The Light Curtain That Tripped When Nobody Was There
A stamping press had a safety light curtain across the operator access. Every time the press cycled, the light curtain tripped out — even when the operator kept his hands out. The maintenance crew replaced the curtain (twice), replaced the safety relay, checked the alignment, and still it tripped. The root cause was nobody’s first guess: the press’s vibration was flexing the curtain mounting posts. The emitters and receivers were bolted to thin 40×40 mm aluminum uprights, and at 120 strokes per minute the posts swayed enough to misalign the beams by a millimeter. We welded gussets on the uprights and added a rigid cross-brace. The phantom trips vanished.
Safety light curtains are not just “install and forget.” The mounting, the resolution, and the blanking logic all determine whether the machine is actually safe — or just looks safe.
Resolution: Finger, Hand, or Body?
Light curtains come in resolutions defined by the beam spacing:
- 14 mm (finger resolution): Detects a finger. Used where hands can enter the point of operation (a press die, a blade). The required safety distance is short (because the finger moves fast).
- 30 mm (hand resolution): Detects a hand. Used at a general access opening. The safety distance is larger.
- 70–90 mm (body resolution): Detects a body or arm. Used around a robot cell or a palletizer. Workers can reach through without tripping, but a torso blocks the beam.
Choosing the wrong resolution is a classic failure. A 90 mm curtain on a press die does not protect fingers — an operator can reach in and lose one. Conversely, a 14 mm curtain around a robot cell is overkill (it trips on dust, insects, and shadows).
Safety Distance: The Critical Calculation
The light curtain must be mounted far enough from the hazard so the machine can stop before the operator reaches the danger point. The formula (per ISO 13855) is:
S = K × T + C
Where:
- S is the minimum mounting distance (mm).
- K is the approach speed (1600 mm/s for hand approach, 2000 mm/s for body).
- T is the total stop time (machine stop + safety relay response).
- C is the intrusion depth (reduced by the curtain resolution — for 14 mm resolution, C can be up to 800 mm; for 90 mm, C = 0).
If the press stops in 500 ms, T = 0.5 s, K = 1600 mm/s, and C = 800 mm (14 mm resolution), then S = 1600 × 0.5 + 800 = 1600 mm. Mounting the curtain 500 mm from the die is a compliance violation — the operator’s hand reaches the die before the press stops.
The distance rule: Calculate S before you drill. The formula is in ISO 13855. A curtain mounted too close is not a safety device — it is a decoration. Get the stop time from the machine builder; do not guess.
Muting and Blanking: When to Ignore the Beams
A conveyor-fed machine needs the light curtain to ignore the product passing through (otherwise it trips every cycle). This is muting: a timed or sensor-triggered bypass that disables specific beams while the product passes, then re-enables them. Muting must use redundant sensors (two independent inputs) and must not allow a human to pass. Fixed blanking (ignoring beams blocked by a permanent fixture) is different — it is programmed once, and the curtain reduces its effective resolution.
Both functions are powerful and dangerous. If your muting logic is wrong, the machine runs when a hand is in the way. Always verify the muting by passing a test finger through the muting zone — not a product.
Cascade and Series Connection
For a long opening (a palletizer, a large press), you can chain multiple curtain segments. The “cascade” input lets you connect segments so the receiver only needs one cable back to the safety relay. The rule: do not mix resolutions in a cascade (the system takes the coarsest), and the segments must overlap by at least one beam.
Common Installation Mistakes
- Flexible mounting: Thin uprights sway under vibration. The phantom trips return. Weld gussets or use rigid steel posts.
- No blanking for background: A conveyor behind the curtain gets read as a blocked beam. Use fixed blanking for known obstructions.
- Wrong resolution: 90 mm on a finger hazard. 14 mm on a body opening (over-sensitive).
- Safety distance too small: The most common audit finding. Recalculate S after any change to the machine stop time.
- No restart interlock: After a trip, the machine restarts automatically. The operator may still be reaching in. Require a manual reset.
A Compliance Checklist
- Is the resolution matched to the hazard (finger vs body)?
- Has S been calculated per ISO 13855 with actual stop time?
- Is the mounting rigid (no sway under vibration)?
- Is muting redundant (two sensors) and tested with a test finger?
- Is the restart interlock present (manual reset after trip)?
- Is the safety relay dual-channel (PL d/e)?
- Are the emitter/receiver alignment LEDs green (steady)?
- Is the curtain protected from damage (a physical guard around it)?
- Has a documented risk assessment been filed?
The Bottom Line
A light curtain is a safety instrument, not a sensor. The phantom-trip press had the right curtain on the wrong (flexible) posts. Mount it rigidly, size the resolution to the body part, calculate the safety distance, and test the muting with a finger. The curtain that actually protects people is the one that was engineered, not just installed.
