Light Curtain Safety Distance and Muting on a Form-Fill-Seal Line
At a rigid plastic packaging plant, a horizontal form-fill-seal machine was fitted with a Type 4 light curtain at the infeed. The curtain has 40 mm detection capability and a 30 ms response time; the machine's braking system stops the sealing head in 120 ms, measured with a stop-time analyzer. Maintenance installed the curtain so that the innermost beam is 400 mm from the nearest hazard point. During pallet changeover, a muting function suspends the curtain; the mute timer is set to 12 seconds although a pallet pass takes about 8 seconds. Operators routinely reach through the curtain to clear jammed film because the key-reset button is mounted on the frame inside the guarded area, so clearing a jam requires a second person. Over three months, two operators reported the curtain 'stopped the machine for no reason.' Production pressure is high; the line runs three shifts, and the plant had a near-miss last quarter when an operator's sleeve was caught.
1. Is the light curtain mounted at a safe distance from the sealing head? Show the calculation and state the minimum acceptable distance.
Read the worked answer
No - it is short by about 100 mm.
Apply ISO 13855: S = K x T + C.
- K = 2000 mm/s for a detection capability of 40 mm and a resulting distance of 500 mm or less.
- T = 0.030 s (curtain) + 0.120 s (machine stop) = 0.150 s. Use the worst-case measured stop time (top dead centre of the sealing head, full line speed, warm brakes), and add any measurement or brake-wear allowance stated in the validation record.
- C = 8 x (d - 14) = 8 x (40 - 14) = 208 mm for 40 mm detection capability.
First pass: S = 2000 x 0.150 + 208 = 508 mm. Since this exceeds 500 mm, ISO 13855 permits recalculation at K = 1600 mm/s: S = 1600 x 0.150 + 208 = 448 mm, which is below 500 mm, so 500 mm is the minimum. The installed 400 mm leaves roughly 100 mm of reach-through - enough for a hand or forearm to reach the sealing head before it stops.
Corrective options: relocate the curtain to at least 500 mm, measured from the innermost beam to the hazard point along the actual approach path; or reduce stopping time (brake upgrade, faster clutch/brake control) and re-measure; or add a fixed tunnel guard that prevents reach-over and reach-through, which changes the geometry. Re-verify with a stop-time analyser after any change and file the calculation in the machine risk assessment. Do not accept a verbal 'it has never happened' as validation.
2. Evaluate the muting arrangement and the jam-clearing design. What engineered changes and administrative controls would you require, and in what order?
Read the worked answer
Muting. Muting is only acceptable if it is impossible for a person to be in the hazard zone while the curtain is suspended. Two defects stand out:
- A 12-second time window on an 8-second pallet pass leaves a 4-second hole in which a person can walk through undetected. Muting should be position-dependent, using a sequenced sensor arrangement (for example four muting sensors in the correct order) that proves the pallet - not a person - is in the zone, and should end as soon as the pallet clears.
- Muting must be monitored: if the sequence is violated or the mute exceeds a maximum time, the safety function must trip and require a manual reset. A muting lamp should show when the curtain is suspended so others know the guard is inactive.
Jam clearing. A reset button mounted inside the guarded area is a design error: it forces someone to reach in or walk in, and it rewards defeating the guard. Reset must be outside the hazard zone with full view of it. Better still, provide an interlocked hinged guard with guard locking for the film path, a hold-to-run inch/jog mode at reduced speed with a three-position enabling device, and long-handled cutters so nobody reaches in.
Order of work (hierarchy). First eliminate the jam (film tension control, guide adjustment), then engineer the guarding and muting, then write the safe jam-clearing procedure, then train. Lockout is still required for any jam that needs the guard to be removed or the head to be blocked. The 'nuisance stops' are the curtain doing its job - fix the muting and reach geometry, not the curtain.
3. The plant manager proposes to add a warning sign at the infeed and run refresher training for operators. How would you respond?
Read the worked answer
Insufficient, and it misdiagnoses the problem. A sign and a refresher talk address operator behaviour, but the dominant causes here are engineering: a curtain mounted about 100 mm too close, a muting window longer than the pallet pass, and a reset button placed where a person must reach into the hazard zone. Training cannot close a 100 mm reach-through gap, and it cannot prevent someone entering during the 4-second muting overrun. Warning signs are the weakest element of the hierarchy of controls and cannot substitute for a safety function.
Worse, treating this as an operator discipline issue is likely to suppress reporting: two operators have already reported nuisance stops, and if those reports are met with retraining, the next report may not happen at all.
My response would be:
- Present the ISO 13855 calculation and the muting logic review in writing, framed as a machine-safety gap rather than an operator failing.
- Require a documented management-of-change record for the mute timer change and for the curtain installation, plus a re-validation with stop-time measurement.
- Fix the engineering items first (distance, muting sequence, reset location, safe inch mode); use training only to explain the new jam-clearing method and why reaching in is no longer needed.
- Verify effectiveness by auditing jam-clearing behaviour and by confirming that curtain stops no longer correlate with reach-in events.
If the manager still resists, the argument is legal as well as technical: the employer must provide guarding that protects against reach-in without relying on employee care.