The aim of detection design is not to "put detectors everywhere" but to detect a fire at the right moment and in the right place. Too much sensitivity produces false alarms; too little arrives late.
Step 1 — Define the protection objective
- Life safety. Buying the time needed to evacuate; sensitivity rises where occupants sleep.
- Property protection. Limiting damage through early intervention.
- Business continuity. Protecting critical equipment; very high sensitivity in a data centre.
- Suppression release. Detection for a gaseous system or pre-action system.
The objective directly determines detector type and threshold settings.
Step 2 — Choose the detector type
| Type | Where it suits | False alarm source |
|---|---|---|
| Optical smoke | General spaces, corridors, offices | Dust, steam, exhaust |
| Ionisation | Fast flaming fires | Air movement, chemical vapour |
| Heat (fixed / rate of rise) | Kitchens, boiler rooms, car parks | Process heat |
| Linear heat cable | Cable galleries, conveyors, tunnels | Mechanical damage |
| Beam | Large, high-ceilinged spaces | Structural movement, obstruction, contamination |
| Aspirating | Data centres, cleanrooms, archives | Filter contamination, airflow |
| Flame (UV/IR) | Flammable liquids, hangars, process areas | Welding arcs, sunlight, hot surfaces |
| Gas | Battery rooms, process spaces | Calibration drift |
Step 3 — Set out the layout
- Meet the maximum coverage area and detector spacing rules.
- Check ceiling height limits; point detectors lose effectiveness at high ceilings.
- Allow for ceiling slope and beam depth; deep beams behave like separate compartments.
- Do not place detectors near air inlets or outlets; smoke is diluted or diverted.
- Assess concealed spaces (ceiling voids, raised floor voids) separately.
- Accessibility: reachable for maintenance and testing.
Air movement is detection's biggest enemy. In spaces with a high air change rate, smoke is diluted and a point detector responds late. In those spaces, aspirating detection or sampling in the return duct is preferred.
Step 4 — Zone it
A zone tells you where to look when an alarm arrives. The rules:
- A zone should be limited to one floor and a reasonable area.
- Zone boundaries should coincide with fire compartment boundaries.
- In an addressable system each device carries its own address, but zones are still defined for operational grouping.
- Zones that release suppression are kept separate.
- The zone plan must be displayed next to the panel.
Step 5 — Position the warning devices
Detection is useless if the alarm is not heard. What to check:
- Adequate sound level in every space; measure behind closed doors and in noisy areas.
- Adequacy for waking occupants where people sleep.
- Visual alarm devices in noisy areas and where users may be hearing impaired.
- Speech intelligibility where voice alarm is required.
- Zone-by-zone control where phased evacuation is used.
Step 6 — Cabling and supply
- Fire-resisting cable and protection of the route.
- A loop architecture where a single break does not divide the system.
- Mains supply and battery standby, with battery duration calculated.
- Fire-stopping at floor penetrations.
- Routing away from sources of electromagnetic interference.
Step 7 — Write the integration scenarios
The panel does not only raise an alarm; it manages the building. The scenarios to write:
- Lift recall and holding doors open.
- Release of fire doors from magnetic hold-open devices.
- Starting smoke control fans and dampers.
- Stopping ventilation or switching it to fire mode.
- The delay, warning and discharge chain for gaseous suppression.
- Release of access control doors.
- Transmission to the alarm receiving centre.
Every scenario must be tried end to end at acceptance testing. "A signal appeared at the panel" is not enough; the device must be verified as actually responding.
Reducing false alarms
- Match the detector type to the environment (heat, not smoke, in a kitchen).
- Use cross-zoning or two-device confirmation, especially where suppression is released.
- Choose devices with contamination compensation and monitor the dirty-detector signal.
- Apply a temporary disablement procedure during refurbishment and hot work.
- Record the causes of false alarms; a point that recurs indicates a design problem.
False alarms are a safety problem, not a comfort problem. In a building with frequent false alarms, occupants stop responding to real ones. Lowering the false alarm rate is one of the most effective ways to shorten evacuation time.
Frequently Asked Questions
What decides the detector type?
The protection objective and the characteristics of the environment. Heat rather than smoke in a kitchen, aspirating detection in a data centre, beam detectors in high-ceilinged spaces.
How does air movement affect detection?
A high air change rate dilutes smoke and a point detector responds late. Those spaces use aspirating detection or sampling in the return duct.
What is the basic rule of zoning?
A zone should be limited to one floor and a reasonable area, with its boundary coinciding with a fire compartment boundary. Zones that release suppression are kept separate.
Why are false alarms a safety problem?
In a building with frequent false alarms, occupants stop responding to real ones. Lowering the false alarm rate shortens evacuation time.
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Download MEP Calc on the App StoreNFPA 25 · NFPA 72 · EN 54 series · BS EN 12845:2015+A1:2019 · FM Global Data Sheets. This guide is a general road map; intervals and acceptance criteria follow the current edition of the relevant standard.