Presence Sensing Safeguarding Device
A Presence Sensing Safeguarding Device (PSSD) is an electronic safety device that creates a defined sensing field (plane, area, or volume) around a hazard and automatically initiates a stop or prevents hazardous motion whenever a person or object enters that field. Examples include light curtains, laser scanners, pressure-sensitive mats, and safety-rated vision systems. It is used to safeguard point-of-operation, perimeter, or area hazards on industrial machinery.
On a manufacturing shop floor, PSSDs are applied in three main ways: point-of-operation safeguarding (e.g., light curtains on press brakes), perimeter/area guarding (e.g., laser scanners around robot cells), and access control for frequent-intervention operations (e.g., pressure mats for walk-in areas). They must be installed at a minimum safe distance based on machine stop time and approach speed, and integrated with safety-rated controls (safety relays/PLCs). Proper installation includes documented risk assessment, validation tests, and periodic functional checks. PSSDs are not substitutes for lockout during maintenance.
How is the safety distance for a light curtain or area scanner determined?
The safety distance S is calculated using the formula S = (K × T) + C, where K is the approach speed (mm/s), T is the total stop time of the machine from actuation until hazardous motion ceases, and C is a constant for device resolution/intrusion. Stop-time must be measured with a calibrated meter and periodically re-measured. The device must be mounted to prevent reach-over/under/around.
Can presence-sensing devices be the only safeguard on all machines?
No. Suitability depends on machine type, hazard, and exposure. They must not be used on full-revolution mechanical power presses because those cannot be stopped mid-stroke reliably. OSHA recommends RF presence-sensing devices not be used as the primary or only safeguard where workers are frequently exposed to serious hazards. Often, fixed guards, PSSDs, and interlocked gates are used together.
How do presence-sensing devices differ from other safety devices like two-hand controls or fixed guards?
Fixed guards provide a physical barrier preventing access. Two-hand controls require both hands away from the danger zone to initiate a cycle, controlling hand location but not other body parts. Presence-sensing devices provide non-contact detection; they allow access but stop or prevent hazardous motion when access occurs. All are safeguards, but PSSDs specifically use electronic presence detection.