Helpful video reference. We use GSH Electrical's UK wiring guide "Smoke Alarm Wiring for UK Homes – The Simple Setup Most Sparks Get Wrong" as the video reference here. GSH Electrical is a UK electrical training college with 170,000 subscribers; their short cuts straight to the interlink wiring error that causes alarm chains to fail.
1. Plan the alarm types and ceiling positions
BS 5839-6:2019 sets the minimum standard for domestic smoke alarms. For most homes, Grade D (mains-powered with integral battery backup) Category LD2 is the minimum: smoke alarms on each floor, including every circulation area (hallways and landings).
- Optical alarms detect slow, smouldering fires (upholstered furniture, bedding). Fit these in bedrooms, landings, and living rooms.
- Ionisation alarms detect fast-flaming fires with little visible smoke. Fit one in the living room or at the top of the stairs if you want dual coverage.
- Heat alarms (not smoke alarms) are correct for kitchens — a smoke alarm above a hob will cause constant false alarms from cooking fumes.
Mount alarms on the ceiling, at least 300 mm from any wall or light fitting, and at least 300 mm horizontally from any beam. In a room with a pitched ceiling, position the alarm within 600 mm of the ridge. Never fit a smoke alarm directly above a cooker or in a bathroom.
2. Choose the supply circuit and fusing arrangement
Mains smoke alarms must be supplied from a circuit that is energised at all times. A common and correct method is a spur from the upstairs lighting circuit, taken from a junction box on the permanent live and neutral — not from a switched loop. Fuse the spur at 3 A using a fused connection unit (FCU).
Do not supply the alarms from the same way as the kitchen or bathroom lights, which might be on a separate MCB that could trip while the alarms still need power. A dedicated lighting circuit MCB (usually 6 A) used only for the alarm supply is the most robust option in a new installation or when rewiring.
3. Cable selection and route planning
Use 1 mm² three-core-and-earth cable, grey sheathed (this is standard for alarm interlink wiring and distinguishes the run from ordinary two-core lighting cable). The four conductors carry:
- Brown — permanent mains live
- Blue — neutral
- Black (re-sleeved brown at both ends) — the interlink signal conductor
- Bare copper (sleeved green-and-yellow) — earth
Route the cable through the ceiling void between alarms where possible. If it must run on the surface, clip it neatly using oval conduit or surface-mounted trunking. The cable must not pass through any void where it could be disturbed or mechanically damaged without being protected by conduit.
4. Wire the supply at each alarm base
Isolate the supply circuit at the consumer unit and confirm dead with a voltage tester before working on any connections. Most mains smoke alarms use a twist-lock ceiling base that accepts the cable tails directly.
At each base, connect:
- Brown conductor → Live terminal (L)
- Blue conductor → Neutral terminal (N)
- Green-and-yellow earth → Earth terminal (E or ⏚)
Strip conductors to the length shown on the terminal block (usually 8–10 mm) and tighten each terminal firmly. Loose connections are the primary cause of alarm faults in the first year after installation.
5. Connect the interlink conductor at every alarm
The black conductor (sleeved brown) connects to the alarm's interconnect terminal, which is usually labelled INT, or marked with a chain-link icon. Connect this same conductor at the INT terminal of every alarm in the circuit.
How the interlink works: when any alarm's sensor fires, it applies a low voltage to its INT terminal. Because all alarms share the same INT conductor, they all see that voltage and all sound simultaneously. Without the interlink conductor connected, an alarm will only sound for its own detector — the person in the bedroom may not hear a fire starting downstairs.
The interlink conductor must be connected at both the incoming and outgoing cable on each intermediate alarm (i.e., both the cable arriving from the previous alarm and the cable continuing to the next alarm share the same INT terminal — use a wago connector or a base with dual interlink terminals).
6. Test the interlink before closing up
Restore power and press the test button on the first alarm. Every other alarm in the circuit must sound within two seconds. Move to the second alarm and repeat. If any alarm fails to trigger the others, the interlink conductor on that alarm is either disconnected, incorrectly terminated, or has a break in continuity — check with a multimeter set to continuity.
The battery backup test is separate: hold the test button for eight seconds to simulate a mains failure on some models. The alarm should continue to sound on battery only. Consult the manufacturer's instructions for the exact test sequence.
When to call us
Smoke alarm wiring is one of the most common small jobs Richard undertakes. The supply and interlink for a three-alarm installation typically takes under two hours, and Part P certification is included. Small local jobs in Sandwich are available at the £10 per 10-minute rate.
Need smoke alarm wiring in Sandwich?
Richard wires and certifies mains smoke alarm installations including the Part P certificate. Phone or WhatsApp with the number of alarms needed.
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