Fire Alarm Installation: First Fix and Second Fix
A fire alarm installation happens in two main stages, dictated by the building programme: first fix, when cable and containment go in before finishes conceal them, and second fix, when the devices are fitted and connected once the space is closer to finished. The quality of the first fix largely determines how smoothly everything after it goes. This guide covers what each stage involves, why the split matters, and what to get right.
The essential idea is that first fix is hard to correct once concealed, so it must be done properly to the design the first time.
Who this is for
This is for competent fire alarm engineers installing systems on construction and refurbishment projects. The experience level assumed is competent engineer. Use it for the principles; the specific cabling, routing and device requirements come from the design, BS 5839-1, the wiring regulations and the manufacturer's instructions.
First fix
First fix is the containment and cabling stage, carried out while the building is still open. It involves running and supporting cable, installing back boxes and containment, and leaving cabling ready to receive devices, all before finishes go in and conceal much of it. Because so much of it becomes hidden, first fix has to be done carefully and correctly to the design and the cabling standards — cable type, routing, support and segregation all matter here. Getting first fix right sets up everything that follows, and it should follow BS 5839-1 and the wiring regulations.
Second fix
Second fix is fitting and connecting the devices — detectors, call points, sounders, the panel and interfaces — onto the first-fix cabling, once the building is nearer completion and the space is less likely to damage them. It is followed by commissioning, which proves the whole system works as intended. Second fix must match both the design and the as-built reality, with devices correctly sited and connected. It follows BS 5839-1 and the manufacturer's instructions, and it is where the installation becomes a working system.
Why the split matters
The two-stage split reflects the building programme rather than being an arbitrary convention. Cabling has to be installed before finishes conceal it, which is first fix; devices go on later, once the space is ready and less exposed to damage, which is second fix. From field experience, the reason the split matters so much is that first-fix errors — poor routing, insufficient cable, inadequate support — become hard and costly to correct once they are behind finishes. Coordinating both stages with the design and the wider build is central to a sound installation under BS 5839-1.
What goes wrong
Common problems cluster around the handover between stages. Cable routed or supported poorly at first fix is often only found much later; too little cable is left for the intended device positions; devices end up sited where finishes or fittings interfere; and connections are sometimes made without following the manufacturer's instructions. From field experience, most of these trace back to first fix not being done tightly to the design, or second fix not being checked against reality. Confirming each stage against the design and the standards before moving on prevents the majority.
Common points to check
Recurring issues include inadequate cable routing or support at first fix, insufficient cable left for devices, poorly-sited devices at second fix, and connections not made per the manufacturer's instructions. Confirming each stage matches the design is the essential check.
When not to rely on this alone
When not to use this article: do not treat it as an installation method statement for a specific project. That comes from the design, BS 5839-1, the wiring regulations and the manufacturer's instructions, applied by competent professionals.
Relevant standards
Installation follows BS 5839-1, a code of practice, together with the wiring regulations (BS 7671) and the manufacturer's instructions. The legal duty for fire precautions in most non-domestic premises sits under the Regulatory Reform (Fire Safety) Order 2005. Separate the legal duty from the recommended methods, and always work to current editions.
Professional disclaimer
This is an educational and workflow resource for competent engineers and does not replace the current British Standards, the wiring regulations, the manufacturer's instructions, the design, or competent judgement. Verify installation against current documentation.
Related documentation
Use this with the current BS 5839-1, BS 7671, the design and the manufacturer's instructions. Record what was installed at each stage against the design, and carry accurate information forward into commissioning and the as-built records.