Fire alarm system categories explained (BS 5839-1: M, L and P)

Direct answer

BS 5839-1 classifies non-domestic fire detection and alarm systems into categories that describe what the system is mainly intended to do: M systems are manual (call points only, no automatic detection), L systems are for life protection (with automatic detection, in sub-categories L1 to L5), and P systems are for property protection (sub-categories P1 and P2). This article is an orientation only. The precise definition of each sub-category should be taken from the current edition of BS 5839-1. Which category your premises needs is specified by the party buying or specifying the system (BS 5839-1 presents the categories as a menu for purchasers, users, specifiers, enforcing authorities, insurers and system designers). That choice should be informed by the fire risk assessment and by competent design advice, for example from an organisation registered to the BAFE SP203-1 scheme, which is assessed by a UKAS-accredited certification body.

Applies to

• Products / Models: Not applicable (system design concept, not a product feature)
• Firmware / Version: Not applicable
• Region: UK
• Standards: BS 5839-1 (system categories)

Why systems are categorised

• The category records the purpose and extent of the system so that everyone — designer, installer, responsible person and inspector — shares the same expectation of what it covers.
• BS 5839-1 does not itself say which category a given building needs: it presents the categories as a ‘menu’ from which purchasers, users, specifiers, enforcing authorities, insurers or system designers can select a suitable system. Its informative Annex A gives information on the categories typically installed in various types of premises, though this is background only and not a recommendation. That selection should be informed by the fire risk assessment and competent design advice rather than made arbitrarily.

The three families (general intent only)

• M — manual systems. Provide a means of raising the alarm manually (for example manual call points) without automatic fire detection.
• L — life protection systems. Include automatic fire detection with the primary aim of protecting life. BS 5839-1 defines sub-categories L1, L2, L3, L4 and L5. L1, L2 and L4 are defined mainly by the extent of automatic detection (L1 covers all areas; L4 covers circulation areas of escape routes). L3 is defined by an objective: it should give warning early enough for all occupants, other than possibly those in the room of fire origin, to escape safely. L5 is defined by a specific fire safety objective other than that of L1 to L4, so an L5 system can range from a single detector in one room to comprehensive detection across large parts of a building. The exact definitions should be taken from the current BS 5839-1.
• P — property protection systems. Include automatic fire detection with the primary aim of protecting property, with sub-categories P1 and P2 describing the extent of coverage.

Combined categories

• Category L1 to L4 systems should also satisfy the recommendations for a Category M system, so these systems normally include manual call points as well as automatic detection. A Category L5 system does not automatically do so; where it is combined with a Category M system it is described as L5/M.
• Where a system meets the objectives of more than one category it is described in combined form, for example P1/M, P2/M, L5/M, L2/P2 or L3/P2. You will often see this form on a design certificate or specification.

The sub-categories L1–L5 and P1–P2 are listed above to show that they exist, not to define them. Their exact meanings and the areas each requires to be covered should be taken from the current BS 5839-1, and the choice for your premises should be made on competent advice rather than from this article. Do not treat this article as the authoritative definition.

Who selects the category

• The category is selected by the party specifying the system — BS 5839-1 lists purchasers, users, specifiers, enforcing authorities, insurers and system designers — informed by the fire risk assessment, the building use, and any requirement of the relevant enforcing authority.
• The chosen category should always be included in the system specification, and BS 5839-1 recommends that the design certificate states the category that has been designed — with a brief description of the protected areas for any category other than M, L1 or P1 (see What certificates should a fire alarm system have?). The commissioning engineer works to that specification, and it is reflected at handover. See Fire alarm commissioning and handover: what to expect.
• Changing the building use or layout can change the category required; review it with a competent person rather than assuming the original category still applies.

When should you contact technical support?

• If you do not know what category your system was designed to, check the system documentation or ask a competent person.
• If you need to confirm the correct category for your premises: this needs competent design advice and the current BS 5839-1, not a general article.

Safety note

This article is general guidance for Hochiki Europe products. Design, installation, commissioning, maintenance and modification of fire detection and alarm systems should be carried out by, or under the responsibility of, a competent person in accordance with the applicable standards (for example BS 5839-1 in the UK). Statutory requirements imposed by the relevant enforcing authority also apply. Always refer to the current product manual and data sheet for the exact model before carrying out any work.

References

• BS 5839-1 (current edition) — system categories (M, L1–L5, P1–P2) and their definitions
• The system's design documentation (records the designed category)

Last reviewed: 2026-10-21 — Reviewed by: Hochiki Europe Technical Support