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)

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 closes haz­ard?


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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Author: Doug Nix

+DougNix is Managing Director and Principal Consultant at Compliance InSight Consulting, Inc. ( in Kitchener, Ontario, and is Lead Author and Managing 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.

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