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Calculation Example of Safe Distance

Safe Distance for Using a Safety Light Curtain

When installing a safety light curtain, the minimum safe distance specified in respective countries must be observed. The following describes the examples of safe distances calculated according to the ISO and ANSI Standards for the case of the SL-C Series (KEYENCE product).

Example 1 EN999 (ISO13855)

Calculation of safe distance based on EN999-1999 (ISO13855-2002)
(For normal approach into detection zone)
Calculation formula (A): S = K x T + C
S: Safe distance (mm)
K: Speed at which a body or part of a body enters the detection zone (mm/s)
T: Total of response time (sec) (T = t1 + t2)
t1: Maximum response time of the SL-C Series (15 ms)

t2:	Maximum stop time (sec) when the machine stops after receiving the signal from the safeguarding device (SL-C)

C: Additional distance (mm) calculated from the diameter of the smallest detectable target of the SL-C Series

Calculation example when the smallest detectable target size is 40 mm or less

EN999 (ISO13855) specifies parameters as: K = 2,000 mm/s and C = 8（d - 14 mm).
Use formula (A): S = K x T + C with these parameters.
C is a value determined from "d: Diameter (mm) of the smallest detectable target of the SL-C Series" and is assumed to be greater than or equals to 0.
(B): S = 2,000 mm/s x (t1 + t2) + 8(d - 14 mm)
When t1 = 15 ms, t2 = 50 ms, and d = 25 mm,
S = 2,000 mm/s x (0.015 s + 0.05 s) + 8(25 mm - 14 mm)
= 218 mm

*	The safe distance calculated by formula (B) above must be within the range between 100 mm and 500 mm. When the resulting safe distance is less than 100 mm, the safe distance is determined as S = 100 mm. In the case above, the safe distance obtained by formula (B) above is determined as S = 218 mm. (EN999 Section 6.1.1 and ISO13855 Section 6.1.1) If the safe distance calculated by formula (B) above is more than 500 mm, repeat the calculation of formula (A) by setting K＝1,600 mm/s.

	(C): S = 1,600 mm/s x (t1 + t2) + 8(d - 14 mm) When t1 = 15 ms, t2 = 300 ms, and d = 25 mm, S = 1,600 mm/s x (0.015 s + 0.3 s) + 8(25 mm - 14 mm) = 592 mm

*	The safe distance calculated by formula (C) above must be 500 mm or longer. When the result is less than 500mm, the safe distance is determined as S = 500mm. In the case above, the safe distance obtained by formula (C) above is determined as S = 592mm. (EN999 Section 6.1.1 and ISO13855 Section 6.1.1)

*	When the SL-C Series is used for the purpose other than industrial purposes, the minimum safe distance will be the value obtained by adding 75 mm to the value of the result obtained by formula (B). This time, formula (C) cannot be used. As a result, to use the SL-C Series for non-industrial purposes, the necessary safe distance is S = 218 mm + 75 mm = 293 mm.

Calculation example when the smallest detectable target size is more than 40 mm but 70 mm or less

When the SL-C**L or the combination of the SL-C**H and Application Unit (SL-R12EX) is used and the floating blanking function is enabled, the smallest detectable target size exceeds 40 mm. This example is the case where the SL-C**L is used.
EN999 (ISO13855) specifies parameters as:
K = 1,600 mm/s and C = 850 mm. Use formula (A): S = K x T + C with these parameters.

S = 1,600 mm/s x (t1 + t2) + 850 mm
When t1 = 15 ms and t2 = 50 ms,
S = 1,600 mm/s x (0.015 s + 0.05 s) + 850 mm
= 954 mm

As a result, the safe distance is S = 954 mm.

Example 2 ANSI B11.19-2003

Calculation of safe distance based on ANSI B11.19-2003
(Case where objects enter the detection zone moving in a vertical direction) 　

Calculation formula: Ds = K x T + Dpf

Ds:	Safe distance (inch). (1 inch = 25.4 mm)
K:	Maximum speed at which a body or part of a body approaches the danger zone or the source of hazard
T:	Total time required until the dangerous action of the machine stops or the dangerous period in a cycle of the machine operation finishes. This value changes depending on the type of the machine and safeguarding device being used.
Dpf:	Additional distance which varies depending on the smallest detectable target size of the SL-C. Dpf = 3.4 (S - 0.275) inches (Refer to the graph below.)

* The value which can be used for K represents the speed of a certain movement of a hand. (Normally, it is considered to be the horizontal movement of hands or arms when a person is sitting.) In general, a value of 1.6 m/s (63 inches/s) is used. Body actions, or the actions which may affect the actual entry speed, is not considered in the speed of a certain movement of a hand. Consequently, it is necessary to determine an optimum value based on the actual usage of the safeguarding device including the value above. OSHA 1910.217(c) specifies a recommended value as K = 63 inches/s (= 1,600 mm/s).

For detailed rules, the calculation concerning value "T" in particular, refer to ANSI B11.19-2003, Appendix D.

Example 3 ANSI/RIA R15.06-1999

Calculation of safe distance based on ANSI/RIA R15.06-1999
(Case where objects enter the detection zone moving in a vertical direction)

Calculation formula: S = [K x (Ts + Tc + Tr)] + Dpf

S:	Safe distance (mm)
K:	Speed at which a body or part of a body enters the detection zone (= 63 inches/s)
Ts:	Stop time (sec) required for the control to completely stop the machine
Tc:	Maximum response time (sec) of the control system of the machine
Tr:	Maximum response time (sec) of the SL-C Series and its interface
Dpf:	Additional distance (mm) obtained with the entry coefficient in the graph below. This value varies depending on the smallest detectable target size of the SL-C.

* ANSI/RIA R15.06-1999 specifies K = 63 inches/s (= 1,600 mm/s）
as the minimum speed.

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