Under what condition will beam detectors activate an alarm?

Beam detectors are designed to detect smoke in a specific area by monitoring the light beam transmitted from a transmitter to a receiver. When smoke interferes with this light beam, it disrupts the path between the transmitter and receiver, which triggers the activation of the alarm. This detection method is particularly effective in larger areas, such as warehouses or atriums, where traditional point smoke detectors might not be as efficient.

The other alternatives do not accurately represent how beam detectors function. For instance, while a disrupted power supply would prevent the detectors from operating, it would not cause the alarm to activate. Similarly, temperature thresholds are relevant for other types of detectors, like heat detectors, but are not the trigger mechanism for beam detectors, which specifically rely on the presence of smoke. Finally, movement in the area does not impact the operation of beam detectors, as they are focused solely on the smoke obscuring the beam of light.