How to Calculate Safety Distance for Light Curtains: ISO 13855 Formula Explained
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- DADISICK Safety Team
- Issue Time
- Jul 2,2026
Summary
Step-by-step guide to ISO 13855 safety distance calculation for light curtains including formula breakdown, examples, common mistakes, and free DADISICK engineering consultation.
Calculating the correct safety distance between a light curtain and the nearest hazard is the most critical step in any machine guarding installation. This guide explains the ISO 13855 formula in plain language with real-world examples.
The ISO 13855 Formula: S = (K x T) + C
S = (K x T) + C
| Variable | Meaning | Unit | Typical Value |
|---|---|---|---|
| S | Minimum safety distance | mm | Calculated result |
| K | Approach speed toward hazard | mm/s | 2,000 or 1,600 |
| T | Total system stopping time | seconds | 0.1 - 0.3 |
| C | Intrusion distance before detection | mm | 0 - 850 |
Parameter K: Approach Speed
- K = 2,000 mm/s: For finger/hand detection (resolution d ≤ 40 mm)
- K = 1,600 mm/s: For body detection (40 < d ≤ 70 mm)
Crucial: If S ≥ 500 mm with K = 2,000, you must recalculate using K = 1,600 mm/s. This mandatory step is frequently overlooked.
Parameter T: Total Stopping Time
| Element | Example Value |
|---|---|
| Light curtain response time | 6 - 30 ms |
| Safety relay processing | 10 - 30 ms |
| Contactors | 10 - 40 ms |
| Machine mechanical stop time | 50 - 500 ms (measure, don't assume!) |
Critical: Measure actual stopping time with a calibrated device. Take 10 readings and use the maximum.
Parameter C: Intrusion Distance
| Resolution (d) | Body Part | K (mm/s) | C (mm) |
|---|---|---|---|
| 14 mm | Finger | 2,000 | 0 |
| 20 mm | Finger | 2,000 | 48 |
| 25 mm | Hand | 2,000 | 88 |
| 30 mm | Hand | 2,000 | 128 |
| 40 mm | Arm | 2,000 | 208 |
| 50+ mm | Body | 1,600 | 850 |
For d ≤ 40 mm: C = 8 x (d - 14). For 40 < d ≤ 70 mm: C = 850 mm.
Worked Example A: Finger Detection
14mm curtain, response 10ms, machine stop 100ms.
C = 8(14-14) = 0 mm. T = 0.010+0.100 = 0.110 s.
S = 2,000 x 0.110 + 0 = 220 mm. (No recalc needed)
Worked Example B: Hand Detection
30mm curtain, response 15ms, machine stop 150ms.
C = 8(30-14) = 128 mm. T = 0.015+0.150 = 0.165 s.
S = 2,000 x 0.165 + 128 = 458 mm. (No recalc needed)
Worked Example C: The Recalculation Trap
25mm curtain, response 12ms, machine stop 250ms.
C = 8(25-14) = 88 mm. T = 0.012+0.250 = 0.262 s.
First: S = 2,000 x 0.262 + 88 = 612 mm. (S ≥ 500 mm, recalc needed!)
Recalc: S = 1,600 x 0.262 + 88 = 507 mm.
Mistake avoided: Without recalc the safety distance would be 612mm — 105mm more than necessary, wasting factory floor space.
Common Calculation Mistakes
- Wrong K value — using 1,600 for hand detection (should be 2,000)
- Incomplete T — ignoring relay and machine stopping time
- Missing recalc — not re-running with K = 1,600 when S ≥ 500 mm
- Confusing ISO 13855 and ISO 13857
- Wrong installation orientation — horizontal vs vertical C formula
- Ignoring circumvention — top/side penetration distances
DADISICK Free Engineering Consultation
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