

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 SLC Series (KEYENCE product). 

Example 1 EN999 (ISO13855) 

Calculation of safe distance based on EN9991999 (ISO138552002) (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 SLC Series (15 ms)
t2: 
Maximum stop time (sec) when the machine stops after receiving the signal from the safeguarding device (SLC) 
C: Additional distance (mm) calculated from the diameter of the smallest detectable target of the SLC 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 SLC 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 SLC 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 SLC Series for nonindustrial 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 SLC**L or the combination of the SLC**H and Application Unit (SLR12EX) is used and the floating blanking function is enabled, the smallest detectable target size exceeds 40 mm. This example is the case where the SLC**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 B11.192003 


Calculation of safe distance based on ANSI B11.192003
(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 SLC. 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 B11.192003, Appendix D. 
Example 3 ANSI/RIA R15.061999 

Calculation of safe distance based on ANSI/RIA R15.061999 (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 SLC 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 SLC. 


* ANSI/RIA R15.061999 specifies K = 63 inches/s (= 1,600 mm/s） as the minimum speed. 
