Fire Alarm Systems (BS 5839)
Fire alarm system design, categories, detection types, and wiring requirements per BS 5839
Fire Alarm System Categories
BS 5839-1 defines system categories based on whether the system protects life (L), property (P), or both. Domestic dwellings fall under BS 5839-6 with LD categories.
System categories — Life protection (L) and Property protection (P)
BS 5839-1, Table 1| Category | Protection | Coverage |
|---|---|---|
| L1 | Life | Detectors throughout the entire building |
| L2 | Life | Detectors in defined areas plus escape routes |
| L3 | Life | Detectors in escape routes and rooms opening onto them |
| L4 | Life | Detectors in escape routes only |
| L5 | Life | Engineered system to satisfy a specific fire strategy |
| M | Manual | Manual call points only — no automatic detection |
| P1 | Property | Detectors throughout the entire building |
| P2 | Property | Detectors in defined high-risk areas only |
Most commercial buildings require at least Category L2 or L3. Category M alone is rarely acceptable.
Detector Types and Selection
Choosing the correct detector for each location is critical for reliable detection and minimising false alarms.
Detector types by application
BS 5839-1, Section 4| Detector Type | Best For | Avoid In |
|---|---|---|
| Optical smoke | Offices, corridors, bedrooms | Kitchens, garages, dusty areas |
| Heat (fixed temp) | Kitchens, boiler rooms, garages | Areas needing early warning |
| Heat (rate of rise) | Plant rooms, stores | Areas with rapid temperature changes |
| Multi-sensor | Hotels, HMOs, mixed-use | High-humidity areas |
| Aspirating (ASD) | Data centres, heritage buildings | Outdoor or open environments |
Reducing False Alarms
Wiring Requirements
Fire alarm circuits demand fire-resistant cables and careful segregation from other services.
All fire alarm wiring must use cables that maintain circuit integrity during a fire. Standard fire-resistant cable rated to PH30 (30 minutes) is acceptable for most installations. Enhanced PH120 cable (120 minutes) is specified where extended operation is essential, such as phased evacuation buildings or systems controlling smoke ventilation.
Cable Segregation
Sounder and Alarm Requirements
Fire alarm sounders must achieve a minimum of 65 dBA at any occupiable point, or 75 dBA at the bedhead in sleeping accommodation. Visual alarm devices (VADs) must be provided for hearing-impaired occupants. The alarm must be at least 5 dBA above sustained background noise.
Minimum sound level = Background noise + 5 dBA (or 65 dBA, whichever is greater)- 65 dBA
- = Minimum for general areas
- 75 dBA
- = Minimum at bedhead in sleeping risk premises
BS 5839-1, Section 16