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 safety distance. 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.
	One-side protection
	Three-side protection
	using deflection mirrors

GENERAL FORMULA FOR THE DETERMINATION OF THE MINIMUM SAFETY DISTANCE

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.

C takes into account:

1. Possible intrusion of parts of the body in the sensitive area before they are detected.
In this case:
• C = 8 x (d-14) If D (light curtain resolution) <= 40 mm
• C = 850 If D (light curtain resolution) > 40 mm and for 2-, 3-, 4-beam light curtains
• C = 1200 - (0,4 x H) for horizontal light curtains

2. Eventuality that the dangerous point is reached by leaning over the upper edge of the sensitive area of a vertical light curtain.
In this case C is obtained from Table 2 of ISO 13855 / EN 999

1 = Dangerous area 2 = Reference plane 3 = Light curtain

Table 2 of ISO 13855/EN 999

• Interpolation is not allowed
• If distances a, b or C fall between values listed in the table, use the higher
• CRO (reaching over) calculated using Table 2 of ISO 13855 / EN 999 must be compared to C as conventionally calculated (see paragraph 1).
Always select the higher value.

For combined mechanical and electrosensitive protections (as shown), calculate parameter C as per Table 1 (for low-risk applications) or Table 2
(for high-risk applications) of ISO 13857:2007 (ex EN 294), here not mentioned, because in this case it is possible to lean on to the mechanical protection.

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 D≤40
	S=2000xT+8x(D-14) if S>500 then S=1600xT+8x(D-14)	• The distance S must not be lower than 100 mm. • If the distance S is greater than 500 mm it is possible to re-calculate the distance through the following formula. • 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. 40<D≤70
	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. D>70
	S= 1600xT+850	Number and height of the beams N. recommended height 2 3 4 400 - 900 mm 300 - 700 - 1100 mm 300 - 600 - 900 - 1200 mm.
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 different heights 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 protected plane. 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)