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 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!


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)

Buy CSA Stan­dards

[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
Down­load ANSI stan­dards

[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:

[14] Bryce Fas­ten­er, Inc. home page. (2012). Avail­able:

[15] Tam­per­proof Screw Co., Inc., home page. (2013). Avail­able:

Presence Sensing Devices — Reaching over sensing fields

This entry is part 2 of 7 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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Acknowl­edge­ments: Fig­ures from CSA Z432, Cal­cu­la­tions f more…
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Trapped Key Interlocking

This entry is part 3 of 7 in the series Guards and Guard­ing

Many machine design­ers think of inter­locks as exclu­sive­ly elec­tri­cal devices; a switch is attached to a mov­able mechan­i­cal guard, and the switch is con­nect­ed to the con­trol sys­tem. Trapped Key Inter­lock­ing is a way to inter­lock guards that is equal­ly effec­tive, and often more appro­pri­ate in dif­fi­cult envi­ron­men­tal con­di­tions. Con­tin­ue read­ing “Trapped Key Inter­lock­ing”