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 is 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 redicu­lous.

References

[1]     Engineers Canada, “Engineers Canada con­cerned Ontario gov­ern­ment decision will neg­at­ively impact work­place health and safety”, (online). 2015. Available: https://​www​.engin​eer​scanada​.ca/​n​e​w​s​/​e​n​g​i​n​e​e​r​s​-​c​a​n​a​d​a​-​c​o​n​c​e​r​n​e​d​-​o​n​t​a​r​i​o​-​g​o​v​e​r​n​m​e​n​t​-​d​e​c​i​s​i​o​n​-​w​i​l​l​-​n​e​g​a​t​i​v​e​l​y​-​i​m​p​a​c​t​-​w​o​r​k​p​l​a​c​e​-​h​e​a​lth. Accessed: 25-​Feb-​16.

[2]     Ontario. Legislative Assembly of Ontario, Professional Engineers Act, (R.S.O. 1990, c. P.28). Toronto. 1990. Available: https://​www​.ontario​.ca/​l​a​w​s​/​s​t​a​t​u​t​e​/​9​0​p​2​8​.​A​c​c​e​s​sed: 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: http://www.ontla.on.ca/web/bills/bills_detail.do?locale=en&BillID=1800&isCurrent=false&ParlSessionID=36%3A2. Accessed: 25-​Feb-​16.

[4]    Ontario. Legislative Assembly of Ontario, Industrial Establishments. Ontario Regulation 851. Toronto. 1990. Available: https://​www​.ontario​.ca/​l​a​w​s​/​r​e​g​u​l​a​t​i​o​n​/​9​0​0​851. Accessed: 8-​Mar-​16.

Presence Sensing Devices – Reaching over sensing fields

This entry is part 2 of 2 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 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 <=12Square/​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)

where
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

where
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

6.2.3.1 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)

where

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]

Then

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)

where

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).

Then

S = (1 600 x T ) + 8(d – 14)

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

Key

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″]

there­fore

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.

Summary

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 closes haz­ard?

Acknowledgements

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.

References

[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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Interlocking Devices: The Good, The Bad and the Ugly

This entry is part 1 of 2 in the series Guards and Guarding

Note: A short­er ver­sion of this art­icle was pub­lished in the May-​2012 edi­tion of  Manufacturing Automation Magazine.

When design­ing safe­guard­ing sys­tems for machines, one of the basic build­ing blocks is the mov­able guard. Movable guards can be doors, pan­els, gates or oth­er phys­ic­al bar­ri­ers that can be opened without using tools. Every one of these guards needs to be inter­locked with the machine con­trol sys­tem so that the haz­ards covered by the guards will be effect­ively con­trolled when the guard is opened.

There are a num­ber of import­ant aspects to the design of mov­able guards. This art­icle will focus on the selec­tion of inter­lock­ing devices that are used with mov­able guards.

The Hierarchy of Controls

The Hierarchy of Controls as an inverted pyrimid.
Figure 1 – The Hierarchy of Controls

This art­icle assumes that a risk assess­ment has been done as part of the design pro­cess. If you haven’t done a risk assess­ment first, start there, and then come back to this point in the pro­cess. You can find more  inform­a­tion on risk assess­ment meth­ods in this post from 31-​Jan-​11. ISO 12100 [1] can also be used for guid­ance in this area.

The hier­archy of con­trols describes levels of con­trols that a machine design­er can use to con­trol the assessed risks. The hier­archy is defined in [1]. Designers are required to apply every level of the hier­archy in order, start­ing at the top. Each level is applied until the avail­able meas­ures are exhausted, or can­not be applied without des­troy­ing the pur­pose of the machine, allow­ing the design­er to move to the next lower level.

Engineering con­trols are sub­divided into a num­ber of dif­fer­ent sub-​groups. Only mov­able guards are required to have inter­locks. There are a num­ber of sim­il­ar types of guards that can be mis­taken for mov­able guards, so let’s take a minute to look at a few import­ant defin­i­tions.

Table 1 – Definitions

International [1] Canadian [2] USA [10]
3.27 guard phys­ic­al bar­ri­er, designed as part of the machine to provide pro­tec­tion.NOTEA guard may act either alone, in which case it is only effect­ive when “closed” (for a mov­able guard) or “securely held in place” (for a fixed guard), or  in con­junc­tion with an inter­lock­ing device with or without guard lock­ing, in which case pro­tec­tion is ensured whatever the pos­i­tion of the guard.NOTE 2Depending on its con­struc­tion, a guard may be described as, for example, cas­ing, shield, cov­er, screen, door, enclos­ing guard.NOTE 3 The terms for types of guards are defined in 3.27.1 to 3.27.6. See also 6.3.3.2 and ISO 14120 for types of guards and their require­ments. Guard — a part of machinery spe­cific­ally used to provide pro­tec­tion by means of a phys­ic­al bar­ri­er. Depending on its con­struc­tion, a guard may be called a cas­ing, screen, door, enclos­ing guard, etc. 3.22 guard: A bar­ri­er that pre­vents expos­ure to an iden­ti­fied haz­ard.E3.22 Sometimes referred to as bar­ri­er guard.”
3.27.4 inter­lock­ing guard guard asso­ci­ated with an inter­lock­ing device so that, togeth­er with the con­trol sys­tem of the machine, the fol­low­ing func­tions are per­formed:
  • the haz­ard­ous machine func­tions “covered” by the guard can­not oper­ate until the guard is closed,
  • if the guard is opened while haz­ard­ous machine func­tions are oper­at­ing, a stop com­mand is giv­en, and
  • when the guard is closed, the haz­ard­ous machine func­tions “covered” by the guard can oper­ate (the clos­ure of the guard does not by itself start the haz­ard­ous machine func­tions)

NOTE ISO 14119 gives detailed pro­vi­sions.

Interlocked bar­ri­er guard — a fixed or mov­able guard attached and inter­locked in such a man­ner that the machine tool will not cycle or will not con­tin­ue to cycle unless the guard itself or its hinged or mov­able sec­tion encloses the haz­ard­ous area. 3.32 inter­locked bar­ri­er guard: A bar­ri­er, or sec­tion of a bar­ri­er, inter­faced with the machine con­trol sys­tem in such a man­ner as to pre­vent inad­vert­ent access to the haz­ard.
3.27.2 mov­able guard
guard which can be opened without the use of tools
Movable guard — a guard gen­er­ally con­nec­ted by mech­an­ic­al means (e.g., hinges or slides) to the machine frame or an adja­cent fixed ele­ment and that can be opened without the use of tools. The open­ing and clos­ing of this type of guard may be powered. 3.37 mov­able bar­ri­er device: A safe­guard­ing device arranged to enclose the haz­ard area before machine motion can be ini­ti­ated.E3.37 There are two types of mov­able bar­ri­er devices:
  • Type A, which encloses the haz­ard area dur­ing the com­plete machine cycle;
  • Type B, which encloses the haz­ard area dur­ing the haz­ard­ous por­tion of the machine cycle.
3.28.1 inter­lock­ing device (interlock)mechanical, elec­tric­al or oth­er type of device, the pur­pose of which is to pre­vent the oper­a­tion of haz­ard­ous machine func­tions under spe­cified con­di­tions (gen­er­ally as long as a guard is not closed) Interlocking device (inter­lock) — a mech­an­ic­al, elec­tric­al, or oth­er type of device, the pur­pose of which is to pre­vent the oper­a­tion of machine ele­ments under spe­cified con­di­tions (usu­ally when the guard is not closed). No defin­i­tion
3.27.5 inter­lock­ing guard with guard lock­ing guard asso­ci­ated with an inter­lock­ing device and a guard lock­ing device so that, togeth­er with the con­trol sys­tem of the machine, the fol­low­ing func­tions are per­formed:
  • the haz­ard­ous machine func­tions “covered” by the guard can­not oper­ate until the guard is closed and locked,
  • the guard remains closed and locked until the risk due to the haz­ard­ous machine func­tions “covered” by the guard has dis­ap­peared, and
  • when the guard is closed and locked, the haz­ard­ous machine func­tions “covered” by the guard can oper­ate (the clos­ure and lock­ing of the guard do not by them­selves start the haz­ard­ous machine func­tions)

NOTE ISO 14119 gives detailed pro­vi­sions.

Guard lock­ing device — a device that is designed to hold the guard closed and locked until the haz­ard has ceased. No defin­i­tion

As you can see from the defin­i­tions, mov­able guards can be opened without the use of tools, and are gen­er­ally fixed to the machine along one edge. Movable guards are always asso­ci­ated with an inter­lock­ing device. Guard selec­tion is covered very well in ISO 14120 [11]. This stand­ard con­tains a flow­chart that is invalu­able for select­ing the appro­pri­ate style of guard for a giv­en applic­a­tion.

5% Discount on ISO and IEC Standards with code: CC2012

Though much emphas­is is placed on the cor­rect selec­tion of these inter­lock­ing devices, they rep­res­ent a very small por­tion of the hier­archy. It is their wide­spread use that makes them so import­ant when it comes to safety sys­tem design.

Electrical vs. Mechanical Interlocks

Mechanical Interlocking
Figure 2 – Mechanical Interlocking

Most mod­ern machines use elec­tric­al inter­locks because the machine is fit­ted with an elec­tric­al con­trol sys­tem, but it is entirely pos­sible to inter­lock the power to the prime movers using mech­an­ic­al means. This doesn’t affect the por­tion of the hier­archy involved, but it may affect the con­trol reli­ab­il­ity ana­lys­is that you need to do.

Mechanical Interlocks

Figure 2, from ISO 14119 [7, Fig. H.1, H.2 ], shows one example of a mech­an­ic­al inter­lock.  In this case, when cam 2 is rotated into the pos­i­tion shown in a), the guard can­not be opened. Once the haz­ard­ous con­di­tion behind the guard is effect­ively con­trolled, cam 2 rotates to the pos­i­tion in b), and the guard can be opened.

Arrangements that use the open guard to phys­ic­ally block oper­a­tion of the con­trols can also be used in this way. See Figure 3 [7, Fig. C.1, C.2].

Mechanical Interlocking using control devices
Figure 3 – Mechanical Interlocking using machine con­trol devices

Fluid Power Interlocks

Figure 4, from [7, Fig. K.2], shows an example of two fluid-​power valves used in com­ple­ment­ary mode on a single slid­ing gate.

Hydraulic interlock from ISO 14119
Figure 4 – Example of a flu­id power inter­lock

In this example, flu­id can flow from the pres­sure sup­ply (the circle with the dot in it at the bot­tom of the dia­gram) through the two valves to the prime-​mover, which could be a cyl­in­der, or a motor or some oth­er device when the guard is closed (pos­i­tion ‘a’). There could be an addi­tion­al con­trol valve fol­low­ing the inter­lock that would provide the nor­mal con­trol mode for the device.

When the guard is opened (pos­i­tion ‘b’), the two valve spools shift to the second pos­i­tion, the lower valve blocks the pres­sure sup­ply, and the upper valve vents the pres­sure in the cir­cuit, help­ing to pre­vent unex­pec­ted motion from trapped energy.

If the spring in the upper valve fails, the lower spool will be driv­en by the gate into a pos­i­tion that will still block the pres­sure sup­ply and vent the trapped energy in the cir­cuit.

5% Discount on ISO and IEC Standards with code: CC2012

Electrical Interlocks

By far the major­ity of inter­locks used on machinery are elec­tric­al. Electrical inter­locks offer ease of install­a­tion, flex­ib­il­ity in selec­tion of inter­lock­ing devices, and com­plex­ity from simple to extremely com­plex. The archi­tec­tur­al cat­egor­ies cov­er any tech­no­logy, wheth­er it is mech­an­ic­al, flu­id­ic, or elec­tric­al, so let’s have a look at archi­tec­tures first.

Architecture Categories

Comparing ANSI, CSA, and ISO Control Reliability Categories
Figure 5 – Control Reliability Categories

In Canada, CSA Z432 [2] and CSA Z434 [3] provide four cat­egor­ies of con­trol reli­ab­il­ity: simple, single chan­nel, single-​channel mon­itored and con­trol reli­able. In the U.S., the cat­egor­ies are very sim­il­ar, with some dif­fer­ences in the defin­i­tion for con­trol reli­able (see RIA R15.06, 1999). In the EU, there are five levels of con­trol reli­ab­il­ity, defined as Performance Levels (PL) giv­en in ISO 13849 – 1 [4]: PL a, b, c, d and e. Underpinning these levels are five archi­tec­tur­al cat­egor­ies: B, 1, 2, 3 and 4. Figure 5 shows how these archi­tec­tures line up.

To add to the con­fu­sion, IEC 62061 [5] is anoth­er inter­na­tion­al con­trol reli­ab­il­ity stand­ard that could be used. This stand­ard defines reli­ab­il­ity in terms of Safety Integrity Levels (SILs). These SILs do not line up exactly with the PLs in [4], but they are sim­il­ar. [5] is based on IEC 61508 [6], a well-​respected con­trol reli­ab­il­ity stand­ard used in the pro­cess indus­tries. [5] is not well suited to applic­a­tions involving hydraul­ic or pneu­mat­ic ele­ments.

The orange arrow in Figure 5 high­lights the fact that the defin­i­tion in the CSA stand­ards res­ults in a more reli­able sys­tem than the ANSI/​RIA defin­i­tion because the CSA defin­i­tion requires TWO (2) sep­ar­ate phys­ic­al switches on the guard to meet the require­ment, while the ANSI/​RIA defin­i­tion only requires redund­ant cir­cuits, but makes no require­ment for redund­ant devices. Note that the arrow rep­res­ent­ing the ANSI/​RIA Control reli­ab­il­ity cat­egory falls below the ISO Category 3 arrow due to this same detail in the defin­i­tion.

Note that Figure 5 does not address the ques­tion of PL’s or SIL’s and how they relate to each oth­er. That is a top­ic for anoth­er art­icle!

The North American archi­tec­tures deal primar­ily with elec­tric­al or fluid-​power con­trols, while the EU sys­tem can accom­mod­ate elec­tric­al, fluid-​power and mech­an­ic­al sys­tems.

From the single-​channel-​monitored or Category 2 level up, the sys­tems are required to have test­ing built-​in, enabling the detec­tion of fail­ures in the sys­tem. The level of fault tol­er­ance increases as the cat­egory increases.

Interlocking devices

Interlocking devices are the com­pon­ents that are used to cre­ate the inter­lock between the safe­guard­ing device and the machine’s power and con­trol sys­tems. Interlocking sys­tems can be purely mech­an­ic­al, purely elec­tric­al or a com­bin­a­tion of these.

Roller cam switch used as part of a complementary interlock
Photo 1 – Roller Cam Switch

Most machinery has an electrical/​electronic con­trol sys­tem, and these sys­tems are the most com­mon way that machine haz­ards are con­trolled. Switches and sensors con­nec­ted to these sys­tems are the most com­mon types of inter­lock­ing devices.

Interlocking devices can be some­thing as simple as a micro-​switch or a reed switch, or as com­plex as a non-​contact sensor with an elec­tro­mag­net­ic lock­ing device.

Images of inter­lock­ing devices used in this art­icle are rep­res­ent­at­ive of some of the types and man­u­fac­tur­ers avail­able, but should not be taken as an endorse­ment of any par­tic­u­lar make or type of device. There are lots of man­u­fac­tur­ers and unique mod­els that can fit any giv­en applic­a­tion, and most man­u­fac­tur­ers have sim­il­ar devices avail­able.

Photo 1 shows a safety-​rated, direct-​drive roller cam switch used as half of a com­ple­ment­ary switch arrange­ment on a gate inter­lock. The integ­rat­or failed to cov­er the switches to pre­vent inten­tion­al defeat in this applic­a­tion.

Micro-Switch used for interlocking
Photo 2 – Micro-​Switch used for inter­lock­ing

Photo 2 shows a ‘microswitch’ used for inter­lock­ing a machine cov­er pan­el that is nor­mally held in place with fasten­ers, and so is a ‘fixed guard’ as long as the fasten­ers require a tool to remove. Fixed guards do not require inter­locks under most cir­cum­stances. Some product fam­ily stand­ards do require inter­locks on fixed guards due to the nature of the haz­ards involved.

Microswitches are not safety-​rated and are not recom­men­ded for use in this applic­a­tion. They are eas­ily defeated and tend to fail to danger in my exper­i­ence.

Requirements for inter­lock­ing devices are pub­lished in a num­ber of stand­ards, but the key ones for indus­tri­al machinery are ISO 14119 [7], [2], and ANSI B11.0 [8]. These stand­ards define the elec­tric­al and mech­an­ic­al require­ments, and in some cases the test­ing require­ments, that devices inten­ded for safety applic­a­tions must meet before they can be clas­si­fied as safety com­pon­ents.
Download stand­ards

Typical plastic-bodied interlocking device
Photo 3 – Schmersal AZ15 plastic inter­lock switch

These devices are also integ­ral to the reli­ab­il­ity of the con­trol sys­tems into which they are integ­rated. Interlock devices, on their own, can­not meet a reli­ab­il­ity rat­ing above ISO 13849 – 1 Category 1, or CSA Z432-​04 Single Channel. To under­stand this, con­sider that the defin­i­tions for Category 2, 3 and 4 all require the abil­ity for the sys­tem to mon­it­or and detect fail­ures, and in Categories 3 & 4, to pre­vent the loss of the safety func­tion. Similar require­ments exist in CSA and ANSI’s “single-​channel-​monitored,” and “control-​reliable” cat­egor­ies. Unless the inter­lock device has a mon­it­or­ing sys­tem integ­rated into the device, these cat­egor­ies can­not be achieved.

Guard Locking

Interlocking devices are often used in con­junc­tion with  guard lock­ing. There are a few reas­ons why a design­er might want to lock a guard closed, but the most com­mon one is a lack of safety dis­tance. In some cases the guard may be locked closed to pro­tect the pro­cess rather than the oper­at­or, or for oth­er reas­ons.

Interlock Device with Guard Locking
Photo 4 – Interlocking Device with Guard Locking

Safety dis­tance is the dis­tance between the open­ing covered by the mov­able guard and the haz­ard. The min­im­um dis­tance is determ­ined using the safety dis­tance cal­cu­la­tions giv­en in [2] and ISO 13855 [9]. This cal­cu­la­tion uses a ‘hand-​speed con­stant’, called K, to rep­res­ent the the­or­et­ic­al speed that the aver­age per­son can achieve when extend­ing their hand straight for­ward when stand­ing in front of the open­ing. In North America, K is usu­ally 63 inches/​second, or 1600 mm/​s. Internationally and in the EU, there are two speeds, 2000 mm/​s, used for an approach per­pen­dic­u­lar to the plane of the guard, or 1600 mm/​second for approaches at 45 degrees or less [9]. 2000 mm/​s is used with mov­able guards, and is approx­im­ately equi­val­ent to 79 inches/​second. Using the International approach, if the value of Ds is great­er than 500 mm when cal­cu­lated using K = 2 000, then [9] per­mits the cal­cu­la­tion to be done using K = 1 600 instead.

Using the stop­ping time of the machinery and K, the min­im­um safety dis­tance can be cal­cu­lated.

Eq. 1              Ds = K x Ts

Using Equation 1 [2], assume you have a machine that takes 250 ms to stop when the inter­lock is opened. Inserting the val­ues into the equa­tion gives you a min­im­um safety dis­tance of:

Example 1             Ds = 63 in/​s x 0.250 s = 15.75 inches

Example 2             Ds = 2000 mm/​s x 0.250 s = 500 mm

As you can see, the International value of K gives a more con­ser­vat­ive value, since 500 mm is approx­im­ately 20 inches.

Note that I have not included the ‘Penetration Factor’, Dpf in this cal­cu­la­tion. This factor is used with pres­ence sens­ing safe­guard­ing devices like light cur­tains, fences, mats, two-​hand con­trols, etc. This factor is not applic­able to mov­able, inter­locked guards.

Also import­ant to con­sider is the amount the guard can be opened before activ­at­ing the inter­lock. This will depend on many factors, but for sim­pli­city, con­sider a hinged gate on an access point. If the guard uses two hinge-​pin style switches, you may be able to open the gate a few inches before the switches rotate enough to detect the open­ing of the guard. In order to determ­ine the open­ing size, you would slowly open the gate just to the point where the inter­lock is tripped, and then meas­ure the width of the open­ing. Using the tables found in [2], [3], [10], or ISO 13857 [12], you can then determ­ine how far the guard must be from the haz­ards behind it. If that dis­tance is great­er than what is avail­able, you could remove one hinge-​pin switch, and replace it with anoth­er type moun­ted on the post oppos­ite the hinges. This could be a keyed inter­lock like Photo 3, or a non-​contact device like Photo 5. This would reduce the open­ing width at the point of detec­tion, and thereby reduce the safety dis­tance behind the guard. But what if that is still not good enough?

If you have to install the guard closer to the haz­ard than the min­im­um safety dis­tance, lock­ing the guard closed and mon­it­or­ing the stand-​still of the machine allows you to ignore the safety dis­tance require­ment because the guard can­not be opened until the machinery is at a stand­still, or in a safe state.

Guard lock­ing devices can be mech­an­ic­al, elec­tro­mag­net­ic, or any oth­er type that pre­vents the guard from open­ing. The guard lock­ing device is only released when the machine has been made safe.

There are many types of safety-​rated stand-​still mon­it­or­ing devices avail­able now, and many variable-​frequency drives and servo drive sys­tems are avail­able with safety-​rated stand-​still mon­it­or­ing.

Environment, failure modes and fault exclusion

Every device has fail­ure modes. The cor­rect selec­tion of the device starts with under­stand­ing the phys­ic­al envir­on­ment to which the device will be exposed. This means under­stand­ing the tem­per­at­ure, humid­ity, dust/​abrasives expos­ure, chem­ic­al expos­ures, and mech­an­ic­al shock and vibra­tion expos­ures in the applic­a­tion. Selecting a del­ic­ate reed switch for use in a high-​vibration, high-​shock envir­on­ment is a recipe for fail­ure, just as select­ing a mech­an­ic­al switch in a dusty, damp, cor­ros­ive envir­on­ment will also lead to pre­ma­ture fail­ure.

Example of a non-contact interlocking device
Photo 5 – JOKAB EDEN Interlock System

Interlock device man­u­fac­tur­ers have a vari­ety of non-​contact inter­lock­ing devices avail­able today that use coded RF sig­nals or RF ID tech­no­lo­gies to ensure that the inter­lock can­not be defeated by simple meas­ures, like tap­ing a mag­net to a reed switch. The Jokab EDEN sys­tem is one example of a sys­tem like this that also exhib­its IP65 level res­ist­ance to mois­ture and dust. Note that sys­tems like this include a safety mon­it­or­ing device and the sys­tem as a whole can meet Control Reliable or Category 3 /​ 4 archi­tec­tur­al require­ments when a simple inter­lock switch could not.

The device stand­ards do provide some guid­ance in mak­ing these selec­tions, but it’s pretty gen­er­al.

Fault Exclusion

Fault exclu­sion is anoth­er key concept that needs to be under­stood. Fault exclu­sion holds that fail­ure modes that have an exceed­ingly low prob­ab­il­ity of occur­ring dur­ing the life­time of the product can be excluded from con­sid­er­a­tion. This can apply to elec­tric­al or mech­an­ic­al fail­ures. Here’s the catch: Fault exclu­sion is not per­mit­ted under any North American stand­ards at the moment. Designs based on the North American con­trol reli­ab­il­ity stand­ards can­not take advant­age of fault exclu­sions. Designs based on the International and EU stand­ards can use fault exclu­sion, but be aware that sig­ni­fic­ant doc­u­ment­a­tion sup­port­ing the exclu­sion of each fault is needed.

Defeat resistance

Diagram showing one method of preventing interlock defeat.
Figure 6 – Preventing Defeat

The North American stand­ards require that the devices chosen for safety-​related inter­locks be defeat-​resistant, mean­ing they can­not be eas­ily fooled with a cable-​tie, a scrap of met­al or a piece of tape.

Figure 6 [7, Fig. 10] shows a key-​operated switch, like the Schmersal AZ15, installed with a cov­er that is inten­ded to fur­ther guard against defeat. The key, some­times called a ‘tongue’, used with the switch pre­vents defeat using a flat piece of met­al or a knife blade. The cov­er pre­vents dir­ect access to the inter­lock­ing device itself. Use of tamper-​resistant hard­ware will fur­ther reduce the like­li­hood that someone can remove the key and insert it into the switch, bypassing the guard.

Inner-Tite tamper resistance fasteners
Photo 6 – Tamper-​resistant fasten­ers

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The International and EU stand­ards do not require the devices to be inher­ently defeat res­ist­ant, which means that you can use “safety-​rated” lim­it switches with roller-​cam actu­at­ors, for example. However, as a design­er, you are required to con­sider all reas­on­ably fore­see­able fail­ure modes, and that includes inten­tion­al defeat. If the inter­lock­ing devices are eas­ily access­ible, then you must select defeat-​resistant devices and install them with tamper-​resistant hard­ware to cov­er these fail­ure modes.

Photo 6 shows one type of tamper res­ist­ant fasten­ers made by Inner-​Tite [13]. Photo 7 shows fasten­ers with uniquely keyed key ways made by Bryce Fastener [14], and Photo 8 shows more tra­di­tion­al tamper­proof fasten­ers from the Tamperproof Screw Company [15]. Using fasten­ers like these will res­ult in the highest level of secur­ity in a threaded fasten­er. There are many dif­fer­ent designs avail­able from a wide vari­ety of man­u­fac­tur­ers.

Bryce Key-Rex tamper-resistant fasteners
Photo 7 – Keyed Tamper-​Resistant Fasteners
Tamper proof screws made by the Tamperproof Screw Company
Photo 8 – Tamper proof screws

Almost any inter­lock­ing device can be bypassed by a know­ledge­able per­son using wire and the right tools. This type of defeat is not gen­er­ally con­sidered, as the degree of know­ledge required is great­er than that pos­sessed by “nor­mal” users.

How to select the right device

When select­ing an inter­lock­ing device, start by look­ing at the envir­on­ment in which the device will be loc­ated. Is it dry? Is it wet (i.e., with cut­ting flu­id, oil, water, etc.)? Is it abras­ive (dusty, sandy, chips, etc.)? Is it indoors or out­doors and sub­ject to wide tem­per­at­ure vari­ations?

Is there a product stand­ard that defines the type of inter­lock you are design­ing? An example of this is the inter­lock types in ANSI B151.1 [4] for plastic injec­tion mould­ing machines. There may be restric­tions on the type of devices that are suit­able based on the require­ments in the stand­ard.

Consider integ­ra­tion require­ments with the con­trols. Is the inter­lock purely mech­an­ic­al? Is it integ­rated with the elec­tric­al sys­tem? Do you require guard lock­ing cap­ab­il­ity? Do you require defeat res­ist­ance? What about device mon­it­or­ing or annun­ci­ation?

Once you can answer these ques­tions, you will have nar­rowed down your selec­tions con­sid­er­ably. The final ques­tion is: What brand is pre­ferred? Go to your pre­ferred supplier’s cata­logues and make a selec­tion that fits with the answers to the pre­vi­ous ques­tions.

The next stage is to integ­rate the device(s) into the con­trols, using whichever con­trol reli­ab­il­ity stand­ard you need to meet. That is the sub­ject for a series of art­icles!

References

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[1] Safety of machinery – General prin­ciples for design – Risk assess­ment and risk reduc­tion, ISO Standard 12100, Edition 1, 2010

[2] Safeguarding of Machinery, CSA Standard Z432, 2004 (R2009)

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[3] Industrial Robots and Robot Systems – General Safety Requirements, CSA Standard Z434, 2003 (R2008)

[4] Safety of machinery — Safety-​related parts of con­trol sys­tems — Part 1: General prin­ciples for design, ISO Standard 13849 – 1, 2006

[5] Safety of machinery – Functional safety of safety-​related elec­tric­al, elec­tron­ic and pro­gram­mable elec­tron­ic con­trol sys­tems, IEC Standard 62061, Edition 1, 2005

[6] Functional safety of electrical/​electronic/​programmable elec­tron­ic safety-​related sys­tems (Seven Parts), IEC Standard 61508-​X

[7] Safety of machinery – Interlocking devices asso­ci­ated with guards – Principles for design and selec­tion, ISO Standard 14119, 1998

[8] American National Standard for Machines, General Safety Requirements Common to ANSI B11 Machines, ANSI Standard B11, 2008
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[9] Safety of machinery — Positioning of safe­guards with respect to the approach speeds of parts of the human body, ISO 13855, 2010

[10] American National Standard for Machine Tools – Performance Criteria for Safeguarding, ANSI B11.19, 2003

[11] Safety of machinery — Guards — General require­ments for the design and con­struc­tion of fixed and mov­able guards, ISO 14120. 2002

[12] Safety of machinery – Safety dis­tances to pre­vent haz­ard zones being reached by upper and lower limbs, ISO 13857. 2008.

[13] Inner-​Tite Corp. home page. (2012). Available: http://​www​.inner​-tite​.com/

[14] Bryce Fastener, Inc. home page. (2012). Available: http://​www​.bryce​fasten​er​.com/

[15] Tamperproof Screw Co., Inc., home page. (2013). Available: http://​www​.tamper​proof​.com