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Written by Ferdinand Anton Principal Fire & Security Consultant
Flame detectors are specialised automatic fire detectors designed to respond to the optical radiation associated with flames. Unlike conventional smoke and heat detectors, which generally depend on the transport of smoke or heated gases from a fire to the detector, flame detectors can detect a fire by directly sensing radiation emitted by the flame.
This characteristic makes flame detectors particularly valuable where a fire is expected to develop rapidly into an open flaming fire, especially where combustible liquids or gases are present. However, flame detectors are not general-purpose substitutes for smoke or heat detectors. Their effectiveness depends on the characteristics of the expected fire, the detector technology, environmental conditions, detection range, field of view and, critically, a suitable line of sight to the protected area.
This article examines the operating principles and principal types of flame detector, their typical applications, the relevance of BS EN 54-10, and the requirements and recommendations concerning flame detectors contained within BS 5839-1:2025. Particular attention is given to Clauses 3.30, 20.6, 20.8, 20.9, 21.7, 33.2.12 and 43.3.12, together with the broader principles of detector selection, false-alarm management, commissioning and maintenance.
Fire detection systems use different detector technologies because fires do not all develop in the same way. A slowly developing smouldering fire may produce substantial smoke before significant flame development, whereas a fire involving a flammable liquid or gas can develop into a rapidly growing flaming fire with comparatively little smoke.
The selection of a fire detector should therefore be based on the fire phenomenon expected from the particular hazard, rather than simply selecting the most sensitive detector available.
BS 5839-1:2025 defines a flame detector in Clause 3.30 as an automatic fire detector that responds to radiation emitted by the flames from a fire.
The fundamental distinction is therefore that a flame detector responds to optical radiation from flame, rather than primarily relying upon smoke particles, combustion gases or temperature.
BS 5839-1:2025 recognises flame detectors as one of the detection technologies that can be considered where early detection of a relatively fast and clean-burning flaming fire is required. In Clause 20.6, flame detectors are identified as one of the options for this type of fire scenario.
Flames emit electromagnetic radiation across a range of wavelengths. Flame detectors exploit this radiation by using radiation-sensitive sensing elements and associated signal-processing algorithms.
The supplied BS 5839-1:2025 text describes flame detectors as detecting infrared and/or ultraviolet radiation emitted by flame. Radiation-sensitive cells are used to detect the fire either directly or through optical arrangements such as lenses or reflectors.
The detector electronics then determine whether the detected radiation has characteristics consistent with a flame rather than an environmental or artificial source.
This distinction is important because an industrial environment can contain sources of optical radiation that might otherwise cause unwanted alarms. Consequently, detector selection and positioning must take account of the environmental conditions surrounding the detector.
Infrared (IR) flame detectors respond to infrared radiation associated with flames.
BS 5839-1:2025 notes that infrared flame detectors can be designed to respond to characteristics such as:
flame flicker;
flame size; and
one or more specific radiation frequencies.
Some detectors intended for outdoor applications can be designed to respond to particular infrared frequency bands that are not characteristic of solar radiation.
One significant advantage of infrared detection is its ability to operate effectively in environments where smoke may be present. The supplied BS 5839-1:2025 commentary notes that infrared radiation can penetrate smoke more effectively than ultraviolet radiation.
This makes IR technology particularly interesting for some high-level or large-volume spaces.
Ultraviolet (UV) flame detectors respond to ultraviolet radiation generated by flames.
The BS 5839-1:2025 commentary notes that UV flame detectors normally operate within wavelengths filtered from solar radiation by the ozone layer. Consequently, they do not normally respond to sunlight and can generally be used outdoors.
However, UV radiation can be significantly attenuated by smoke.
This limitation is reflected directly in Clause 20.9 of BS 5839-1:2025, which states that UV flame detectors should not be used as the sole means of fire detection in areas within buildings where a fire can produce significant quantities of smoke before flaming occurs.
Therefore, UV flame detection should be carefully considered where smoke obscuration is expected before the flame becomes visible to the detector.
Commercial flame detection technology can also incorporate more than one spectral response or multiple detection channels to improve discrimination between genuine flames and sources of unwanted radiation.
However, it is important not to confuse detector technology with EN 54 performance classification.
For example, Class 1, Class 2 and Class 3 under BS EN 54-10 are performance classifications within the product standard; they are not different flame-detector technologies such as UV or IR.
The precise detection technology and performance characteristics should therefore be established from the manufacturer's product documentation and certification.
Video fire detection is a related but distinct technology.
BS 5839-1:2025 addresses video fire detectors separately in Clause 20.10. Where video fire detectors are used as the sole means of detection, the standard recommends following the manufacturer's or supplier's stated detection-performance and application limitations, with specialist knowledge sought where necessary.
The standard also highlights the importance of reliable flame and/or smoke detection when normal building lighting and dedicated lighting supplies are unavailable.
Video fire detection should therefore not automatically be treated as equivalent to a conventional point flame detector complying with BS EN 54-10.
The principal advantage of flame detection is that it can respond rapidly to a developed flaming fire without waiting for smoke or heated gases to travel to a ceiling-mounted detector.
However, this advantage comes with an important limitation: flame detectors are dependent on the characteristics of the flame and the detector's field of view.
Detector technologyPrimary phenomenonTypical strengthPrincipal limitationSmoke detectorSmoke particlesEarly detection of many smouldering firesCan be susceptible to unwanted alarms from environmental conditionsHeat detectorTemperatureRobustness and relatively high resistance to some environmental influencesGenerally slower response and poor response to small/slow firesFlame detectorOptical radiation from flameRapid response to suitable flaming firesRequires appropriate flame characteristics and line of sightCO/combustion-gas detectorCombustion gasesUseful for certain fire phenomenaApplication depends strongly on fire characteristics and environmentVideo fire detectorVisual flame and/or smoke characteristicsLarge-area visual monitoringPerformance depends on environmental and optical conditions
The selection process should therefore begin with the fire risk and expected fire development, not simply with detector sensitivity.
BS 5839-1:2025 states that flame detectors are not suitable as general-purpose detectors because of their inability to detect smouldering fires.
They are instead normally associated with specialised applications.
Typical applications can include:
Areas containing flammable liquids may present a particularly suitable application because ignition can produce a rapidly developing flaming fire.
Examples can include:
fuel storage areas;
chemical processing facilities;
solvent handling areas;
flammable-liquid processing plants; and
industrial production areas involving combustible liquids.
Flame detection can also be considered in environments where combustible gases are handled or stored and where a fire could develop rapidly.
Examples may include:
gas processing facilities;
industrial gas installations;
fuel-handling areas;
compressor installations; and
process plant.
The suitability of a flame detector must, however, be established through the fire-risk assessment and system design rather than assumed simply because a flammable material is present.
Flame detectors can be useful where conventional smoke detection would be unsuitable because of the environmental conditions or where the anticipated fire is predominantly a rapid flaming fire.
BS 5839-1:2025 specifically discusses the use of infrared flame detectors in very high spaces such as cathedrals and atria.
One advantage is that the detector does not necessarily need to be mounted at ceiling level. It can be installed at relatively low levels on walls around the protected space, provided the required field of view and line of sight are achieved.
This can provide an important alternative where conventional ceiling-mounted smoke or heat detectors may have significant limitations due to the height of the space.
Flame detection should not be regarded as a universal replacement for smoke or heat detection.
The most significant limitations include:
A flame detector requires a suitable flame-related radiation signature.
Consequently, a fire that produces smoke without significant flame may remain undetected.
This is one of the fundamental reasons why BS 5839-1:2025 restricts the circumstances in which flame detectors should be used.
Flame detectors require an unobstructed optical view of the protected hazard.
Objects such as:
structural elements;
equipment;
pipework;
storage;
partitions; and
process machinery
can obstruct the detector's field of view.
A detector that is technically capable of detecting a flame may therefore be ineffective if the flame cannot be seen by the detector.
Infrared and ultraviolet radiation from other sources can create the potential for unwanted alarms.
This is expressly addressed by Clause 33.2.12 of BS 5839-1:2025, which states that flame detectors should not be located in areas where sources of infrared or ultraviolet radiation create the potential for unwanted alarms, and that the manufacturer's guidance should be followed.
Although infrared detection can have advantages where smoke is present, detector performance still depends on the particular technology and application.
UV detection in particular can be adversely affected where smoke develops before significant flame becomes visible to the detector.
The principal product standard relevant to conventional point flame detectors is:
BS EN 54-10:2002 — Fire detection and fire alarm systems — Part 10: Flame detectors — Point detectors.
The British Standards Institution currently lists BS EN 54-10:2002 as Current, Under Review. BSI states that the standard specifies requirements, test methods and performance criteria for point-type, resettable flame detectors that operate using radiation from a flame for use in fire detection systems installed in buildings.
The standard does not cover flame detectors operating on fundamentally different principles, although BSI notes that it may be used as guidance in assessing such products.
BS EN 54-10 is a product standard, whereas BS 5839-1 is primarily concerned with the design, installation, commissioning and maintenance of the overall fire detection and fire alarm system.
These should not be confused.
A flame detector having appropriate product conformity evidence does not, by itself, demonstrate that the detector has been correctly selected, positioned and integrated into the fire detection system.
BSI's information for the EN 54 series also highlights the distinction between component conformity and system compatibility: conformity of individual components with their respective EN 54 parts does not automatically demonstrate that they will function correctly together unless system compatibility has been assessed.
The UK regulatory and conformity-assessment route applicable to a particular flame detector should be established for the product and the circumstances in which it is being placed on the market.
Government sources identify EN 54-10:2002 and EN 54-10:2002/A1:2005 in the UK construction-products conformity framework. UK conformity-assessment body information also identifies EN 54-10 within the scope of product certification for fire detection products, including flame detectors.
Therefore, when specifying a flame detector for a UK project, the designer should verify:
the applicable product standard;
the manufacturer's declaration and conformity documentation;
the applicable conformity-assessment route;
the detector's certified performance;
the manufacturer's application limitations; and
compatibility with the fire alarm system.
Product compliance and system compliance are separate considerations.
BS 5839-1:2025 is a code of practice for the design, installation, commissioning and maintenance of fire detection and fire alarm systems in non-domestic premises. BSI lists the 2025 edition as current and states that it was published on 30 April 2025, superseding the previous edition.
The standard contains several provisions directly relevant to flame detectors.
Clause 3.30 defines a flame detector as an:
automatic fire detector which responds to the radiation emitted by the flames from a fire.
This provides the fundamental definition used throughout the standard.
Clause 20.6 addresses areas where early detection of a relatively fast and clean-burning flaming fire is required.
The clause identifies flame detectors as one of the detector options, alongside suitable multi-sensor detectors, ionization chamber smoke detectors and suitably sensitive heat detectors.
This is important because it establishes the fire scenario for which flame detection may be appropriate.
Clause 20.8 is one of the most important flame-detector provisions.
It states that flame detectors should only be used where it is sufficient for the system to respond to flaming fires, rather than fires producing smoke without significant flame, such as smouldering fires.
It also states that flame detectors should have a clear line of sight to the protected area.
This means that the designer needs to consider not merely detector coverage in plan, but the actual three-dimensional field of view.
Clause 20.9 specifically addresses UV flame detectors.
It states that UV flame detectors should not be used as the sole means of fire detection in areas within buildings where a fire can produce significant quantities of smoke before flaming occurs.
This is an important application limitation and should be considered during the fire-risk assessment and detector-selection process.
Clause 21.7 is the principal flame-detector-specific siting clause in BS 5839-1:2025.
The spacing of flame detectors should remain within the maximum limits specified by the manufacturer.
This is significant because the standard does not provide a universal flame-detector spacing distance comparable to the standard spacing values used for some conventional detector types.
The manufacturer's specified performance and coverage therefore become critical design information.
Where flame detectors are intended to provide general area protection, there should be a clear line of sight between all points within the protected area and one or more flame detectors.
This requires careful consideration of:
detector field of view;
detector orientation;
obstructions;
equipment layout;
structural elements; and
changes to the protected area.
Where flame detectors are being used to protect a specific hazard, there should be a clear line of sight between the hazard and one or more flame detectors.
This approach is particularly relevant to equipment-specific protection, such as a defined process hazard or flammable-liquid installation.
Clause 33.2 is titled:
Selection and siting of automatic fire detectors
and forms part of Clause 33 — Measures to limit false alarms and prevent unwanted fire alarm signals in the supplied BS 5839-1:2025 text.
This is important because detector selection is not solely about achieving sensitivity. It is also about achieving an acceptable balance between fire detection and unwanted-alarm resistance.
Specifically, Clause 33.2.12 addresses flame detectors and states that they should not be located where sources of infrared or ultraviolet radiation create the potential for unwanted alarms. The manufacturer's guidance should be followed.
Potential sources therefore need to be identified during the design process.
Depending on the application, these could include artificial optical sources, industrial processes or other equipment capable of producing radiation within the detector's response range.
Flame detectors are also specifically addressed during system testing.
Clause 43.3.12 states that flame detectors should be functionally tested using a method that determines whether the detector responds to a suitable frequency of radiation and produces a fire alarm signal.
The manufacturer's guidance on detector testing should be followed.
This is an important distinction from simply checking that the detector is physically present or that the CIE receives a signal during a superficial test.
Functional testing should demonstrate the intended detector response.
The following table provides a useful reference for the article and for future technical work.
ClauseSubjectRelevance
3.30
Definition of flame detector
Direct
20.6
Detection of fast, clean-burning flaming fires
Direct/relevant
20.8
Use and limitations of flame detectors; line of sight
Direct
20.9
Ultraviolet flame detectors
Direct
20.10
Video fire detectors
Related technology
21.7
Siting of flame detectors
Direct
21.7.1
Maximum spacing
Direct
21.7.2
General area protection / line of sight
Direct
21.7.3
Specific hazard protection / line of sight
Direct
33.2
Selection and siting of automatic fire detectors
General but relevant
33.2.12
IR/UV radiation and unwanted alarms
Direct
43.3.12
Functional testing of flame detectors
Direct
Annex D
Selection and application of fire detectors
Supporting guidance
This distinction is useful because not every clause mentioning automatic fire detectors is a flame-detector-specific requirement.
BS 5839-1:2025 also contains Clause 21.8, Limits of ceiling height, and Table 3.
Table 3 provides ceiling-height limits for several detector technologies. For heat detectors, for example, the table gives:
DetectorGenerally applicable maximum ceiling heightMaximum for 10% of ceiling areaHeat detectors — Class A1
9.0 m
10.5 m
Heat detectors — other classes
7.5 m
10.5 m
However, these figures should not be incorrectly presented as flame-detector spacing or ceiling-height limits.
Table 3 includes an “Other fire detectors” category for which the maximum ceiling height is specified by the manufacturer.
This is consistent with Clause 21.7.1, which requires flame-detector spacing to be within the manufacturer's specified maximum limits.
Therefore, for flame detectors, the designer should not simply apply the heat-detector ceiling-height figures.
False-alarm performance is an important consideration when selecting flame detection.
A flame detector can be highly effective in the correct environment but inappropriate in an environment containing radiation sources that resemble the expected flame signature.
The design process should therefore consider:
expected fire characteristics;
fuel type;
flame characteristics;
detector spectral response;
environmental radiation;
sunlight and other external radiation;
equipment producing IR/UV radiation;
detector field of view;
obstructions;
environmental conditions;
manufacturer application limitations; and
the consequences of unwanted alarms.
The objective should not simply be maximum sensitivity.
The objective is appropriate detection performance for the specific fire risk with an acceptable false-alarm performance.
Line of sight is arguably the most important practical design consideration for flame detectors.
A conventional ceiling-mounted smoke detector can detect smoke that reaches its sensing chamber from different directions. A flame detector, by contrast, needs the relevant radiation from the flame to reach its sensing system.
A designer should therefore consider the protected area in three dimensions.
For example, a large item of plant might obscure a flame detector's view of a hazard behind it. A plan drawing may suggest that the area is covered, while the actual optical field of view contains a significant blind spot.
For this reason, flame-detector design should consider:
Hazard → Flame development → Detector field of view → Optical path → Alarm response
rather than simply:
Detector → Coverage radius
BS 5839-1:2025 places significant importance on manufacturer-specified performance for flame detectors.
This is particularly apparent in Clause 21.7.1, which requires spacing to remain within the maximum limits specified by the manufacturer.
The manufacturer's technical documentation should therefore be examined for information such as:
detection range;
field of view;
spectral response;
minimum detectable flame size;
response time;
environmental limitations;
mounting arrangements;
sensitivity settings;
false-alarm rejection features;
allowable spacing;
testing procedure; and
application restrictions.
The manufacturer's data should not, however, replace the designer's fire-risk assessment.
Correct installation does not by itself demonstrate continued detection performance.
BS 5839-1:2025 specifically addresses functional testing of flame detectors in Clause 43.3.12.
The test should determine whether the detector responds to an appropriate frequency of radiation and generates the required fire alarm signal.
The manufacturer's test procedure should be followed.
Maintenance activities should also consider factors that could degrade detector performance, including:
contamination of optical surfaces;
changes to detector orientation;
newly installed obstructions;
changes to plant layout;
changes to environmental radiation;
changes to the protected hazard; and
changes to the detector's configuration or settings.
A flame detector that was correctly positioned when commissioned may become ineffective if the protected area is subsequently modified.
A competent design process should answer the following questions before flame detectors are selected:
What fuel is likely to burn?
Is the anticipated fire likely to develop rapidly into a flaming fire?
Could significant smoke be generated before flaming?
Is infrared, ultraviolet or another technology appropriate?
What spectral characteristics does the expected flame have?
Are there environmental radiation sources that could produce unwanted alarms?
What is the manufacturer's specified detection range?
Is the required field of view achieved?
Is there a clear line of sight to every relevant part of the protected area?
Are there any current or foreseeable obstructions?
Is the detector suitable for the fire detection and alarm system?
Is appropriate product conformity evidence available?
Are system compatibility requirements satisfied?
Can the detector be functionally tested in accordance with the manufacturer's instructions?
Can the detector remain accessible for inspection and maintenance?
Could future changes to the building or process compromise its field of view?
These questions help demonstrate that flame-detector selection is an engineering decision, rather than simply a product-selection exercise.
The principal lessons from BS 5839-1:2025 can be summarised as follows:
Flame detectors are specialist detectors, not general-purpose fire detectors.
They are particularly relevant to rapid flaming fires.
They are unsuitable where detection of smouldering fires is an essential requirement.
Flame detectors require an appropriate clear line of sight.
Detector spacing should remain within the manufacturer's specified limits.
UV flame detectors should not be the sole detection method where significant smoke could develop before flaming.
Sources of IR or UV radiation must be considered because they can contribute to unwanted alarms.
Product conformity and system design compliance are different matters.
BS EN 54-10 is the principal product standard for point-type, resettable flame detectors operating using radiation from flame.
Flame detectors should be functionally tested in accordance with the requirements of Clause 43.3.12 and the manufacturer's instructions.
The designer should consider the specific fire risk, environment, detector technology and manufacturer application data before selecting the detector.
Flame detectors provide an important specialist capability within modern fire detection and alarm systems. Their ability to respond directly to radiation emitted by flames can provide rapid detection of suitable flaming fires, particularly in environments where combustible liquids or gases are present or where conventional ceiling-mounted detection may be limited by the height or geometry of the protected space.
However, their strengths are also accompanied by important limitations. Flame detectors generally cannot provide reliable detection of smouldering fires, require an appropriate optical path to the protected hazard and may be susceptible to unwanted alarms from sources of infrared or ultraviolet radiation.
BS 5839-1:2025 provides specific guidance on their selection, application, siting and testing. In particular, Clauses 20.8 and 20.9, Clause 21.7, Clause 33.2.12, and Clause 43.3.12 should be considered when designing, commissioning and maintaining systems incorporating flame detectors.
From a product-compliance perspective, BS EN 54-10:2002 provides the principal UK-adopted product standard for point-type resettable flame detectors operating using flame radiation. BSI currently lists the standard as current and under review.
Ultimately, successful flame-detector application depends on integrating fire-risk assessment, detector technology, manufacturer performance data, line-of-sight analysis, unwanted-alarm considerations, system compatibility and appropriate commissioning and maintenance.
Flame detection should therefore be viewed not simply as a highly sensitive detection technology, but as a specialist fire-detection engineering solution that must be matched to the anticipated fire scenario.
According to BS 5839-1:2025, what is a fundamental requirement when flame detectors are used for general area protection?
A. They must always be ceiling mounted. B. They must be spaced at exactly 7.5 m intervals. C. There should be a clear line of sight between all points within the protected area and one or more flame detectors. D. They should always be combined with heat detectors.
Correct answer: C
According to Clause 20.9 of BS 5839-1:2025, where should ultraviolet flame detectors not be used as the sole means of fire detection?
A. In outdoor areas exposed to sunlight. B. In areas where a fire can produce significant quantities of smoke before flaming occurs. C. In areas containing flammable liquids. D. In high-ceiling buildings.
Correct answer: B
Ferdinand Anton Principal Fire & Security Consultant
The information set out in this article is believed to be correct based on the information and standards available at the time of writing. However, it is not guaranteed to be complete or current, and readers should refer to the latest applicable standards, legislation, manufacturer’s instructions and competent professional advice before relying on the information provided.
The author accepts no responsibility or liability for any loss, damage, or consequence arising from the use of, or reliance upon, the information contained in this article.