Machinery Safety 101

Presence Sensing Devices – Reaching over sensing fields

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 (c)]
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 (c)]
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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