Light Curtains, Interlocks & Presence-Sensing Devices
When a machine can’t be fully closed off with a fixed guard, safeguarding devices stop it when a person reaches in or enters the danger zone. They protect only when they’re set up correctly, tested, and never bypassed.
- Manufacturing
- Maintenance
- Machine operators
Types of safeguarding devices
| Device | How it works |
|---|---|
| Light curtain | A screen of infrared beams; breaking it stops the machine |
| Area scanner | A laser scanner that slows or stops a machine or robot when someone enters a zone |
| Safety mat | Stops the machine when stepped on |
| Interlocked guard | Opening a guard door or cover stops the machine and keeps it from starting |
| Two-hand control | Requires both hands on separate buttons to cycle, keeping hands out of the danger area |
| Pullback and restraint devices | Cables attached to the wrists pull hands out or keep them away |
Safety distance
A light curtain must be mounted far enough from the hazard that the machine stops before a hand can reach the danger point. The distance depends on how fast the machine stops. For mechanical power presses, OSHA uses a formula based on a hand speed of 63 inches per second times the stopping time. If a machine’s brakes wear and it stops slower, the distance may no longer be enough, which is why stopping time is checked.
Keep them working
- Test at the start of each shift as the maker describes, like passing a test rod through a light curtain.
- Report any device that doesn’t stop the machine, and tag the machine out.
- Keep sensors clean and aligned.
- Don’t reach around, over or under a light curtain.
Never bypass, tape, jumper or block a safety device to speed up production. Many amputations happen on machines where a guard or interlock was defeated. If a device keeps tripping, there’s a reason; get it fixed.
Muting and blanking
Some systems mute part of a light curtain during safe parts of a cycle, or blank out beams for a fixed obstruction. Only qualified people should set these up, following the maker’s instructions, so no gaps open where a hand could get through.
Not a substitute for lockout
Safeguarding devices protect during normal operation. For maintenance, cleaning and clearing jams, use lockout/tagout. See Clearing Jams Safely and Lockout/Tagout.
Standards & references
- 29 CFR 1910.212: General requirements for all machines
- 29 CFR 1910.217: Mechanical power presses (safety distance)
- OSHA eTool: Machine guarding
- See also: Machine Guarding · Power Presses
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