HMO Smoke Alarm Wiring Diagram — BS 5839-6 Explained

BS 5839-6 and BS 7671 between them require that the mains supply to a Grade D alarm system is taken from a permanently-energised circuit that is not switched, plugged or otherwise interruptible by occupants. In a domestic installation, that means the lighting circuit at the consumer unit.

The lighting circuit is fed via a 6A Type B MCB (or 6A RCBO where the consumer unit has been upgraded to a high-integrity board). The alarm system is wired in parallel with the lighting load — the alarms remain energised even when the lights are switched off.

Sockets circuits are not permitted as a supply source. The reasoning is straightforward: an occupant could disconnect the alarm by removing a plug or tripping a sockets RCD. The lighting circuit is permanently live and is the regulated supply point under BS 7671 Section 559 and the BS 5839-6 supporting guidance.

Where the consumer unit serves multiple lighting circuits — common in larger HMOs — the alarms are typically grouped on the upstairs lighting circuit because that supplies the bedrooms and landing where most detectors sit. The supply circuit must be labelled on the consumer unit with the words 'Fire Alarm — Do Not Switch Off'.

The alarm system is wired as a radial circuit from the supply point to each detector in turn. The supply enters the first detector, the loop continues to the second, then the third, and so on. There is no ring final circuit configuration for alarm systems — radial is the only permitted topology under BS 5839-6.

At each detector position, the cable enters the back-box and connects to the detector terminals: live, neutral, earth, and interconnect. The detector is then plugged into the back-box via its bayonet or screw-fixed mechanism. The interconnect terminal is the orange-coded conductor that carries the signal from one alarm to the next.

Where the radial run is long — over 50m of cable between the first and last detector — voltage drop and signal integrity need checking. We use 1.5mm 3-core-plus-earth for any HMO install over 30m total cable length to give margin on voltage drop and reduce the risk of nuisance signals.

The cable specification for a hard-wired interlinked Grade D alarm system is 1.0mm or 1.5mm 3-core-plus-earth — brown live, blue neutral, grey or orange interconnect signal, and bare/sleeved earth. The grey conductor carries the low-voltage interlink signal between detectors.

Each detector has four terminals: L, N, IC (interconnect), and earth. The IC terminal on detector 1 connects to the IC terminal on detector 2, which connects to the IC terminal on detector 3, and so on. When detector 2 triggers, it pulses the IC line and every other detector on the IC bus sounds.

The earth conductor is essential even though many older Grade D detectors do not require an earth connection internally. BS 7671 Section 411 requires the earth to be present and continuous for fault protection. We always terminate the earth at every detector position regardless of whether the device internally connects to it.

Common error on legacy installs: someone has wired a 2-core cable (live and neutral only) between alarms and the interlink is missing. The alarms then operate as independent point alarms. This is one of the most frequent failures we find on HMO compliance audits.

Grade D under BS 5839-6 requires that each detector has its own integral battery backup capable of operating the alarm for a minimum of 72 hours of standby plus an additional 4 minutes of alarm operation at the end of the 72-hour period. This standby gives the system resilience against extended mains failure during a fire incident.

Modern Grade D detectors use sealed 10-year lithium battery packs that cannot be replaced by the occupant. The pack is monitored continuously and a low-battery warning chirps at the end of life, prompting full detector replacement. The 10-year sealed approach removes the historical problem of tenants removing 9V backup batteries to silence chirps.

Older detectors with user-replaceable PP3 batteries are still in service in some London HMOs. Where these are encountered on a compliance audit, the standard recommendation is to schedule replacement with sealed-battery equivalents at the next refurbishment cycle. They are not strictly non-compliant if functional, but they are obsolete and we treat them as end-of-life.

Hard-wiring an alarm system into a finished HMO is invasive — cables need to be chased through ceilings and walls, plaster damaged, tenants displaced. For occupied properties, the radio-link alternative is the standard solution and is fully compliant with BS 5839-6 when the manufacturer's design is followed.

Each detector receives a dedicated radio module that plugs into its base. The module pairs with every other detector on the same network during commissioning. When one detector triggers, the module broadcasts a digital signal on the dedicated alarm RF band and every other detector on the network sounds within milliseconds.

Mains feed is still required — a single tap into the nearest lighting circuit per detector via a fused spur is the typical install pattern. The interconnect cable is not needed because the RF link replaces it. Per-detector cost is similar to wired (£180 per point) but install time drops from a full day to two hours for a typical 3-bed HMO.

Range is generally sufficient for a 4-storey London terrace from ground to loft. Where the building is unusually large or has reinforced concrete floors, an RF range extender can be added. We verify range and signal strength during commissioning and document the test in the BS 5839-6 commissioning certificate.

BS 5839-6 requires a commissioning certificate at handover documenting the design category (Grade D LD2 in a typical HMO), the detector model, the cable specification, the supply circuit, the radio-link confirmation if applicable, and a test of every alarm via the test button and via smoke or heat application as appropriate.

The certificate is signed by the installing competent person and registered with the alarm system manufacturer where the brand offers cloud monitoring. The certificate is held by the landlord or managing agent and produced on request at council inspection or fire and rescue service audit.

We issue our commissioning certificates digitally via our compliance portal — every certificate is timestamped, signed and attached to the property file. The landlord can download a PDF or share a link with the council, agent or letting platform on demand.

Recertification cadence under BS 5839-6 is annual functional test plus full periodic inspection every five years. The annual test confirms each alarm responds to smoke or heat aerosol and that the interlink network is intact. The five-year inspection reviews cable condition, detector age and any changes to the building layout that affect coverage.

Single-cable feed without interconnect — the most common legacy failure. Someone wired the alarms with standard 2-core-plus-earth cable carrying only live and neutral. The detectors operate as independent point alarms with no interlink. This is a fundamental design failure that requires either rewiring with 3-core cable or retrofitting radio-link modules to every detector.

Sockets-circuit feed — feeding the alarms from a sockets RCD trips the whole system whenever the RCD trips. We have seen this on retrofits where the installer took convenience power from the nearest 13A spur rather than tracing the lighting circuit. The remedy is to relocate the feed to the lighting circuit or to install a dedicated supply.

Mismatched detector ages — detectors installed at different dates within the same network reach end-of-life at different times, producing a rolling sequence of chirp warnings and replacement visits. Best practice is to install or replace the whole network in a single visit so all detectors share a common end-of-life date.

Missing certificate — even where the install is physically compliant, the lack of a BS 5839-6 commissioning certificate makes the system unverifiable at council inspection. We retrospectively certify existing compliant installs for £80 per property where the original installer cannot be traced and the system passes our functional audit.

A three-storey HMO — common in London where Victorian terraces have been converted with loft extensions — triggers additional fire-safety provisions under BS 5839-6 and the LACORS guidance that most London councils still apply. The escape route from a third floor must be protected, the alarm coverage must include the loft level, and the detector at the top of the stairs must be sited to give earliest possible warning of fire developing below.

Loft-conversion alarms are typically the most vulnerable to nuisance triggering from condensation, dust during construction and heat build-up in summer. We use multi-sensor detectors (combined optical and heat) in loft positions to reduce false alarms while maintaining LD2 compliance. Aico's Ei3030 multi-sensor is our standard specification for converted lofts.

Where the loft conversion has a velux escape window as a secondary means of escape, the alarm system design must consider whether the loft occupant will hear the ground-floor or first-floor alarms when sleeping. Our normal solution is to install an additional bedhead alarm in the loft bedroom with a louder sounder than the standard Grade D unit.