Fire alarm · BS 5839-1
Fire alarm categories: L1 to L5, M, P1 and P2 explained
What each fire alarm category means, where detection goes for each one, and what changed for bedrooms and stairway lobbies in BS 5839-1:2025.
What a category is
A fire alarm category says what a system is for and where it detects fire. It is chosen at design stage and recorded on the certificates. The letter tells you the purpose: L is for protecting life, M is manual only, and P is for protecting property.
The categories
| Category | What it means |
|---|---|
| L1 | Detection throughout the building |
| L2 | Detection in defined areas, plus the L3 areas |
| L3 | Detection on escape routes and in the rooms off them |
| L4 | Detection on escape routes only |
| L5 | Designed to meet a specific fire risk objective |
| M | Manual call points only |
| P1 | Detection throughout, to protect property |
| P2 | Detection in defined areas, to protect property |
Add /M to P1, P2 or L5 to show that manual call points are included.
Life or property?
Systems in the L categories are there to protect the people in a building. Systems in the P categories are there to protect the building itself and what is in it. That difference also shows up in how quickly an alarm has to reach an alarm receiving centre: within 90 seconds for L systems and 120 seconds for P systems. A complete failure of that link must show at the receiving centre and on the panel within 3 minutes for L, or 31 minutes for P.
What changed in 2025
- Sleeping rooms: category L2 now includes rooms where people sleep. The standard recommends smoke, multi-sensor or CO detectors there, and heat detectors only if a fire risk assessment justifies them. The same applies to sleeping rooms in L3. It is not retrospective.
- Stairway lobbies: the 2025 edition adds automatic detection in stairway lobbies.
- Variations: every variation from the standard must be recorded, and some, such as leaving out a zone plan, are no longer acceptable.
Where it appears
The category goes on the certificates for the system, so that the person servicing it years later knows what the design was meant to achieve.
Common questions
What is a category L1 fire alarm system?
A system with automatic detection throughout the building, designed to protect life.
What is the difference between L1, L2 and L3?
L1 has detection throughout. L2 has detection in defined areas plus the L3 areas. L3 covers escape routes and the rooms off them. L4 covers escape routes only.
What does P1 mean on a fire alarm system?
Detection throughout the building, to protect property. P2 is detection in defined areas, also to protect property.
What does /M mean after a fire alarm category?
It shows that manual call points are included, as in P1/M.
This guide is general information for UK fire & security companies, not legal or engineering advice. Always work to the current edition of the relevant standard and your own competent-person judgement.
See this on a real certificateKerova builds the register, readings and verdict into the certificate itself.
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