Fire alarm grades explained (BS 5839)
BS 5839 describes fire detection two ways: by Grade (the type of system — for example Grade A is a full panel-based system, Grade D is mains-powered interlinked alarms with battery backup) and by Category (how much of the building is covered — LD1 is everywhere, LD2 is escape routes plus higher-risk rooms, LD3 is escape routes only). The right combination for your property comes from a fire risk assessment.
Grade — the type of system
The Grade describes the kit. At the higher end, a Grade A system is a designed installation with a control panel, separate detectors and sounders, and monitoring — typical of commercial premises and larger HMOs. A Grade D system uses mains-powered alarms with battery backup, interlinked so they all sound together — common in smaller domestic and HMO settings.
Category — how much is covered
The Category (for life protection, the 'LD' series) describes coverage. LD1 means detection throughout the property. LD2 covers escape routes plus rooms that present a higher fire risk. LD3 covers the escape routes only. A higher category means more detectors and more protection.
How the right spec is decided
You don't pick a grade and category from a catalogue — they come from a fire risk assessment, which considers the building, who uses it and how they'd escape. That assessment is the legal basis for what gets installed, and it's where any compliant design starts.
What grade does a small HMO usually need?
Smaller HMOs commonly use a mains-interlinked (Grade D) system, but it depends entirely on the property and its fire risk assessment — that's what sets the requirement, not a rule of thumb.
What's the difference between Grade and Category?
Grade is the type of system (the equipment); Category is how much of the building it covers. You need both specified — e.g. 'Grade D, Category LD2'.
Do I need a fire risk assessment first?
For most commercial premises and HMOs, yes — it's a legal document that determines the system you need. We work to whatever the assessment specifies.