Factory safetyReading time 12 min
Machine Safety: Set the Light Curtain at the Right Distance
A light curtain is only protective if the machine stops before a person can reach the hazard. Start with the risk assessment, then calculate the distance from stopping time and approach speed.
01Assess the risk first
ISO 12100 starts with the machine's limits and foreseeable hazards, then assesses and reduces risk before checking what remains. Safeguards are one part of that process. Their type and position should follow the hazard and stopping behavior of the actual machine.
- Inherently safe design. Remove the hazard, change the mechanism, lower the energy, open up the gap.
- Guards and protective devices. Fixed guards, interlocked doors, light curtains, safety mats.
- Information for use. Warning labels, manuals, training.
02Required safety performance
ISO 13849-1 uses severity, exposure and the possibility of avoiding harm to determine the required performance level, PLr. Work from the machine's specific hazard and use pattern rather than choosing the highest level by default.
| Variable | Options | What it asks |
|---|---|---|
| S, severity of injury | S1 slight / S2 serious or fatal | Is it recoverable? |
| F, frequency of exposure | F1 rare / F2 frequent or prolonged | How often does someone enter the zone? |
| P, possibility of avoidance | P1 possible / P2 scarcely possible | Can you see it coming and get clear? |
The achieved performance level depends on the complete safety function: architecture, component reliability, diagnostic coverage and common-cause measures. A highly rated sensor cannot compensate for a weak circuit or unverified stopping behavior.
03Calculating safety distance
ISO 13855 expresses minimum separation as S = K × T + C. Measure the total stopping time, choose the applicable approach speed and allowance, then verify the installed distance on the real machine.
- S, minimum distance from the hazard zone to the detection device (mm)
- K, approach speed, normally 2000 mm/s for hand movement (1600 is allowed once S exceeds 500 mm)
- T, total stopping time: detection plus control plus mechanical stop (seconds)
- C, intrusion allowance, derived from the device resolution
| Curtain resolution d | C value | Detects |
|---|---|---|
| 14 mm or finer | 0 | Fingers |
| 14 ～ 30 mm | 8 × (d − 14) | Hands |
| 30 ～ 40 mm | 850 mm | Arms / body |
| 40 mm and above | 850 mm | Body passage only |
04Installation mistakes
- You can reach under or around the curtain. Add a supplementary curtain or a physical barrier.
- Reflective surfaces let the beam bypass a person. A stainless panel beside the curtain can bounce the beam around someone standing in it. Mount far enough from reflective surfaces.
- Someone can stand inside the field. Once past, the person no longer breaks the beam and the machine can restart. Add a safety mat, or require reset from outside the guarded area.
- The safety relay gets bypassed. Shorting the contacts for convenience is the most common and most serious violation on the floor.
05Keep the evidence
- Risk assessment sheet, hazard list, S/F/P, PLr, countermeasures, residual risk
- List of safety functions with the PL verification calculation for each
- Safety distance calculations and measured stop-time records
- Periodic inspection records, how often each device gets function tested
- Change records, a modified machine needs its risk assessment redone