Safety light curtains: how do you size the safety distance?
A safety light curtain only protects reliably when resolution, protective field height and safety distance are matched. This guide shows sizing to DIN EN ISO 13855, explains the distance formula S = K x T + C and gives concrete values for finger, hand and body protection.
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What does a safety light curtain do?
A safety light curtain is an electro-sensitive protective equipment (ESPE) that spans a vertical protective field with parallel infrared beams. When a person or body part breaks a beam, the device sends a stop signal to the machine. The key figure is the resolution d, the smallest object that is reliably detected.
Resolution results from beam spacing plus lens diameter and decides whether fingers, hands or whole bodies are detected. The finer the resolution, the closer the curtain may sit to the hazard. Typical classes are 14 mm and 30 mm for finger and hand protection, and 40 mm or more for body and access protection.
How are protective field height and beam count chosen?
The protective field height must cover the whole access zone. For access guarding with multi-beam devices, DIN EN ISO 13855 fixes the beam heights so that reaching over, under or through the field is prevented.
- 2-beam system: beam heights 400 and 900 mm above the reference floor.
- 3-beam system: 300, 700 and 1100 mm for higher access security.
- 4-beam system: 300, 600, 900 and 1200 mm for strict access guarding.
- Full protective field: continuous beams for finger and hand protection at the opening.
- Always size the field so climbing over and crawling under are impossible.
How do you calculate the safety distance?
The safety distance S is the minimum gap between protective field and hazard. It ensures the machine has stopped before the hand reaches the danger. DIN EN ISO 13855 provides the formula S = K x T + C.
For a resolution d up to 40 mm, K = 2000 mm/s. The penalty C equals 8 x (d - 14 mm) but never less than 0 mm. If K = 2000 gives a distance above 500 mm, you may recalculate with K = 1600 mm/s, but S must stay at least 500 mm.
- Always measure the response time T, never estimate it - wear extends it.
- For resolution above 40 mm (body protection) use C = 850 mm and K = 1600 mm/s.
- Also check low crawl-under paths and reflection off shiny surfaces.
Frequently asked questions
Which formula applies to the safety distance?
Per DIN EN ISO 13855 it is S = K x T + C. K is the approach speed (usually 2000 mm/s), T the total response time and C the intrusion penalty that depends on resolution.
How large is the penalty C?
For resolutions up to 40 mm, C = 8 x (d - 14 mm) and is never negative. For body protection with coarser resolution a flat C = 850 mm is used.
Which resolution do I need for finger protection?
Finger protection requires 14 mm resolution, hand protection 30 mm. Only these fine resolutions allow a field directly at the hazard with a small distance.
Why is the response time so important?
T feeds directly into the distance. A longer response time from worn brakes or valves increases the required safety distance, so it must be measured regularly.
Need to size a safety light curtain?
We supply finger, hand and body protection light curtains to DIN EN ISO 13855 and help with resolution, field height and distance calculation.
Standards compliant
Sizing to DIN EN ISO 13855 and EN 61496.
Right resolution
Finger, hand and body protection in stock.
Distance verified
Calculation with measured response time.
Expert advice
Automation specialists guide the selection.