The LWF Blog

Fire Safety Engineering for Design – Fire Detection & Alarm Systems – Part 161

December 4, 2023 12:02 pm

LWF’s Fire Safety Engineering blog series is written for Architects, building designers and others in the construction industry to highlight and promote discussion on all topics around fire engineering. In part 160, LWF continued to discuss the placement of detectors as per BS 5839-1 and NFPA 72. In part 161, we will talk about beam detectors and flame detectors.

Beam Detectors

In the context of fire detection and fire safety, a beam detector detects smoke by using an infrared beam. A detector is placed on one wall and a reflector on the opposite wall. The detector shoots an infrared beam across the open area to the reflector, which sends it straight back. An interruption in the beam indicates the presence of smoke.

Such detectors are useful particularly in very large or high volumes and may be addressable so as to pinpoint the location of the signal.

The beam should not pass closer than within 500 mm of a wall, partition or duct (but excluding within 500 mm of the transmitter and receiver, or reflectors). Where there is the potential for people to pass through the area of the beam, then the detector should be installed more than 2.7 m above the floor. People are not the only moving object that should be considered when installing beam detectors. Any object that may pass through the beam must be taken into account, whether it is on a permanent basis, such as a forklift passing by regularly, or occasional, during periods of maintenance.

NFPA guidance refers the installer to the manufacturers’ instructions with regards to beam detectors. It may be noted that there are a number of specific requirements for point detector coverage that equally apply to beam detector coverage.

Flame Detectors

Flame detectors work by monitoring the frequency of light in the protected area. They look for the specific frequencies of Ultra Violet (UV) and/or Infra-Red (IR). The detectors only operate in the designated frequency range and will activate the alarm when the frequency of light indicating flame is detected.

Flame detectors do not rely on smoke or heat being carried to them and so do not need to be ceiling mounted. More than one flame detector may be installed in a single area (to cover angles) to ensure a flame is detected in the least possible time.

All flame detectors should be installed strictly in accordance with the manufacturer instructions.

In part 162 of LWF’s series on fire engineering, we will discuss ceiling height limits before moving on to look at control equipment. In the meantime, if you have any questions about this blog, or wish to discuss your own project with one of our fire engineers, please contact us.

Lawrence Webster Forrest has been working with their clients since 1986 to produce innovative and exciting building projects. If you would like further information on how LWF and fire strategies could assist you, please contact the LWF office on 0800 410 1130.

While care has been taken to ensure that information contained in LWF’s publications is true and correct at the time of publication, changes in circumstances after the time of publication may impact on the accuracy of this information.