Helpful video reference. John Ward (jwflame), a qualified electrician based in Dorset, covers the full wiring arrangement for a PIR sensor with a three-position override in his video "Lighting Sensor Wiring with Override Facility". John runs Flameport Electrical and has been making UK-specific electrical education videos for over a decade. This video explains how the three switch positions (off, auto and permanently on) are wired, and how the lux and time delay adjustments work in practice.
1. Choose the right PIR switch for the job
PIR occupancy switches come in two main form factors. Wall-mounted units replace a standard single-gang switch plate; ceiling-mounted units sit in a standard conduit box or surface housing and cover a wider detection arc. Choose the type that suits the room geometry.
Check the current rating carefully. Most domestic PIR switches are rated at 6 A or 10 A resistive, but the effective load rating for LED circuits is usually lower. Many units have a minimum load requirement — often 40 W or more — that a handful of LED downlights will not meet. Look for a unit labelled "LED compatible" or with a minimum load below 5 W, and confirm the maximum load stays within the switch rating.
Before buying anything, confirm what wiring is available at the switch position. Only replace a standard switch with a PIR switch when the cable at that position carries a permanent live, a switched live and a neutral. Loop-in-at-switch wiring delivers all three. Loop-in-at-ceiling-rose wiring runs the switched live and neutral back to the rose; neutral may not reach the switch back box at all. Open the existing switch and count the conductors before you order.
2. Isolate the circuit and confirm dead
Go to the consumer unit and switch off the MCB for the lighting circuit. If the board has a hasp, lock the MCB in the off position and pocket the key. In an older rewireable fuse board, pull the lighting fuse and keep it with you.
Return to the switch position and test both existing terminals with an approved voltage tester. Confirm there is no voltage present on either. Only then is it safe to proceed.
3. Remove the existing switch and photograph
Unscrew the two faceplate screws and ease the plate forward from the back box. Do not pull hard — the conductors are short. Hold the plate steady and take a clear, well-lit photograph of the wiring before you touch a single terminal. This photograph is your reference for the rest of the job. If you have any doubt about which conductor is which, the photo resolves it.
4. Identify the three conductors
A PIR switch needs three conductors: permanent live (line), switched live (load) and neutral.
In a modern installation using 6242Y twin-and-earth with a separate neutral, you will typically see brown (line), blue (neutral) and a third conductor that may be grey or black with a brown or red sleeve identifying it as the switched live (load). In older loop-in-at-switch wiring the colours can vary: red and black are the pre-2006 standard, and the returning switched conductor often has a red or brown sleeve over the black to indicate it is live when the circuit is on.
If you cannot positively identify all three conductors, do not guess. Call an electrician.
5. Connect to the PIR switch terminals
Terminal labelling varies between manufacturers but the three you need are: L or Line (permanent live), N (neutral) and Load or COM (switched output to the luminaire). Match each conductor to the correct terminal using the manufacturer's wiring diagram and your photograph.
Strip only as much insulation as the terminal requires — the manufacturer's instructions will specify the stripping length, often 6 mm to 8 mm. Overtightening a screw on too much bare copper is one of the most common faults in switch work. Insert each conductor, tighten the screw firmly and give the conductor a gentle tug to confirm it is held. Sleeve any re-used bare earth conductors with green-and-yellow sleeving.
6. Set the sensitivity controls
Most PIR switches have two or three rotary adjustment dials on the body:
- LUX dial (lux threshold): controls the ambient light level below which the sensor will activate. Set it high in a room with good natural light so the sensor only triggers when the room is genuinely dark. In a room with no windows, set it lower or to maximum sensitivity.
- TIME dial (time delay): sets how long the light stays on after motion stops. Thirty seconds works in a hallway; five to ten minutes suits a utility room or garage; up to thirty minutes may be appropriate in a home office if the sensor has a limited detection arc.
- Sensitivity range (if fitted): reduces the detection distance if the sensor is picking up movement in an adjacent area or outside the room.
Initial settings do not need to be perfect. Set the time delay to around two minutes and the lux threshold to mid-range, then refine once the unit is installed and in use.
7. Test the three override positions
Most PIR switches include a three-position override: Off, Auto and On. Before folding the unit into the back box, restore power at the consumer unit and test each position in turn.
- Off: the light should remain off regardless of movement in front of the sensor.
- Auto: wave a hand in front of the sensor. The light should come on within a second or two. Stop moving and wait. After the set time delay the light should go off.
- On: the light should come on and remain on permanently, ignoring the sensor entirely.
If the light flickers, dims instead of switching cleanly, or the MCB trips when you restore power, switch back off at the consumer unit and recheck all connections. Flickering on LED loads almost always means the switch is not rated for LED or the load is below the switch's minimum. A tripping MCB points to a short — almost always a conductor touching something it should not inside the back box.
Once all three positions test correctly, fold the conductors gently into the back box and fix the faceplate. Do not trap any insulation under the edge of the plate.
When to call us
A PIR occupancy switch retrofit is usually a quick job once you know you have neutral available at the switch position. In older homes with loop-in ceiling rose wiring, checking whether neutral reaches the switch is the first thing to establish. If it does not, running a new cable or fitting a smart switch that works without neutral are both options Richard can advise on. Sandwich small-job call-out rate is £10 per 10 minutes.
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Richard fits PIR occupancy switches, dimmer upgrades and smart lighting across east Kent.
Contact RichardFrequently asked questions
Do I need a neutral wire to fit a PIR occupancy switch?
Most mains-voltage PIR switches need a permanent live, a switched live (load) and a neutral. Loop-in-at-switch wiring delivers all three; loop-in-at-ceiling-rose wiring often does not bring neutral down to the switch back box. Check before buying the switch.
Will a PIR switch work with LED bulbs?
Only if it is rated for LED loads. Many older or cheaper PIR switches have a minimum load — often 40 W or 60 W — that a 5 W LED bulb will not meet. Look for a switch rated "LED compatible" or with a minimum load of 0 W or under 5 W.
Is fitting a PIR switch notifiable under Part P?
Like-for-like switch replacement in a general-purpose room is not notifiable. If you are adding new wiring, altering a circuit or working in a kitchen, bathroom or outside, the work falls under Part P and should be done by a registered electrician.