The Light Curtain That Didn’t Catch the Finger: Safety Distance and Resolution
A light curtain guarding a pressing station. The operator reached in to clear a jam. The press didn’t stop. The light curtain didn’t trigger. The operator’s finger passed between the beams. The curtain was mounted too far from the danger point, and the beam spacing was too wide. This is the safety calculation that the install skipped.
The safety distance formula
A light curtain must be mounted far enough from the danger point that the operator can’t reach the hazard before the machine stops. The minimum distance is:
S = K · T + C
Where K is the hand speed (2000 mm/s per ISO 13855), T is the total stop time (machine response + brake stop, in seconds), and C is the additional distance based on the curtain’s resolution. For our press: T = 0.2 s (PLC response) + 0.3 s (brake stop) = 0.5 s. K = 2000 mm/s. C = 8 × (resolution – 14) mm, for resolution above 14 mm. With a 30 mm resolution curtain: C = 8 × (30 – 14) = 128 mm. S = 2000 × 0.5 + 128 = 1128 mm. The curtain was mounted at 500 mm. Way too close.
The resolution problem
The curtain’s resolution (beam spacing) determines what it can detect. A 30 mm resolution curtain detects objects larger than 30 mm — a hand, an arm. It doesn’t detect a finger (about 15 mm). The operator’s finger passed between the beams. The curtain didn’t see it.
For finger detection, use a 14 mm resolution curtain. For hand detection, use 30 mm. For arm detection, use 70 mm. The resolution determines the C term in the safety distance formula — smaller beams require a closer mount (C is smaller), but they detect smaller objects.
| Resolution | Detects | C term (ISO) | Use for |
|---|---|---|---|
| 14 mm | Fingers, thumbs | 0 mm (at 14 mm or less) | Point-of-operation guarding |
| 30 mm | Hands | 128 mm | Body entry, not point-of-op |
| 70 mm | Arms, body | 488 mm | Access to hazardous areas |
What we changed
1. Move the curtain. We moved it to 1200 mm from the press die. The safety distance (1128 mm) is now met. The operator can’t reach the die before the press stops.
2. Upgrade to 14 mm resolution. We replaced the 30 mm curtain with a 14 mm finger-resolution curtain. Now a finger breaks the beam. The C term drops to 0 mm. The required distance is S = 2000 × 0.5 + 0 = 1000 mm. We mounted at 1100 mm. Margin.
3. Check the stop time. We measured the press’s actual stop time. It was 0.5 seconds (PLC + brake). If the stop time were faster (0.2 s), the curtain could be closer (400 mm). We used the measured time, not the catalog’s estimate.
The mute and blanking
The curtain shouldn’t mute (ignore beams) during normal operation. If the curtain is muted, the operator can reach in without stopping the machine. We verified the mute function was disabled. The curtain must be active 100% of the time during production. For automatic cycles where parts pass through the curtain, use blanking (programmed beam suppression for the part path) — not mute.
The safety rule: Calculate the safety distance (S = K·T + C) per ISO 13855. Use 14 mm resolution for finger protection at point-of-operation. Mount the curtain at least S mm from the hazard. A curtain that doesn’t stop the press when a finger reaches in is mounted wrong or has the wrong resolution. This isn’t an adjustment — it’s a safety calculation.
