BS 5839 Fire Alarm Categories Explained: Choosing the Right System for Your Building

Category M (Manual systems) – Alarms activated by people (no automatic detectors). 

Category L (Life Protection Systems) – Automatic fire detection to protect lives (subdivided into L1 to L5 levels). 

Category P (Property Protection Systems) – Automatic fire detection to protect property (subdivided into P1 and P2). 

Usage: Category M might be acceptable in small workplaces or warehouses where fires would be quickly noticed by occupants who are awake and alert. Every exit route will have call points (within 45m travel distance, per BS 5839) so that occupants can raise the alarm quickly. 

Limitations: No automatic detection means if a fire breaks out unseen (e.g. in a closed room), the alarm won’t sound until someone discovers it. Therefore, M systems are not suitable for sleeping areas or places where a fire could smell unnoticed. They provide no early warning unless a person is present and reacts. 

P1 – Whole Building Coverage: A Category P1 system has detectors throughout all areas of the building. The goal is to detect a fire anywhere on the premises as early as possible, ideally before it grows. This offers maximum property protection and may reduce insurance premiums or damage costs by enabling a swift response. 

P2 – Partial Coverage: A Category P2 system has detectors only in defined high-risk or high-value areas. For example, you might install AFD in areas like server rooms, plant rooms, storage of critical inventory, or archives – places where a fire would cause major loss or where fires are likely to start. The idea is to cover the important zones rather than the entire building. 

L1 – Maximum Life Protection: Detectors are installed throughout all areas of the building, covering all rooms, corridors, void spaces, and storage areas. Every part of the building is monitored so a fire anywhere will be detected at the earliest opportunity. In practice, L1 means even small cupboards and roof spaces have detection if accessible. This category is often used in high-risk properties like large hotels, hospitals, or care homes where any delay in detection is dangerous. The 2025 standard also specifies that rooms where people sleep must have smoke or equivalent detectors (not just heat detectors) in an L1 system. 

L2 – Additional Life Protection: Detectors are installed as in L3 (see below) plus extra detection in specified areas that pose higher risk or have sleeping occupants. In plain terms, an L2 system covers all escape routes and adjoining rooms (like L3) and adds detectors to other high-risk areas or any rooms where people sleep. For example, an office building with an L2 system might include smoke detectors in an on-site server room or kitchen (high fire risk), and if there’s a sleeping area, it must be protected. New in BS 5839-1:2025: L2 now explicitly requires coverage in rooms where people sleep, treating them as high-risk areas. This means you can no longer rely on just heat detectors in bedrooms; smoke detection is required in any sleeping quarters for an L2 system. 

L3 – Standard Life Protection: Detectors are installed on all escape routes (like corridors, stairways) and in all rooms that open onto those escape routes. The idea is to ensure anyone on an escape route is alerted to fire in time, and if a fire starts in a room off a corridor, the detector in that room will warn others before smoking pours into the corridor. L3 is a common requirement for many non-residential buildings where the main risk is ensuring safe exit (e.g. offices, schools during the day). However, L3 does not cover rooms with no direct exit to a corridor (if they don't open onto an escape route, they might not have detectors in an L3 system). 

L4 – Modest Life Protection: Detectors are installed only along escape routes (corridors, stairways, circulation areas). L4 gives a basic level of protection by ensuring the alarm will sound when smoke is affecting the escape routes, even if the fire started elsewhere. Update in 2025: An L4 system must now also include detectors at the top of any lift shafts or other vertical shafts that could funnel smoke. This is to catch fires that might otherwise spread unseen via these “flue-like” structures. L4 systems are less comprehensive; they might be found in lower-risk workplaces or small buildings where fire is likely to be noticed quickly. Keep in mind, L4 will not detect a fire in a room off the corridor until smoke enters the corridor. 

L5 – Custom Life Protection: This is a customised detection system designed to fulfill a specific fire safety objective beyond the scope of L1-L4. In other words, L5 is a “designed to suit” category. It might cover only particular areas or hazards that a risk assessment has identified as crucial. For example, if a unique risk (maybe a piece of machinery or an atrium) requires special detection, an L5 system can be implemented targeting that. L5 systems are less prescriptive – the protected areas and detector types are determined by the special risk need, as documented in the fire risk assessment. 

Buildings with Sleeping Occupants (Hotels, Care Homes, Student Housing, Apartments): Because people sleep and may not notice fires, these buildings demand a high level of life protection. Typically, an L1 system (full coverage) or L2 system is recommended so that every bedroom and escape route has detection. For example, in a hotel, detectors in all bedrooms, hallways, plus any plant rooms or kitchens (L2 covers those extra risks) will ensure guests are alerted in time. Never use a purely manual or L4 system where people sleep – that would violate best practice and likely fail compliance due to inadequate warning in sleeping areas. 

Offices, Retail Stores, Schools (No Sleeping Risk On-site): These occupied buildings still need life protection, but you might opt for an L3 system (covering escape routes and adjacent rooms) or L4 (escape routes only) depending on size and layout. An L3 system is often a good standard for most workplaces – it will warn all occupants if any office or storeroom connected to a corridor catches fire, giving people time to evacuate. L4 provides a basic warning on the exits but might not detect a fire in a closed room until the corridor is affected, so L3 is safer for most scenarios. Always consider if there are any high-risk rooms (server room, boiler, archives); if so, L2 might be warranted to cover those additionally. 

Industrial Sites and Warehouses: If a building is not continuously occupied (e.g., a warehouse at night) but holds valuable goods or equipment, a P1 or P2 system could be appropriate for property protection. For instance, an automated warehouse might have a P1 system, so any fire triggers an alarm and automatic call to the fire service even if no one is there. However, if workers are present in the day, you should also have L-category protection for life safety during working hours. An example approach: L4 or L3 for life safety when occupied, combined with P1 for property protection 24/7 – many fire alarm control panels can be configured to meet both life (L) and property (P) criteria simultaneously. 

Small Low-Risk Premises: In a very small shop or single-story business with good visibility, a Category M system (manual call points only) might be deemed acceptable if a risk assessment shows a fire would quickly be noticed by staff. However, adding even a few automatic smoke detectors (upgrading to an L4 or L3 system) greatly increases safety. Given how affordable detectors are relative to potential loss of life or property, most premises err on the side of caution and install at least some automatic detection. Remember: relying on manual detection means no one is alerted until a person discovers the fire, which can be too late – so M should be chosen only in very limited cases. 

Sleeping Areas in L2 and L3: In the new standard, any L2 system must cover rooms where people sleep (this was not explicitly required before). Likewise, heat detectors are no longer allowed in rooms used for sleeping under L2 or L3 – you need to smoke or multi-sensor detectors there. This change was driven by safety concerns, as smoke detectors give earlier warning to sleeping occupants than heat detectors. 

Detection in Lift Shafts (L4): Category L4 now explicitly requires detectors at the top of lift shafts or similar vertical shafts. Previously, these might have been considered low-risk areas, but now they are recognised as critical points to detect fire/smoke spread early. 

Stairwell & Lobby Detection: Areas like stairway lobbies that were once considered “low risk” now must have detection if they are part of escape routes. The 2025 update tightened what’s considered acceptable unprotected space on an escape route. 

Alarm Monitoring and Transmission: The standard now specifies maximum times for alarm signals to reach an Alarm Receiving Centre (if your system is monitored). For example, in Category L systems, fire signals should reach the ARC within 90 seconds, and any total system failure should be reported within 3 minutes. In Category P systems, alarm signals must be received within 120 seconds and total failure is reported within 31 minutes. These ensure faster responses for life safety systems. 

Other Updates: BS 5839-1:2025 includes various other improvements (beyond category definitions) – for instance, all manual call points now must have protective covers to reduce false alarms, and control panels must display a “False Alarm Notice” if they auto-dial the fire service. There’s also a greater emphasis on ongoing maintenance, documentation of any variations, and competency of installers/maintainers. While these are not all directly about categories, they round out the compliance picture.