Beam detectors provide a cost effective method of covering wide open plan areas such as galleries and atria, however care should be taken that activities in the space do not obstruct the beam and that the building structure is such that the beam does not ‘move’ or false operation may result. This detector consists of two components, a light transmitter and a receiver, that are mounted at some distance (up to 300 ft/100m) apart. As smoke migrates between the two components, the transmitted light beam becomes obstructed and the receiver is no longer able to see the full beam intensity. This is interpreted as a smoke condition, and the alarm activation signal is transmitted to the fire alarm panel.
If optical beam detectors are mounted within 2feet/ 600mm of the ceiling level, they should be positioned such that no point in a protected space is more than 25 ft / 7.6m from the nearest part of the optical beam. Should the beam detector be mounted more than 600mm below ceiling level then spacing should be altered to 12.5% of the height of the beam detector above the highest likely seat of any fire. Other than the part of the beam within 500mm of the beam’s transmitter or receiver, if any other section of a beam which runs closer than 500mm to any wall partition or other obstruction to the flow of hot gasses, that section of the beam should be discounted from providing protection.