When using exterior photoelectric detectors, an alarm signal shall be initiated only when:

When using exterior photoelectric detectors, an alarm signal shall be initiated only when:

• A. Two or more detectors anywhere in the same room
• B. One detector is activated
• C. A minimum of two detectors on the same vertical plane are activated
• D. All detectors in the zone are activated

Answer: (C)

Exterior photoelectric detectors are used to guard a boundary by detecting interruptions along a line of sight. To avoid nuisance alarms from wind, debris, or animals, the system requires redundancy: at least two detectors on the same vertical plane must be activated before an alarm is issued. This setup ensures a disturbance is true and consistent across the same plane, reducing false positives that a single detector could produce. The other options either apply to interior spaces, rely on a single sensor, or demand every detector to trigger, which would be overly strict and prone to misses.

Exterior photoelectric detectors are used to guard a boundary by detecting interruptions along a line of sight. To avoid nuisance alarms from wind, debris, or animals, the system requires redundancy: at least two detectors on the same vertical plane must be activated before an alarm is issued. This setup ensures a disturbance is true and consistent across the same plane, reducing false positives that a single detector could produce. The other options either apply to interior spaces, rely on a single sensor, or demand every detector to trigger, which would be overly strict and prone to misses.