Glossary
L5 fire detection system.
A category of fire detection and alarm system under BS 5839-1. L5 is the custom category: a system designed to address a specific life-protection risk that is not adequately covered by the standard L1 to L4 categories.
When L5 is used.
L5 is used where a specific risk needs targeted detection that the standard categories do not provide. Examples include detection in specific high-risk plant rooms (lift motor rooms, electrical intakes), detection in concealed spaces (voids, ducts) where a slow-developing fire could go undetected, or detection tailored to an unusual occupancy or escape strategy. The category designation does not by itself describe the coverage; the design specification does.
How to read an L5 specification.
For an FRA assessor reading a fire strategy that specifies an L5 system, the relevant question is what specifically the L5 was designed to detect. The fire strategy report and the BS 5839 design certificate together explain the rationale. Without that documentation, the assessor needs to evaluate the as-built coverage on its merits and judge whether it meets the current fire strategy.
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