21.1 Commentary

21.1.1 General

Detection Characteristics:

Fire detectors sense one or more of the following:
Heat
Smoke
Combustion gases (e.g., carbon monoxide)
Infrared or ultraviolet radiation

Detection Methods:

Point Detectors: Detect characteristics at specific points within a protected area.
Line Detectors:
Integrating Line Detectors: Measure characteristics along a line and integrate the effects.
Non-Integrating Line Detectors: Function like multiple point detectors arranged along a line.

Multi-Sensor Detectors:

Combine multiple sensors (e.g., heat and smoke) to reduce false alarms while providing effective fire detection.
When applying the recommendations in 21.2, refer also to Annex E for guidance on the selection of detection type and any settings of mode or sensitivity. Clause 40.2b)1) recommends that a designer record the selection of the detector type and configuration using Table E.1.

21.1.2 Heat detectors

Types:

Point Heat Detectors: Detect heat at specific points.
Line Heat Detectors: Detect heat along a defined line.

Operation:

Can trigger an alarm based on reaching a fixed temperature or an abnormal rate of temperature rise.
Heat detectors conforming to BS EN 54-5 respond to either a fixed temperature or an abnormal rate of rise.
Detectors that only respond to a rate of rise and not a fixed temperature do not meet BS EN 54-5 standards and may miss slowly developing fires.

21.1.3 Smoke detectors

Point smoke detectors operate based on two principles:

Ionization chamber smoke detectors: These detect smoke by measuring the reduction in current flow between electrodes in an ionization chamber caused by smoke.
Optical smoke detectors: These detect smoke by measuring the light scatter caused by the presence of smoke within the detector.

Optical beam smoke detectors function as line type detectors, using a light source (typically infrared) and a receiver. These can be separated or housed together with reflectors. They detect smoke by sensing the obscuration of light due to smoke. Some can also detect heat through the "shimmer effect" caused by turbulence between hot and cold air.

Aspirating smoke detection systems draw air samples through pipework to a central detector using a pump or fan. The central detector can use an ionization chamber or optical principles to detect smoke. The sampling points draw minimal air, not affecting smoke flow in the protected area, and each point is considered equivalent to a point type smoke detector if it has similar sensitivity.

21.1.4 Combustion gas detectors

Type: Point-type detectors.

Function: Detects gases produced by fires, such as carbon monoxide, which results from incomplete combustion.

Spread: Can detect gases at a distance from the fire and on different floors.

Considerations: Must ensure accurate information for responders to avoid misleading signals.

Maintenance: Electrochemical sensors have a limited lifespan and need replacement; users should be aware of their expected lifetime.

21.1.5 Flame detectors

Function: Detects infrared and/or ultraviolet radiation emitted by flames.

Infrared Detectors: Respond to flame characteristics like flicker, size, or specific radiation frequencies. Outdoor models may filter out solar radiation frequencies.

Ultraviolet Detectors: Operate within wavelengths filtered by the ozone layer, avoiding response to sunlight, making them suitable for outdoor use.

21.1.6 Multi-sensor fire detectors

Function: Combine multiple sensors to detect different fire characteristics (e.g., heat, smoke) and reduce false alarms.
Performance: Enhances fire detection and reduces false alarms by integrating signals from different sensors.
Configuration: Some systems allow disabling of individual sensors to further reduce false alarms. When sensors are disabled, detectors should be spaced according to the least sensitive sensor's range.

21.1.7 Video fire detectors

Function: Use video cameras to monitor and analyze images for smoke and flames.

Detection: Requires continuous illumination, which can be normal or infrared lighting.

Verification: Can verify the presence of fire, but must be designed and assessed by specialists if used as the sole detection method.

Standards: No specific product standards; rely on manufacturer recommendations and are often used as supplemental detection in special applications.

21.1.8 Choice of fire detection principle

Selecting the appropriate fire detector depends on:

Speed of response
Minimizing false alarms
Nature of the fire hazard
Other considerations include cost, environmental suitability, and maintenance requirements. Sometimes a combination of detectors is ideal to optimize detection speed or reduce false alarms.

Detector Types and Their Characteristics:

Heat Detectors

Sensitivity: Less sensitive to most fires.
Best Use: Clean-burning fires like flammable liquids.
Limitations: Ineffective for smouldering fires; flames must reach about one-third of the distance to the ceiling.
Advantages: Greater resistance to environmental conditions and minimal maintenance.

Ionization Chamber Smoke Detectors

Sensitivity: Sensitive to small particles from rapidly burning fires.
Best Use: Rapidly burning flaming fires.
Limitations: Less effective for dense smoke with larger particles.

Optical Smoke Detectors

Sensitivity: Sensitive to dense smoke.
Best Use: Escape routes and areas with visible smoke hazards.
Limitations: Less effective for clean-burning fires with little visible smoke.

Optical Beam Smoke Detectors

Best Use: Large open spaces with high ceilings.
Limitations: Require stable mounting to prevent misalignment; not effective for clean-burning fires without visible smoke.

Aspirating Smoke Detection Systems

Sensitivity: Higher sensitivity than point or beam detectors.
Best Use: Critical electronic equipment rooms, aesthetic considerations, high ceilings, extreme temperatures, or difficult maintenance access.
Classes: Class A (high sensitivity), Class B (enhanced sensitivity), Class C (normal sensitivity).

Multi-Sensor Fire Detectors

Functionality: Combine sensors for smoke, carbon monoxide, and heat.
Advantages: Versatile, can reduce false alarms, and adapt to varying risks.

Carbon Monoxide Detectors

Sensitivity: Most sensitive to smouldering fires and fires with controlled burning rates.
Advantages: Less prone to false alarms from environmental factors like dust or steam.

Flame Detectors (Infrared and Ultraviolet)

Best Use: Specialized applications like plants handling flammable liquids or gases.
Limitations: Not suitable for general use due to inability to detect smouldering fires.

Considerations for Selection:

Speed of detection: Based on fire risk assessment.
Combustible materials: Type, quantity, and combustion properties.
Environmental factors: Humidity, temperature, pollutants.
Evacuation strategy: Need for rapid response.
Height and layout: Geometry of the protected area.
Other fire protection measures: Existing active and passive systems.
Response time: Of fire services and other responders.
False alarms: Minimizing disruptions and ensuring reliability.

Specific Applications:

Transformer Chambers: Heat detection for oil-filled transformers; smoke detection for cable fires.
Bedrooms in Residential Buildings: Heat detectors for general warning; smoke or multi-sensor detectors for earlier warning, especially for disabled occupants.

Conclusion:

Early and tailored consultation between stakeholders and a thorough fire risk assessment are crucial in designing an effective fire detection system that balances rapid response and false alarm prevention.

21.2 Recommendations

The following recommendations are applicable.

General Considerations:

Select detectors to protect occupants and property while minimizing false alarms.
Consider maintenance and user requirements, including aesthetics.

Heat Detectors:

Suitable for most areas, except:
Areas with small fires causing significant damage (Category P).
Escape routes (Category L).
Areas where smoke is a significant threat.
High potential for false alarms.

NOTE 1 Products of combustion might be corrosive and cause considerable damage without burning the contents of the affected area.

Smoke Detectors:

Suitable for most areas, except:
Areas with flammable liquids/gases producing little smoke.
High potential for false alarms unless necessary and no alternative is faster.

NOTE 2 It is generally accepted that occupants in a room of fire origin are likely to be aware of a fire before it is detected by any form of fire detector, unless they are asleep. Even if occupants are asleep, the risk to occupants of typically sized bedrooms might not be such as to warrant smoke detection within the bedrooms, except in the case of dormitory accommodation or rooms intended for mobility-impaired disabled people who require additional time to escape from a fire in their bedroom.

Corridors and Stairways:

Optical smoke detectors preferred unless prone to false alarms.

NOTE 3 If fire is likely to involve carbonaceous materials, such as wood, paper, textiles, etc., in the early stages, smoke detection might be suitable.

NOTE 4 This does not imply that a high rate of false alarms is ever acceptable; methods, such as filtering, might be necessary to avoid an unacceptable rate of evacuation signals and calls to the fire and rescue service (see Section 3).

NOTE 5 Use of an optical smoke detector instead of an ionization chamber smoke detector (or vice versa) can change the potential for false alarms considerably (see Section 3).

NOTE 6 Multi-sensors containing a smoke detector might be suitable in circumstances referred to in 21.2c) and 21.2d).

Early Smouldering Fire Detection:

Use optical smoke detectors, beam detectors, aspirating systems, CO fire detectors, or suitable multi-sensor detectors.

Early Flaming Fire Detection:

Use ionization smoke detectors, suitable multi-sensor systems, flame detectors, or sensitive heat detectors.

Carbon Monoxide Fire Detectors:

Suitable for:
Areas where heat detectors are acceptable, excluding flammable liquid hazards.
Rooms opening onto escape routes (Category L3).
Escape routes in Category L3 or L4 with smoke detectors.
Areas with fire hazards where CO detectors are proven effective.

NOTE 7 Use of carbon monoxide fire detectors in bedrooms provides a higher standard of protection of sleeping occupants than heat detectors, but the potential for false alarms will be less than in the case of smoke detectors.

Flame Detectors:

Use for flaming fires, not smouldering fires.
Require clear line-of-sight to the protected area.

Ultraviolet Flame Detectors:

Not to be used as the sole detection method in areas with significant smoke before flaming.

Video Fire Detectors:

Follow manufacturer recommendations and seek specialist advice.
Ensure capability in absence of normal lighting and mains power.

NOTE 8 It is important that video fire detectors are capable of detecting flame and/or smoke reliably in the absence of the normal lighting in the building and the absence of a mains power supply to any lighting provided specifically to aid the detection of flame and/or smoke.

Multi-Mode Detection Systems:

Document response characteristics in the operating and maintenance manual.

21 Types of fire detector and their selection