Fire Alarm Detection Devices
Purpose
This document covers the types of detection devices used in fire alarm systems — smoke detectors, heat detectors, multi-sensor detectors, manual call points, and sounders/beacons. Understanding what each device does and when to use it is fundamental to compliant fire alarm design and installation.
Smoke Detectors
Optical (Photoelectric) Smoke Detectors
Detect light scatter from smoke particles inside a detection chamber. An infrared LED shines light across the chamber — when smoke particles enter, they scatter the light onto a photodiode receiver, triggering the detector.
Responds best to: Slow, smouldering fires — burning upholstery, bedding, electrical faults.
Less sensitive to: Fast-flaming fires producing little visible smoke initially.
Common use: Bedrooms, living rooms, corridors, offices.
Caution: Can be triggered by steam, dust, or cooking fumes — do not site near kitchens or bathrooms.
Ionisation Smoke Detectors
Use a small amount of radioactive material (Americium-241) to ionise air in a sensing chamber. Smoke disrupts the ionisation current and triggers the detector.
Responds best to: Fast-flaming fires — paper fires, chip pan fires.
Common use: Less common now — largely replaced by optical in most applications.
Note: Must be disposed of correctly due to radioactive material — not in general waste.
Multi-Sensor Detectors
Combine optical smoke with a heat sensor (and sometimes CO sensing). The panel cross-references readings — reducing false alarms while maintaining sensitivity.
Best for: Areas prone to false alarms — near kitchens, dusty workshops, areas with vapour or steam.
Heat Detectors
Heat detectors respond to elevated temperature, not smoke. Use in areas where smoke detectors would cause constant false alarms.
Fixed Temperature Heat Detectors
Trigger when ambient temperature reaches a fixed threshold — typically 58°C or 83°C.
Use for: Kitchens, boiler rooms, garages.
Limitation: Slower response than smoke detectors — not suitable for life protection in sleeping areas.
Rate-of-Rise Heat Detectors
Trigger when temperature rises rapidly (typically more than 10°C per minute). Some include a fixed temperature backup.
Use for: Storage areas, rooms where sudden heat development would indicate fire.
CO (Carbon Monoxide) Detectors
Detect carbon monoxide produced in early stages of some fires (slow-combustion, incomplete burning). These are fire-system CO detectors — not the same as domestic gas-appliance CO alarms.
Manual Call Points (Break-Glass)
Allow any person to manually activate the fire alarm. Placed on escape routes and at exits so anyone evacuating can trigger the alarm as they leave.
BS 5839-1 requirement: No point in a building should be more than 45m from an MCP on the same floor.
Siting: At every exit from a floor, at stairwell doors, and at the main exit. Mounted 1.2–1.4m above finished floor level.
Types: Flush-mounted (recessed) or surface-mounted. Break-glass or key-operated resettable element.
Sounders and Beacons
Sounders
Produce the audible alarm throughout the building. BS 5839-1 requires 65 dB(A) or 5 dB(A) above background noise at any occupied point (whichever is greater).
Types: Wall-mounted sounder; combined sounder/strobe; sounder base (fits under detector).
Visual Alarm Devices (Beacons/Strobes)
Required in:
- Areas where background noise exceeds 90 dB(A) — workshops, plant rooms
- WC/toilet facilities accessible to disabled persons
- Where specified by BS 8300 or BS EN 54-23
Plan sounder placement carefully — walls and closed doors can create dead spots below the required sound level.
Device Siting Rules — Key Points from BS 5839-1
Smoke detectors:
- Maximum floor area per detector: 60m² at standard ceiling height (up to 6m)
- Minimum distance from walls: 300mm
- Not within 300mm of a light fitting or HVAC outlet
- Not in dead air spaces (corners near sloping ceilings)
Heat detectors:
- Maximum floor area per detector: 30m²
- Otherwise similar spacing rules to smoke detectors
Ceiling height:
- Above 6m: specialist high-ceiling detector types or beam detectors required
- Sloped ceilings: detector should be within 600mm of the apex