According to UL 681, door contacts should trigger if the door is opened ___ inches.

The correct answer is 2 inches, which aligns with UL 681's requirements for door contacts in alarm systems. According to the standard, door contacts are designed to detect when a door opens or closes, and a gap of 2 inches is specified to ensure reliable operation. This measurement is critical because it sufficiently accommodates the natural movement of a door, minimizing false alarms while ensuring that the system can accurately detect unauthorized access.

Choosing a distance less than 2 inches, such as 1 or even 1.5 inches, may lead to challenges in ensuring the sensor accurately captures the door's state, particularly due to slight misalignments or variations in installation, which might not trigger the alarm. On the other hand, selecting a distance greater than 2 inches could unnecessarily increase the chances of a security breach going undetected, as the sensor may not activate until the door is opened much further than necessary.

Overall, adhering to the 2-inch requirement not only enhances the operational reliability of the door contacts but also follows recognized safety standards, ensuring effective security measures are in place.