|The effectiveness of the protection depends greatly on the correct positioning of the
light curtain with respect to the danger.
The light curtain must be located at a distance greater than or equal to the minimum safety distance, S, so that reaching the dangerous point will be possible only when the dangerous action of the machine has been stopped.
The light curtain must be positioned so that:
• It is impossible to reach the dangerous point without going through the zone controlled by the light curtain.
• A person cannot be present in the dangerous zone without his/her presence being detected. To this end, it might be necessary to resort to additional safety devices (i.e..: photoelectric light curtains arranged horizontally).
European Standard EN 999 provides the elements for the determination of the
If the machine in object is governed by a specific C type Standard, the latter shall be taken into due account.
If the distance S determined in this manner is too big, it is necessary:
a) to reduce the total stopping time of the machine,
b) to improve the detection capability (resolution) of the light curtain.
|using deflection mirrors|
|S = K x T + C|
|S||minimum safety distance between the protection and hazardous point, expressed in mm.|
|K||speed of approach of the body or parts of the body, expressed in mm / sec.|
|T||total stopping time of the machine, consisting of:
t1 reaction time of the protective device in seconds
t2 reaction time of the machine in seconds, until it stops the hazardous action.
|C||additional distance in mm.|
|1 = Dangerous area 2 = Reference plane 3 = Light curtain|
|Table 2 of ISO 13855/EN 999|
1 = Dangerous area
2 = Reference plane
3 = Light curtain
4 = Mechanical protection
When calculating the safety distance, also consider installation tolerances, accuracy of the measured response time and possible decay of the brake
system performance of the machine.
Where brake system decay is possible, use a stopping performance monitor device (SPM).
|DIRECTION OF APPROACH PERPENDICULAR TO THE PROTECTED PLANE WITH α=90° (± 5°)|
|Light curtains with resolution equal to or lower
than 40 mm for the detection of hands and fingers
|• The distance S must not be lower than
• If the distance S is greater than 500 mm it is
possible to re-calculate the distance through the
• In these circumstances, the distance
must in no case be lower than 500 mm.
|Light curtains with a resolution greater than 40 mm
and less than 70 mm for detection of arms and legs.
|S= 1600xT+850||• The height of the lowermost beam must
be equal to or lower than 300 mm.
• The height of the uppermost beam must
be equal to or higher than 900 mm.
|Light grids for body detection through access control
with a resolution of over 70 mm.
|S= 1600xT+850||Number and height of the beams
|DIRECTION OF APPROACH PARALLEL TO THE PROTECTED PLANE WITH α=0° (± 5°)|
|Horizontal light curtains for presence control
in a dangerous area.
|S=1600xT+(1200-0,4xH)||• 1200 - (0,4 x H) must be equal to or greater
than 850 mm.
• The height H depends on the resolution D of
the light curtains and is determined through
the following formula:
H = 15 x (D - 50)
• This can also be used to determine the
maximum resolution that can be used at the
D = H / (15 + 50)
• Knowing that maximum height H must be 1000
mm, the maximum resolution limits will be:
for H = 1000 mm D = 116 mm
for H = 0 mm D = 50 mm
• If H is greater than 300 mm, at the stage of
risk assessment it becomes necessary to take
into consideration the possibility of access
from beneath the beams.
|DIRECTION OF APPROACH ANGLED TO THE PROTECTED PLANE WITH 5°<α<85°|
|Slanted light curtains to detect hands and
arms and for presence control
in the dangerous area.
|With angle α >30° refer to the case
of approach perpendicular to the
The distance S refers to the closest beam to the hazardous point.
|• With angle α <30° refer to the case of approach
parallel to the protected plane.
-The distance S refers to the beam farthest
away from the hazardous point.
-The height of the beam away farthest from
the hazardous point must not be greater than 1000 mm.
-For the determination of height H or resolution D apply the following formulas to the lowermost beam:
H = 15 x (D - 50)
D = H / (15 + 50)