Interlocking Devices: The Good, The Bad and the Ugly

This entry is part 1 of 7 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

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

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

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

[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.brycefastener.com/

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

Series Nav­ig­a­tionPres­ence Sens­ing Devices – Reach­ing over sens­ing fields

Author: Doug Nix

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