence; (“ingénieur”)

12. (1) No per­son shall engage in the prac­tice of pro­fes­sion­al engi­neer­ing or hold him­self, her­self or itself out as engag­ing in the prac­tice of pro­fes­sion­al engi­neer­ing unless the per­son is the hold­er of a licence, a tem­po­rary licence, a pro­vi­sion­al licence or a lim­it­ed licence.  R.S.O. 1990, c. P.28, s. 12 (1); 2001, c. 9, Sched. B, s. 11 (16).

12. (2) No per­son shall offer to the pub­lic or engage in the busi­ness of pro­vid­ing to the pub­lic ser­vices that are with­in the prac­tice of pro­fes­sion­al engi­neer­ing except under and in accor­dance with a cer­tifi­cate of autho­riza­tion.  R.S.O. 1990, c. P.28, s. 12 (2).

12. (3) Sub­sec­tions (1) and (2) do not apply to pre­vent a per­son,
(f) from using the title “engi­neer” or an abbre­vi­a­tion of that title in a man­ner that is autho­rized or required by an Act or reg­u­la­tion.  R.S.O. 1990, c. P.28, s. 12 (3); 2001, c. 9, Sched. B, s. 11 (17); 2010, c. 16, Sched. 2, s. 5 (18).

Regard­less of your qual­i­fi­ca­tions or expe­ri­ence, or your job respon­si­bil­i­ties, you can­not use the term “engi­neer” with­out risk­ing the wrath of PEO. PEO has sig­nif­i­cant pow­ers under the PEA and can take you to court or impose oth­er penal­ties as described in the PEA.

I believe that a sig­nif­i­cant part of the prob­lem with the PEA is the breadth of the def­i­n­i­tion of pro­fes­sion­al engi­neer­ing, and the lack of clar­i­ty cre­at­ed by the unde­fined “appli­ca­tion of engi­neer­ing prin­ci­ples”. We could debate the def­i­n­i­tions for hours, but instead, I want to focus on the impact that engi­neer­ing has on man­u­fac­tur­ing.

Industrial Exception

Ontario has an inter­est­ing excep­tion built into the Pro­fes­sion­al Engi­neers Act, unof­fi­cial­ly called the “indus­tri­al excep­tion”. So, what is the “indus­tri­al excep­tion”? This excep­tion is laid out in clause 12.(3)(a) [2]:

(3) Sub­sec­tions (1) and (2) do not apply to pre­vent a per­son,

(a) from doing an act that is with­in the prac­tice of pro­fes­sion­al engi­neer­ing in rela­tion to machin­ery or equip­ment, oth­er than equip­ment of a struc­tur­al nature, for use in the facil­i­ties of the person’s employ­er in the pro­duc­tion of prod­ucts by the person’s employ­er;

The exemp­tion per­mits unli­censed per­sons to do work cov­ered by the def­i­n­i­tion of pro­fes­sion­al engi­neer­ing if they are doing it for their employ­er on equip­ment owned and used by their employ­er for the work that employ­er does.

This sit­u­a­tion allows employ­ers to save mon­ey on wages by allow­ing low­er paid work­ers to do work nor­mal­ly reserved for high­er paid licenced pro­fes­sion­als. The down­side to this for employ­ers is that you have no guar­an­tee that the per­son doing the work is ade­quate­ly qual­i­fied, and if any­thing goes wrong, they will be unlike­ly to car­ry insur­ance that could reduce the impact of any loss cre­at­ed.

The Indus­tri­al Excep­tion does not per­mit unli­censed per­sons to con­duct Pre-Start Health and Safe­ty Reviews.

Based on this, a per­son can under­take any act gov­erned by the PEA relat­ed to machin­ery or equip­ment, oth­er than struc­tur­al engi­neer­ing, on behalf of their employ­er, as long as the equip­ment is owned by their employ­er and will be used in pro­duc­tion by their employ­er. This opens up mod­i­fi­ca­tions and design of machin­ery and equip­ment to unli­censed per­sons, as long as the machin­ery or equip­ment is intend­ed for pro­duc­tion use by their employ­er. It does not per­mit unli­censed per­sons to design machin­ery or equip­ment and then sell that equip­ment to oth­ers. It also does not per­mit unli­censed per­sons to con­duct Pre-Start Health and Safe­ty Reviews. The exis­tence of the indus­tri­al exemp­tion is, in part, respon­si­ble for the exis­tence of Ontario’s Pre-Start Health and Safe­ty Review [4, Sec­tion 7].

Is there a problem?

Con­sid­er that the exemp­tion has been part of the PEA since 1984. In that time, the work­place acci­dent rates in Ontario have declined. Six­teen years lat­er, in 2000, the “Pre-Start Health and Safe­ty Review” was cre­at­ed, and a whole new line of busi­ness for engi­neers was cre­at­ed. Acci­dent rates have con­tin­ued to decline at about the same rate as they did pri­or to 2000. There is lit­tle evi­dence to show that the indus­tri­al excep­tion had any sig­nif­i­cant effect on work­place safe­ty in the time since its incep­tion.

The excep­tion cre­ates NO bar­ri­ers for licenced engi­neers.

The excep­tion cre­ates NO bar­ri­ers for licenced engi­neers. The claims that the con­tin­u­a­tion of the excep­tion cre­ates a bar­ri­er to licenced engi­neers who want to move to Ontario and con­tin­ue their prac­tice is com­plete­ly unsup­port­ed. The kinds of work that licenced engi­neers do are com­plete­ly unaf­fect­ed by the excep­tion. The claim that the excep­tion cre­ates bar­ri­ers for licenced engi­neers mov­ing from Ontario to oth­er Provinces and Ter­ri­to­ries is also unfound­ed, since Cana­di­an engi­neer­ing licences are trans­fer­able, and there may be MORE work in oth­er provinces because the work done in Ontario under the exemp­tion must be done by licenced engi­neers in oth­er Provinces or Ter­ri­to­ries.

Elim­i­nat­ing the exemp­tion would force hun­dreds of small and medi­um-sized employ­ers to hire licenced pro­fes­sion­al engi­neers to con­duct work that they may have been doing suc­cess­ful­ly for years. This would increase labour costs for these employ­ers, assum­ing that they could actu­al­ly find an engi­neer to hire. This would also dis­place all of the work­ers already doing the work. Employ­ers might have to halt projects part way com­plet­ed until they could hire a licenced pro­fes­sion­al engi­neer to over­see the com­ple­tion of the project. In a let­ter dat­ed 22-Feb-2013, Ian How­croft, Vice Pres­i­dent of Cana­di­an Man­u­fac­tur­ers and Exporters Ontario, said “…busi­ness­es that gen­er­ate $270 bil­lion in GDP and employ over 700,000 Ontar­i­ans, are writ­ing to request that the imple­men­ta­tion of the “Repeal of the Indus­tri­al Excep­tion”, cur­rent­ly sched­uled to be in force March 1st, 2013, be dis­con­tin­ued until a full reg­u­la­to­ry impact analy­sis can be con­duct­ed.” This request was made because “…busi­ness­es have iden­ti­fied a num­ber of issues that could have sig­nif­i­cant cost impli­ca­tions for busi­ness­es and neg­a­tive con­se­quences for the Ontario Econ­o­my.” This let­ter was under­signed by 24 oth­er trade and busi­ness asso­ci­a­tions. Clear­ly, elim­i­nat­ing the “indus­tri­al excep­tion” could have sig­nif­i­cant impacts on Ontario’s econ­o­my and work­force.

Unfor­tu­nate­ly, both Engi­neers Cana­da and Pro­fes­sion­al Engi­neers Ontario are attempt­ing to use fear, uncer­tain­ty, and doubt to per­suade the gen­er­al pub­lic that an immi­nent risk to life and health is being cre­at­ed by this 32-year-old leg­is­la­tion. If it was going to hap­pen, the prob­lem would have shown itself years ago. Claim­ing any­thing else is ridicu­lous.


[1]     Engi­neers Cana­da, “Engi­neers Cana­da con­cerned Ontario gov­ern­ment deci­sion will neg­a­tive­ly impact work­place health and safe­ty”, (online). 2015. Avail­able: Accessed: 2018-01-10.

[2]     Ontario. Leg­isla­tive Assem­bly of Ontario, Pro­fes­sion­al Engi­neers Act, (R.S.O. 1990, c. P.28). Toron­to. 1990. Avail­able:  Accessed: 25-Feb-16.

[3]     Ontario. Leg­isla­tive Assem­bly of Ontario, An Act respect­ing the Ontario Asso­ci­a­tion of Cer­ti­fied Engi­neer­ing Tech­ni­cians and Tech­nol­o­gists, Mr. Baird. (36:2 Bill PR25) Toron­to. 1998. Avail­able: Accessed: 25-Feb-16.

[4]    Ontario. Leg­isla­tive Assem­bly of Ontario, Indus­tri­al Estab­lish­ments. Ontario Reg­u­la­tion 851. Toron­to. 1990. Avail­able: Accessed: 8-Mar-16.

Presence Sensing Devices — Reaching over sensing fields

This entry is part 2 of 3 in the series Guards and Guard­ing

I recent­ly heard about an appli­ca­tion ques­tion relat­ed to a light cur­tain where a small gap exist­ed at the top of the sens­ing field, between the last beam in the field and the sur­round­ing struc­ture of the machine. There was some con­cern raised about the gap, and whether or not addi­tion­al guard­ing might be need­ed to close the gap. To answer this ques­tion, we need to split it into a few small­er pieces that we can deal with using the tools in the stan­dards.

The first piece to con­sid­er is the gap at the top of the sens­ing field. For this part of the analy­sis, I’m going to assume that the light cur­tain is a fixed bar­ri­er guard, and we’ll analyse the gap based on that idea.

The sec­ond piece of the puz­zle is the place­ment of the light cur­tain, and we’ll look at that sep­a­rate­ly. Once we under­stand the two pieces, we’ll put them togeth­er to see if there are any oth­er issues that may need to be addressed.

The Application

For the pur­pose of this arti­cle, I’ve sketched up the fol­low­ing fig­ures to illus­trate the ideas in the arti­cle. These draw­ings don’t rep­re­sent any actu­al robot cell or appli­ca­tion. Note that the light cur­tain in the sketch is shown with zero safe­ty dis­tance to the robot enve­lope. This is NEVER per­mit­ted.

Cell Elevation View
Fig­ure 1 — Cell Ele­va­tion View show­ing Gap above Light Cur­tain


Cell Plan View
Fig­ure 2 — Cell Plan View

Analyzing The Gap

Light cur­tains are treat­ed the same way that mov­able guards are treat­ed, so the answer to this ques­tion starts with deter­min­ing the size of the gap. I’m going to ref­er­ence two sets of stan­dards in answer­ing this ques­tion: CSA and ISO.

Safety Distances for fingers reaching through an opening
Fig­ure 3 — Fin­ger-to-Knuck­le Reach­ing through a Reg­u­lar Open­ing [1, C.4]
Z432 Reaching Through Regular Openings
Fig­ure 4 — Arm-up-to-Shoul­der Reach­ing through Reg­u­lar Open­ing [1, C.4]
Ref­er­enced Stan­dards
CSA Z432 2004 [1] ISO 13857 2008 [2]
Table 3 — Min­i­mum dis­tance from haz­ard as a func­tion of bar­ri­er open­ing size Table 4 — Reach­ing through Reg­u­lar open­ings
Open­ing Size (e) Safe­ty Dis­tance (sr) Open­ing Size (e) Safe­ty Dis­tance (sr)

11.1– 16.0mm [0.376″–0.625″]

Slot­ted >= 89.0 mm [3.5″] Square >= 66 mm [2.6″] Slot
10 < e <=12 Square/Round
10 < e <=12
>= 100 mm >= 80 mm
49.1–132.0 mm [1.876–5.000″] Slotted/Square <= 915.0 mm [36.0″] Slot/Square/Round 40 < e <= 120 mm <= 850 mm

The first thing to notice is that CSA and ISO use slight­ly dif­fer­ent open­ing sizes (e) and safe­ty dis­tances (sr). These dif­fer­ences have their ori­gin in slight­ly dif­fer­ent anthro­po­met­ric data used to devel­op the tables. In both cas­es, the max­i­mum val­ue for e defines the largest open­ing per­mit­ted with­out addi­tion­al guard­ing.

Let’s look at the appli­ca­tion to see if the gap between the top-most beam and the edge of the phys­i­cal guard falls into the bands defined for e.

Cell Elevation Close Up
Fig­ure 5 — Cell Ele­va­tion Close-Up

Based on the sketch­es of the appli­ca­tion, we have a prob­lem: The gap shown above the light cur­tain is right at the edge of the robot enve­lope, i.e., the dan­ger zone. We are going to have to either, a) Move the fence back 915 mm to get the nec­es­sary safe­ty dis­tance or, b) close the gap off com­plete­ly, either with hard guard­ing or by extend­ing the light cur­tain to close the gap.

Know­ing the size of the gap, we can now decide if the gap should be reduced, or the light cur­tain moved or enlarged. Since light cur­tains run about $125/linear inch, adding an addi­tion­al plate to reduce the size of the gap is like­ly the most cost effec­tive choice. We also need to know the dis­tance from the top-most beam of the light cur­tain to the haz­ard behind the guard. If that dis­tance is less than 915/850 mm, then we have anoth­er prob­lem, since the guard­ing is already too close to the haz­ard.

Analyzing the Light Curtain

The light cur­tain posi­tion­ing is dri­ven by the stop­ping per­for­mance of the machine. Again, let’s ref­er­ence both CSA and ISO for the rel­e­vant cal­cu­la­tions.

Ref­er­enced Stan­dards
CSA Z432 2004 ISO 13855 2005 [3]
5.1 Over­all sys­tem stop­ping per­for­mance
The over­all sys­tem stop­ping per­for­mance com­pris­es at least two phases.Thetwophasesare linked by Equa­tion (1):

T = t1 + t2                             (1)

T is the over­all sys­tem stop­ping per­for­mance;
t1 is the max­i­mum time between the occur­rence of the actu­a­tion of the safe­guard and the out­put sig­nal achiev­ing the OFF-state;
t2 is the stop­ping time, which is the max­i­mum time required to ter­mi­nate the haz­ardous machine func­tion after the out­put sig­nal from the safe­guard achieves the OFF-state. The response time of the con­trol sys­tem of the machine shall be includ­ed in t2.

t1 and t2 are influ­enced by var­i­ous fac­tors, e.g. tem­per­a­ture, switch­ing time of valves, age­ing of com­po­nents.

t1 and t2 are func­tions of the safe­guard and the machine, respec­tive­ly, and are deter­mined by design and eval­u­at­ed by mea­sure­ment. The eval­u­a­tion of these two val­ues shall include the uncer­tain­ties result­ing from the mea­sure­ments, cal­cu­la­tions and/or con­struc­tion.

Clause 10.11 — Safe­guard­ing device safe­ty dis­tanceThe­cal­cu­la­tion­formin­i­mum safe dis­tance between a safe­guard­ing device and the dan­ger zone of a machine shall be as fol­lows:

S = [K (Ts + Tc + Tr + Tbm)] + Dpf

Ds = min­i­mum safe dis­tance between the safe­guard­ing device and the haz­ard

K = speed con­stant: 1.6 m/s (63 in/s) min­i­mum, based on the move­ment being the hand/arm only and the body being sta­tion­ary.
Note: A greater val­ue may be required in spe­cif­ic appli­ca­tions and when body motion must also be con­sid­ered.
Ts = worst stop­ping time of the machine/equipment

Tc = worst stop­ping time of the con­trol sys­tem

Tr = response time of the safe­guard­ing device, includ­ing its inter­face
Note: Tr for inter­locked bar­ri­er may include a delay due to actu­a­tion. This delay may result in Tr being a deduct (neg­a­tive val­ue).

Note: Ts + Tc + Tr are usu­al­ly mea­sured by a stop-time mea­sur­ing device if unknown.

Tbm = addi­tion­al stop­ping time allowed by the brake mon­i­tor before it detects stop-time dete­ri­o­ra­tion beyond the end users’ pre­de­ter­mined lim­its. (For part rev­o­lu­tion press­es only.)

Dpf = max­i­mum trav­el towards the haz­ard with­in the pres­ence-sens­ing safe­guard­ing device’s (PSSD) field that may occur before a stop is sig­naled. Depth pen­e­tra­tion fac­tors will change depend­ing on the type of device and appli­ca­tion. See Fig­ure 5 for spe­cif­ic val­ues. (If applic­a­ble, based on the style of safe­ty device.)

Clause 6.2.3 — Elec­tro-sen­si­tive pro­tec­tive equip­ment employ­ing active opto-elec­tron­ic pro­tec­tive devices with a sen­sor detec­tion capa­bil­i­ty of  < 40 mm  in diam­e­ter Cal­cu­la­tion

The min­i­mum dis­tance, S, in mil­lime­tres, from the detec­tion zone to the haz­ard zone shall not be less than that cal­cu­lat­ed using Equa­tion (2):

S = (K x T ) + C                             (2)


K = 2 000 mm/s;

C = 8 (d — 14), but not less than 0;

d is the sen­sor detec­tion capa­bil­i­ty of the device, in mil­lime­tres (mm).

[Author’s Note — T comes from 5.1, above]


S = (2 000 x T ) + 8(d-14)               (3)

Equa­tion (3) applies to all min­i­mum dis­tances of S up to and includ­ing 500 mm. The min­i­mum val­ue of S shall be 100 mm.

Where the val­ues for S, cal­cu­lat­ed using Equa­tion (3), exceed 500 mm, Equa­tion (4) can be used. In this case, the min­i­mum val­ue of S shall be 500 mm.

S = (K x T ) + C                          (2)


K = 1 600 mm/s;

C = 8 (d - 14), but not less than 0;

d is the sen­sor detec­tion capa­bil­i­ty of the device, in mil­lime­tres (mm).


S = (1 600 x T ) + 8(d — 14)

ISO 13855 Fig. 3 a) Normal Approach
Fig­ure 6 — ISO 13855 Fig. 3 a) Nor­mal Approach


1 haz­ard zone

2 detec­tion zone

3 fixed guard

S min­i­mum dis­tance

a Direc­tion of approach

The two cal­cu­la­tion meth­ods shown above are essen­tial­ly the same, with the pri­ma­ry dif­fer­ence being the val­ue of K, the “hand-speed con­stant”. ISO uses a high­er val­ue of K for light cur­tain instal­la­tions where the field is ver­ti­cal or angled as low as 45º. If the cal­cu­lat­ed val­ue of S is >500 mm, then the val­ue of K is reduced to 1600 mm/s. Using the high­er val­ue of K for a North Amer­i­can instal­la­tion is not wrong, and will result in a more con­ser­v­a­tive instal­la­tion result. Use of 1 600 mm/s for machines going into inter­na­tion­al mar­kets is wrong if S is <500 mm when cal­cu­lat­ed using 2 000 mm/s.

Let’s assume some val­ues so we can do a rep­re­sen­ta­tive cal­cu­la­tion:

Stop­ping Time of the sys­tem (T) = 265 ms [0.265 s]

Light cur­tain res­o­lu­tion (d) = 30 mm [1.2″]

Cal­cu­lat­ing Dpf

Dpf = 8 x (d — 14) = 8 x (30 — 14) = 128

Using K = 2 000 mm/s

S = (2000 x 0.265) + 128 = 658 mm

Since appli­ca­tions where S > 500 mm can be recal­cu­lat­ed using K = 1600 mm/s

S = (1 600 x 0.265) + 128 = 552 mm

So, from the above cal­cu­la­tion, we can see that the dis­tance from the plane of the light cur­tain to the edge of the robot enve­lope (i.e., the dan­ger zone) must be at least 552 mm [21.75″]. That dis­tance is enough that some peo­ple might be able to stand between the light cur­tain field and the fix­ture in the cell, so we should prob­a­bly add a hor­i­zon­tal light cur­tain to pro­tect against that pos­si­bil­i­ty. See Fig­ure 7.

Figure 7 - Vertical Light Curtain with Horizontal segment
Fig­ure 7 — Ver­ti­cal Light Cur­tain with Hor­i­zon­tal seg­ment [1, Fig. B.15 ©]
Anoth­er alter­na­tive to adding a hor­i­zon­tal sec­tion is to slope the light cur­tain field, so that the plane of the light cur­tain is at 45 degrees above the hor­i­zon­tal, with the high­est beam as far away from the haz­ard as pos­si­ble. See Fig­ure 8.

Figure 8 - Sloped light curtain installation [1, CSA Z432 Fig B.15 (c)]
Fig­ure 8 — Sloped light cur­tain instal­la­tion [1, CSA Z432 Fig B.15 ©]
This type of instal­la­tion avoids the need to replace the exist­ing light cur­tain, as long as the field depth is enough to meet the cal­cu­lat­ed Ds.

The field could also be laid hor­i­zon­tal­ly, with no ver­ti­cal com­po­nent. This will change the Dpf cal­cu­la­tion as high­light­ed by the note in Fig­ure 8. Dpf for a hor­i­zon­tal field is cal­cu­lat­ed using the fol­low­ing equa­tion:

Dpf = 1 200 mm [48″]


S = (1 600 x 0.265) + 1200 = 1 624 mm

Note also that there is a height restric­tion placed on hor­i­zon­tal devices based on the object res­o­lu­tion as well, so the 0.3 m max­i­mum height may not apply to an exclu­sive­ly hor­i­zon­tal appli­ca­tion. Note that ISO 13855 allows H a max­i­mum val­ue of 1 000 mm, rather than cut­ting the val­ue off at 990 mm as done in CSA Z432. Using either the 14 mm or the 30 mm res­o­lu­tion cur­tains yields a min­i­mum height of 0 mm and a max­i­mum of 990 mm (CSA) or 1 000 mm (ISO). Note that the 3rd Edi­tion of CSA Z432 is like­ly to har­mo­nize these dis­tances with the ISO cal­cu­la­tions, elim­i­nat­ing these dif­fer­ences.

Also, note that field heights where H > 300 mm may require addi­tion­al safe­guards in con­junc­tion with the Pres­ence-Sens­ing Safe­guard­ing Device (PSSD) field.

Figure 8 - Calculating "H" [1, Fig. B.15 (g)]
Fig­ure 8 — Cal­cu­lat­ing “H” [1, Fig. B.15 (g)]
Going back to our orig­i­nal ver­ti­cal field instal­la­tion, there is one more option that could be con­sid­ered: Reduce the object res­o­lu­tion of the light cur­tain. If we go down to the small­est object res­o­lu­tion typ­i­cal­ly avail­able, 14 mm, the cal­cu­la­tion looks like this:

Dpf = 8 x (14–14) = 0

S = (2 000 x 0.265) + 0 = 530 mm

Since S > 500,

S = (1 600 x 0.265) + 0 = 424 mm [16.7″]

While we have sub­stan­tial­ly reduced the safe­ty dis­tance, it looks like we will still need the hor­i­zon­tal light cur­tain to ensure that no one can stand behind the cur­tain with­out being detect­ed.

If the design of the machin­ery allows, it might be pos­si­ble to reduce the stop­ping time of the machine. If you can reduce the stop­ping time, you will be able to short­en the safe­ty dis­tance required. Note that the safe­ty dis­tance can nev­er go to zero, and can nev­er be less than that deter­mined by the object res­o­lu­tion applied to the reach­ing-through tables. In this case, a 14 mm open­ing results in an 89 mm [3.5″] min­i­mum safe­ty dis­tance (CSA). Since the stop­ping time of the machine can nev­er be zero, 89 mm works out to a stop­ping time of 44.5 ms using K=2 000 mm/s, or 55.6 ms if K= 1 600 mm/s. Very few machines can stop this quick­ly.

The cal­cu­lat­ed safe­ty dis­tance is about half of the safe­ty dis­tance required for the gap, at 915 mm. Clear­ly, clos­ing the gap with the light cur­tain or hard guard­ing will be prefer­able to mov­ing the fence away from the dan­ger zone by 915 mm.

Here’s one more fig­ure to help illus­trate these ideas.

Z432 Figure B.14 a
Fig­ure 9 — CSA Z432 Fig­ure B.15 a)

Fig­ure 9 shows the dif­fer­ence between the reach­ing-through or reach­ing-over light cur­tain appli­ca­tions. Notice that with­out a restrict­ing guard above the cur­tain as we have in our exam­ple, the Dpf val­ue goes out to 1 200 mm [48″], rather than the 915 mm val­ue used in our exam­ple.

The low­er fig­ures show light fence appli­ca­tions, where two or three beams are used, rather than a full cov­er­age light cur­tain.


Here are some of the more impor­tant con­sid­er­a­tions:
1) Is the field of the light cur­tain placed cor­rect­ly, based on the stop­ping per­for­mance of the machine?
2) What is the object res­o­lu­tion of the sens­ing field? This dimen­sion may be used to assess the size of the “open­ings” in the field if this becomes rel­e­vant.
3) What is the height of the low­est and high­est beams or the edges of the sens­ing field?
4) What are the dimen­sions of the gap above the field of the cur­tain, and the dis­tance from the open­ing to the clos­est haz­ard?

ed. note: This arti­cle was reviewed and updat­ed 28-Aug-17.


I’d like to acknowl­edge my col­league, Chris­t­ian Bid­ner, who sug­gest­ed the idea for this arti­cle based on a real-world appli­ca­tion he had seen. Chris­t­ian works for OMRON/STI in their Toron­to office.


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

[2]     Safe­ty of machin­ery — Safe­ty dis­tances to pre­vent haz­ard zones being reached by upper and low­er limbs. ISO 13857.International Orga­ni­za­tion for Stan­dard­iza­tion (ISO). Gene­va. 2008.

[3]     Safe­ty of machin­ery — Posi­tion­ing of safe­guards with respect to the approach speeds of parts of the human body. ISO 13855. Inter­na­tion­al Orga­ni­za­tion for Stan­dard­iza­tion (ISO). Gene­va. 2010.

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