Access Control, Sensors, and Entrapment Protection Devices
What you must know
Entrapment protection is the heart of UL 325 compliance. Devices include photoelectric sensors (through-beam and retroreflective), safety edges (contact), and intrinsic entrapment protection (inherent force limiting). Know the difference between primary and secondary entrapment protection and that UL 325 requires both for most operators. Access control devices - keypads, card readers, loop detectors, radio receivers, telephone entry - must integrate with the operator's control inputs. Understand loop detector types (safety, shadow, exit) and their placement.
Traps
Questions test device placement: a photo-eye mounted too high misses a child; a safety edge on one side only leaves the other side unprotected. Another trap is confusing a safety loop (reverses or holds open) with a shadow loop (prevents closing). Watch for questions where a single photo-eye is assumed sufficient - many operators require two independent means of entrapment protection. Also, sensors must be tested after any adjustment; a sensor that passes a static test may fail dynamically.
Rule of thumb
Match the device to the hazard: photo-eyes for presence detection, safety edges for contact, loops for vehicle presence. Verify placement covers the full travel path, including the closing edge.
Worked example
A swing gate has one photo-eye at 24 inches. A child crawls under it. Correct answer: the device does not provide adequate entrapment protection; add a second means (safety edge or lower-mounted sensor) per UL 325.