Flame Detectors: UV, IR and Where They Belong
Most fire detection senses smoke or heat, but some fires develop so fast, or in such open spaces, that a detector which sees the flame itself is the better answer. Flame detectors respond to the optical radiation a flame emits and can react in seconds across a distance. They are a specialist tool, powerful where they fit and prone to trouble where they don't. This guide covers how they work, where they belong, and what to check.
The defining feature is speed against fast-flaming, high-energy fire risks that other detection would catch too late.
Who this is for
This is for competent fire alarm engineers selecting, siting or maintaining flame detectors. The experience level assumed is competent engineer. Use it for the principles; the specific selection and siting come from the fire risk assessment, BS 5839-1 and the manufacturer's data.
How flame detectors work
Flame detectors sense the ultraviolet and/or infrared radiation emitted by a flame, rather than the smoke or heat a fire produces. UV, single-IR, and multi-spectrum IR types each detect different parts of the spectrum with different immunities to interference. Because they respond to the flame directly, they act very quickly and can cover a distance and a wide field of view. Flame detectors for fire alarm use are covered by BS EN 54-10, and their application follows BS 5839-1 and the fire risk assessment.
Where they belong
Flame detectors suit spaces with a fast-flaming fire risk, frequently with high ceilings or open structures where smoke would take too long to reach a ceiling detector — fuel handling, some process and industrial areas, and warehouses holding flammable goods. In these settings a detector that sees the flame responds far faster than one waiting for smoke to travel. Whether flame detection is the right choice, and which type, is a design decision from the fire risk assessment and BS 5839-1, matched to the specific hazard.
False-alarm sources
The flip side of optical detection is optical interference. Depending on the technology, flame detectors can respond to sunlight, hot surfaces, welding, arc flashes, certain artificial lighting and reflections. UV, single-IR and multi-spectrum types differ in what fools them, which is exactly why type selection and careful siting to avoid interfering sources are central to a reliable installation. From field experience, an unmanaged optical source in the field of view — a new rooflight, a welding bay, a hot process — is a classic cause of nuisance activations.
Servicing flame detectors
Servicing centres on the detector's line of sight and cleanliness. Confirm the field of view to the risk is unobstructed, the lens or window is clean, the detector is tested by the manufacturer's method, and no new obstruction or interfering optical source has appeared since installation. Contamination of the optical window and blocked sightlines are common causes of quietly degraded performance. Record test results and any change to the area that alters coverage.
Common points to check
Recurring issues include blocked or obstructed sightlines to the risk, contaminated optical windows, new interfering optical sources, and the wrong detector type for the environment. Confirming a clean, unobstructed view of the intended risk is the essential check.
When not to rely on this alone
When not to use this article: do not use it to select or site flame detection for a specific hazard. That comes from the fire risk assessment, BS 5839-1, BS EN 54-10 and the manufacturer's data, applied by competent professionals.
Relevant standards
Flame detectors are covered by BS EN 54-10, and their application within a system by BS 5839-1, a code of practice, with the manufacturer's data authoritative for siting and type. The legal duty for fire precautions in relevant premises sits under the Regulatory Reform (Fire Safety) Order 2005. Separate the legal duty from the recommended methods, and always work to current editions.
Professional disclaimer
This is an educational and workflow resource for competent engineers and does not replace the current British and European Standards, the manufacturer's data, the fire risk assessment, or competent judgement. Verify flame-detection selection and siting against current documentation.
Related documentation
Use this with the current BS 5839-1 and BS EN 54-10, the fire risk assessment, and the manufacturer's data. Record detector type, field of view, test method and results, and re-check coverage after any change to the area.