Determination of the safety distance

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 ISO 13855 provides the elements for the determination of the safety distance.

If the machine in object is governed by a specific C type Standard, it shall be taken into due account.

If the distance S determined in this manner is too big, it is necessary:

  • To reduce the total stopping time of the machine
  • 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. The K values can be:

  • K = 2000 mm / sec. for safety distance up to 500 mm (forearm movement speed)
  • K = 1600 mm / sec. for safety distance higher than 500 mm (body movement speed)

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

Direction of approach perpendicular to the protected plane with α=90° (± 5°)

Scenario 1 – Possibility to reach the hazardous point only through the sensitive area

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Light curtains with resolution for the detection of hands and fingers.
Light curtains resolution (d): 14 – 20 – 30 – 40 mm

S = K x T + C

Where:
K = 2000 or 1600
T = t1 + t2 
C = 8x(d-14)

S = 2000xT + 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 using K=1600 but In these circumstances, the distance must, in no case, be lower than 500 mm

S = 1600xT + 8x(d-14)

Light curtains resolution: 14 – 20 – 30 – 40 mm

1: Hazardous point

2: Sensitive area

3: Reference plane

a: Height of the hazardous point

S: Safety distance

C: additional distance in mm = 8 x (d-14)

K: speed of approach of the body or parts of the body, expressed in mm / sec
(1600 or 2000)

T: Total stopping time of the machine

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Light curtains with a resolution for detection of arms and legs.
Light curtains resolution (d): 50 – 90 m

S = K x T + C

Where:
K = 1600
T = t1 + t2 
C = 850

S = 1600xT + 850

Light curtains resolution: 50 – 90 mm

1: Hazardous point

2: Sensitive area

3: Reference plane

a: Height of the hazardous point

S: Safety distance

C: additional distance in mm = 850

K: speed of approach of the body or parts of the body, expressed in mm / sec
(1600)

T: Total stopping time of the machine

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Light curtains for accesso control in dangerous area. Light curtains with 2 – 3- 4 beams

S = K x T + C

Where:
K = 1600
T = t1 + t2 
C = 850

S = 1600xT + 850

Note for 2 beams light curtains:

  • H lower beam = 400 mm (can be used if allowed by risks analysis).
  • H higher beam = 900 mm

Note for 3 beams light curtains:

  • H lower beam = 300 mm
  • H middle beam = 700 mm
  • H higher beam = 1100 mm

Note for 4 beams light curtains:

  • H lower beam = 300 mm
  • H middle beam 1 = 600 mm
  • H middle beam 2 = 900 mm
  • H higher beam = 1200 mm

Height of the beams from the reference plane (eg. floor).

Light curtains resolution: 2, 3 or 4 beams

1: Hazardous point

3: Reference plane

S: Safety distance

Hra: Height of the highest beam

Hrb: Height of the lower beam

 

Scenario 2 – Possibility to reach the hazardous point by leaning over the edge of the sensitive area

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Light curtains with a resolution for detection of hands, fingers, arms and legs.

S = K x T + C

Where:
K = 2000 or 1600 (see following calculations)
T = t1 + t2 
CRO = See the following Table 1

  • In this case C, called “CRO” is obtained from the following Table 1 of ISO 13855:2010.
  • Interpolation is not allowed.
  • If distances a, b or CRO fall between values listed in the table, use the higher.
  • CRO (reaching over) calculated using Table 1 of of ISO 13855:2010 must be compared to C as conventionally calculated (see previous section).
    Always select the higher value.

Light curtains resolution: 14 – 20 – 30 – 40 – 50 – 90 mm

1: Hazardous point

2: Sensitive area

3: Reference plane

a: Height of the hazardous point

b. Height of the highest beam

S: Safety distance

CRO: additional distance in mm see table 1

K: speed of approach of the body or parts of the body, expressed in mm / sec
(1600 or 2000)

T: Total stopping time of the machine

Table 1 of of ISO 13855:2010
Height of Hazard
zone “a”		Height “b” of upper edge of area protected by light curtain
90010001100120013001400160018002000220024002600
Alternative distance CR0
2600000000000000
25004004003503003003003003002501501000
24005505505505004504504004003002501000
220080075075070065065060055040025000
2000950950850850800750700550400000
1800110011009509508508007505500000
160011501150110010009008007504500000
1400120012001100100085080065000000
12001200120011001000850800000000
1000120011501050950750700000000
80011501050950800500450000000
600105095075055000000000
4009007000000000000
20060000000000000
0000000000000

Scenario 3 – Possibility to reach the hazardous point by leaning against the mechanical protection and bypass the light curtain

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For combined mechanical and electrosensitive protections (as shown), where it would be possible to lean against the mechanical protection and bypass the light curtain

S = K x T + C

Where:
K = 2000 or 1600 (see following calculations)
T = t1 + t2 
C = See tables 1 or 2 of ISO 13857:2007

For the calculation of the parameter C should use

  • Table 1 (for low risk applications) or
  • Table 2 (for high-risk applications)
    of ISO 13857:2007 (formerly EN 294) in place of the table on the previous page.

In this guide the two tables of ISO 13857:2007 (formerly EN 294) – Safety distances to prevent danger zones being reached by upper and lower limbs – are not mentioned.

Light curtains resolution: 20 – 30 – 40 – 50 – 90 mm

1: Hazardous point

2: Sensitive area

3: Reference plane

a: Height of the upper edge

b. Height of the highest beam

S: Safety distance

CRO: additional distance in mm. See ISO 13857:2007

K: speed of approach of the body or parts of the body, expressed in mm / sec
(1600 or 2000)

T: Total stopping time of the machine

Direction of approach parallel to the protected plane with α=0° (± 5°)

Horizontal light curtains for presence control in a dangerous area

S = K x T + C

Where:
K = 1600
T = t1 + t2 
C = 1200 – (0,4 x H)

S = 1600xT + (1200 – 0,4xH)

Note:

  • C = 1200 – (0,4 x H) must be equal to or greater than 850 mm
  • The maximum height allowed is: Hmax = 1000 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 formula can also be used to determine the maximum resolution that can be used at the different heights: d = H / 15 + 50
  • For example, 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
  • When using the light curtain as a combination of trip and presence sensing device, the distance x must be less than or equal to the detection capability

Light curtains resolution: 50 – 90 mm

1: Hazardous point

2: Sensitive area

3: Reference plane

a: Height of the hazardous point

X. Distance between end of the detection zone and machine edge

S: Safety distance

H: Height of the sensitive area

Direction of approach angled to the protected plane with 5°< α <85°

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Slanted light curtains to detect hands and arms or presence control in the dangerous area
  • With angle α >30° refer to the case of “Approach perpendicular to the protected plane”
  • With angle α <30° refer to the case of “Approach parallel to the protected plane”.

Note:

  • The distance S refers to the beam farthest away from the hazardous point
  • The height of the beam farthest away 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
  • When using the light curtain as a combination of trip and presence sensing device, the distance x must be less than or equal to the detection capability.

Light curtains resolution: 20 – 30 – 40 mm or 50 – 90 mm for presence in dangerous area

1: Hazardous point

2: Sensitive area

3: Reference plane

a: Height of the hazardous point

X. Distance between end of the detection zone and machine edge

S: Safety distance

Hra. Height of the highest beam

Hrb. Height of the lower beam