Introduction
After 45 years and 11 revisions, BS 5839-1 remains the cornerstone of fire detection and alarm system design in the UK. The 2025 edition represents a comprehensive review of this mature standard, introducing important changes that will affect how we design, install, commission and maintain fire detection & fire alarm systems. As technical consultants, we've been working through the implications of these updates, and I wanted to share the key changes that will impact your projects.
A New Structure and Approach
The most immediately noticeable change is the complete renumbering of the standard. BSI has introduced a new numbering format, replacing the previous lettered subclauses with a numerical system. While the core recommendations remain largely unchanged, you'll need to familiarise yourself with the new clause references.
The standard now includes an expanded introduction that clarifies an important principle: fire alarm system design should support the required fire evacuation procedures, not the other way around. This reinforces that the need for fire detection systems should be determined by fire risk assessments and relevant legislation, not prescribed by the standard itself.
Enhanced Protection for Sleeping Accommodation
One of the most significant changes affects Category L2 systems. Rooms where people sleep are now explicitly defined as high-risk areas requiring automatic detection. heat detectors are no longer permitted in sleeping accommodation under L2 systems. The same restriction now applies to Category L3 systems, despite their more limited objective.
It's important to note that this change is not retrospective. Existing systems don't require immediate upgrading unless you're carrying out new works, replacement or system upgrades. However, for any new installations or modifications, you'll need to specify smoke detection in these areas.
Stairway lobbies have also been reclassified, now requiring automatic detection where previously they could be excluded as low-risk areas.
Stricter Rules on Variations
While BS 5839-1 remains a code of practice that allows for variations, the 2025 edition introduces non-negotiable restrictions. Two specific departures are now deemed unacceptable:
- The absence of a zone plan in multi-zone premises (particularly where people sleep)
- The absence of alarm transmission to an Alarm Receiving Centre (ARC) in supported housing or residential care homes where this facility is considered necessary
All agreed variations must now be recorded in the system logbook, removing the previous ambiguity around what constituted a "major" variation.
Preparing for the All-IP Future
With the public switched telephone network scheduled for shutdown in 2027, the standard now provides clear requirements for alarm transmission equipment. Maximum transmission timings have been specified:
- Category L systems: fire alarm signals must reach the ARC within 90 seconds
- Category P systems: within 120 seconds
- Catastrophic transmission failures must be indicated within 3 minutes (L systems) or 31 minutes (P systems)
A new recommendation requires a warning label on or adjacent to the control and indicating equipment, reminding premises managers that the system connects to the fire and rescue service via an ARC. This aims to reduce false alarms caused by testing without prior notification.
Multi-Sensor Detectors and False Alarm Management
The growing use of multi-sensor detectors has prompted greater emphasis on Annex D (previously Annex E), which covers detector selection and application. Where detectors can operate in different modes, designers must now record their chosen configuration, making this information available to commissioning technicians and including it in the operating and maintenance manual.
The false alarm investigation process has been streamlined, with trigger points aligned to the calculation recommended at each service visit:
- Preliminary investigation: when false alarms exceed 4 per 100 detectors per annum
- In-depth investigation: for systems with over 40 detectors when false alarms exceed 5 per 100 detectors per annum
The standard now recommends investigating all false alarms, with the commissioning organisation required to provide guidance on how this should be carried out.
Practical Installation Changes
Several practical aspects of installation have been clarified:
Cable colour: All fire alarm cables and low-voltage mains supply should be a single, common colour, with red being preferred. The functional earth conductor should now be identified in pink or marked "FE" (replacing the previous cream colour).
Call point positioning: The mounting height tolerance has been clarified to 200mm above or 300mm below the standard 1.4m height. Maximum distances remain 30m straight line or 45m actual travel distance.
Obstacles: The definition of "closely spaced beams" has been clarified as approximately 1m or less, measured centre to centre. New guidance addresses the increasingly common issue of ceiling-mounted obstructions like ductwork.
Ceiling voids: A new figure clarifies detector positioning and mounting depth in voids.
Maintenance and Servicing
The servicing interval has been aligned with established custom and practice. Systems should be serviced within one month either side of the six-month interval, with the acceptance date serving as the datum point.
At each service visit, remote indicators must be checked to ensure they haven't been obstructed or painted over. For systems with ARC connections, alarm signal transmission must be verified at every visit.
The standard now acknowledges remote servicing and cyber security, introducing recommendations for preventing unauthorised access and requiring risk assessments before performing remote service functions.
Documentation Requirements
A new requirement mandates the provision of a cause-and-effect matrix or text description with system documentation. This could be as simple as "this system operates as a simultaneous evacuation" or a detailed matrix for complex strategies.
The model logbook format has been updated to reflect all these changes, and commissioning organisations must now advise users on the importance of keeping documentation current and accessible.
Looking Ahead
These changes reflect the evolution of fire safety technology and best practices, particularly the shift toward IP-based communications and the increasing sophistication of detection devices. While some changes are immediately applicable, others represent a gradual shift in approach that will become standard practice over time.
For those working on new projects, familiarising yourself with the new structure and requirements is essential. For existing systems, understanding which changes apply retrospectively and which only affect new works will help you advise clients appropriately.
The full standard provides extensive detail on all these points and more. As always, thorough design documentation and clear communication with all stakeholders remain the foundation of compliant, effective fire alarm systems.
