Project Engineering
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5.1.3 Safety Clearance
5.1.3.1 General Information
The safety clearance is the minimum distance between the safety field of an ESPE and the danger zone.
It’s task is to prevent the danger zone from being reached before the hazardous motion is completed.
In accordance with ISO 13855, the safety clearance is influenced by the following factors:
• Stopping time of the machine (time from the triggering of the sensor to the completion of the hazardous
motion)
• Response time of the entire safety equipment (ESPE, machine, downstream safety evaluation)
• Approach speed
5.1.3.2 Calculating the Safety Clearance
The general formula for calculating the safety clearance S is:
S = (K × T ) + C or S = K × (t
1
+ t
2
+ t
3
) + C
S [mm]
Safety clearance, measured from the danger zone to the safety field
K [mm/s]
Approach speed
C
Additional clearance depending upon beam clearance / resolution in mm
T [s]
Total response time (t
1
+ t
2
)
T [s]
Total response time
T = (t
1
+ t
2
+ t
3
)
t
1
[s]
Response time of the ESPE
t
2
[s]
Response time of the safety switching device
t
3
[s]
Machine over-travel time
Sample calculation:
An ESPE with 3 beams and 400 mm beam distance (SEFB423) is to be used for protection. The required
safety clearance must be calculated.
• Response time of the ESPE
t
1
= 15 ms
• Over-travel time of the safety switching device
t
2
= 15 ms
• Machine over-travel time
t
3
= 300 ms
Calculate safety clearance
S = 1,600 mm/s × ( t
1
+ t
2
+ t
3
) + C
S = 1,600 mm/s × (0.015 s + 0.015 s + 0.3 s) + 850 mm
S = 1,378 mm