PIR Sensors, Occupancy Detectors, and Daylight Switching — Lighting Controls for UK Electricians

Automatic lighting controls are standard specification on commercial, industrial, and residential new-build projects in 2026. Part L of the Building Regulations requires energy-efficient lighting controls in non-domestic buildings, and Part P covers the safe installation of all fixed wiring accessories. Understanding the difference between passive infrared (PIR) detectors, microwave/ultrasonic occupancy sensors, and photocell daylight switches — and knowing which to specify where — is core competency for any UK electrician.

This guide covers sensor types, wiring topologies, zone coverage, common faults, and compliance requirements under BS 7671:2018 (18th Edition) and the IEE Code of Practice for Energy Efficiency in Buildings.

How PIR Sensors Work

Passive infrared sensors detect the infrared radiation emitted by warm bodies. A Fresnel lens divides the sensor's field of view into alternating hot and cold zones; when a person moves through the zones, the differential in detected IR triggers the switching output. PIR sensors are passive — they emit no signal of their own — which makes them low-energy and long-lasting.

Key parameters:

• Detection angle: Typically 180° for wall-mounted sensors; 360° for ceiling-mounted occupancy detectors
• Detection range: 6–12 m for standard sensors; up to 24 m for long-corridor types
• Sensitivity: Most sensors have an adjustable sensitivity trim-pot to reduce false triggering from HVAC air movement or small animals
• Hold time: Adjustable 10 seconds to 30 minutes — the period the light stays on after last detected movement
• Lux threshold: Integrated photocell can inhibit the PIR during daylight hours, avoiding unnecessary switching

Types of Occupancy Sensor

Passive Infrared (PIR)

Best for: corridors, entrance halls, staircases, toilet facilities, external areas. Detects gross body movement — not suitable where occupants are stationary for extended periods (office workstations, meeting rooms).

Microwave (Doppler Radar)

Emits a 5.8 GHz or 10.5 GHz microwave field and detects frequency shifts from moving objects. Detects minor movement including breathing — suitable for offices and meeting rooms where occupants may be stationary. Microwave signals penetrate thin walls and partitions, which can cause false triggering in adjacent spaces.

Ultrasonic

Emits 32–40 kHz ultrasonic pulses and detects Doppler shift from moving objects. Excellent for spaces with poor line-of-sight (open-plan with high racking, toilet cubicles). Ultrasonic sensors can be triggered by HVAC air turbulence at high sensitivity settings.

Dual Technology (PIR + Microwave)

Requires both PIR and microwave to trigger simultaneously before switching on — dramatically reduces false activation. Both technologies must agree to switch off — the light stays on until both confirm no occupancy. Specified for critical areas where inadvertent switching off is unacceptable (operating theatres, server rooms).

Photocell / Twilight Switch

A photocell (also called a daylight sensor or twilight switch) switches loads based on ambient lux level rather than occupancy. Used for external lighting, streetlighting, security floodlights, and as an override inhibitor within occupancy sensor circuits. Lux switching threshold is typically adjustable between 10–100 lux.

Wiring Configurations

Standard PIR Wiring (Lighting Control)

Most occupancy sensors for lighting control are wired in series with the switched live:

Live (permanent) → PIR L (live input)
PIR SL (switched live output) → Light fitting
Neutral → Light fitting
Earth → PIR and fitting

Some PIR sensors draw their operating power from the load (filament lamp or low-voltage transformer); LED loads may require a bypass capacitor or a specific "LED-compatible" sensor. Always check manufacturer compatibility data before specifying with LED drivers.

PIR in Multi-Way Switching

In a staircase or corridor requiring switching from multiple points, the PIR replaces one of the two-way switching positions. A common approach:

• PIR at the top of stairs acts as master control
• Override switch at the bottom (push-to-make, momentary) allows manual on
• Some PIRs include a manual override function built in

DALI and Addressable Systems

In commercial installations, ceiling-mounted occupancy sensors often interface with DALI (Digital Addressable Lighting Interface) systems. The sensor outputs a 0–10V or DALI control signal rather than switching the supply directly. DALI allows per-zone programming, dimming, daylight-linked dimming, scene setting, and energy logging. Wiring is low-voltage control cable (DALI uses a 2-wire polarity-independent bus) alongside separate power wiring to drivers.

External PIR Sensors and Security Lighting

Outdoor PIR sensors are rated to at least IP44 (splashproof); many specify IP65 (dustproof/jetproof). In UK weather conditions, select IP65 minimum for exposed locations.

Key considerations for external PIR:

• Temperature range: Operation down to -20°C is standard; verify for exposed northern sites
• Wind and foliage: Moving foliage generates false triggers; mount sensors away from trees or reduce sensitivity
• Heat sources: Exhaust vents, boiler flues, or hot car bonnets can trigger PIR — avoid pointing sensors at heat sources
• Zone coverage: Driveway sensors need 180° sweep with 12 m+ range; porch sensors typically require 10–12 m at 160°

External security floodlights with integrated PIR are commonly wired on a fused spur from a ring final or on their own lighting circuit. Always ensure the floodlight supply is protected by a 30 mA RCD — especially where the cable is clipped externally or run underground per BS 7671 Regulation 411.3.3.

For more on outdoor electrical installations see our guide to outdoor and garden electrical installations.

Zone Coverage and Sensor Positioning

Ceiling-Mounted 360° Sensors

Mount at 2.4–3.0 m height. Coverage radius at 2.5 m mounting height is typically 6 m radius (so 12 m diameter circle). For larger spaces, multiple sensors with overlapping zones are required. Do not mount above partitions over 1.5 m high — detection is blocked by tall furniture and racking.

Wall-Mounted 180° Sensors

Mount at 1.8–2.2 m height. Ideal for corridors: point across the corridor (not along it) to maximise detection. Mounting along the axis of travel gives the shortest detection time as subjects move in and out of the Fresnel zones rapidly.

Corner Mounting

A corner-mounted PIR covers the maximum floor area with a single device. Recommended for rectangular rooms up to 4 m × 6 m.

Daylight Linking

Daylight-linked switching (also called daylight harvesting) reduces energy consumption by dimming artificial lighting in proportion to available natural light. In Part L compliant non-domestic installations, manual switching alone is not acceptable in zones adjacent to windows — automatic daylight controls are required.

Implementation options:

• Integrated photocell in PIR: Inhibits sensor below the set lux level. Simple and cheap but on/off only.
• Separate photocell + DALI sensor: Provides continuous dimming proportional to daylight. The DALI driver dims the fitting from 100% to a minimum output as daylight increases.
• Closed-loop vs open-loop: Closed-loop measures light at the work surface and compensates for variations in lamp output and surface reflectance. Open-loop measures daylight at the facade and applies a pre-calculated dimming curve.

LED Compatibility

PIR sensors designed for filament or fluorescent loads may not function correctly with LED drivers. Problems include:

• Ghost switching: The LED driver's capacitive input triggers the PIR's load-sensing circuitry even when the sensor output is open-circuit
• Flicker: The sensor's own power-draw through the LED load causes visible flickering at the minimum dimming level
• Failure to switch off: Very low LED wattage falls below the sensor's minimum load requirement

Solutions: use a sensor specifically rated for LED loads, add a bypass capacitor (some sensors include one), or use a separate control relay. Always check the sensor's minimum load specification against the total connected LED wattage.

See our guide to surge protection devices and consumer unit wiring for related 18th Edition compliance topics.

Common Faults and Troubleshooting

Light Stays On Permanently

• Hold time set too long — reduce with trim-pot
• Sensitivity set too high — triggering from HVAC, passing vehicles, or sunlight reflecting off surfaces
• Sensor permanently triggered by a heat source in its field of view (hot pipe, radiator)
• Faulty relay stuck closed — replace sensor

Light Fails to Trigger

• Lux threshold set too high — daylight inhibit is active even in dim conditions
• Sensitivity set too low — sensor does not detect slow movement
• Sensor mounted behind a glass panel — glass blocks IR
• Wiring fault — check switched live continuity to fitting

Intermittent False Triggering

• Moving foliage or flags visible through a window
• Microwave sensor detecting movement in adjacent rooms through thin partitions
• HVAC diffusers creating warm air plumes within the detection zone — reposition sensor
• Poorly earthed metal conduit causing electrical interference

Regulations and Compliance

BS 7671:2018 (18th Edition)

All wiring accessories including occupancy sensors must be installed in accordance with BS 7671. Key relevant regulations:

• Reg 411.3.3: Additional protection (30 mA RCD) for socket outlets, mobile equipment outdoors, and cables concealed in walls at less than 50 mm depth
• Reg 421.1.7: Electronic dimmers and switching devices — selection to match load type
• Reg 537.3.2: Every circuit must have a means of isolation — sensors in series with the switched live satisfy this if they have a manual override; otherwise a separate isolator is required for maintenance

Part L — Building Regulations (Non-Domestic)

Non-Domestic Building Services Compliance Guide requires automatic lighting controls in most non-domestic zones. Minimum requirements typically include:

• Local manual override in every zone
• Automatic switch-off when spaces are unoccupied
• Daylight-linked dimming in zones with significant daylight contribution

Part P — Electrical Safety (Domestic)

Replacement of ceiling roses, light fittings, and switches is generally notifiable under Part P if it involves the fitting of a new circuit. Adding a PIR to an existing lighting circuit is notifiable if it involves new cable runs. Check with Building Control or use a Part P Competent Person Scheme (NICEIC, NAPIT, SELECT) for self-certification.

See our full guide to Part P notifiable electrical work.

Specifying the Right Sensor

Use this checklist when specifying occupancy controls:

1. Application type: Corridor → PIR; Office → Dual-tech or microwave; Toilet/cubicle → Ultrasonic; External → IP65 PIR with photocell
2. Mounting height: Ceiling or wall; 2.4 m vs 3.5 m will dramatically affect zone coverage
3. Load type: LED, fluorescent, or mixed — check minimum/maximum load ratings
4. Control output: Direct switching, volt-free relay, 0–10 V dimming, or DALI
5. IP rating: IP20 minimum for interior; IP44+ for protected exterior; IP65 for exposed exterior
6. Override: Does the client need manual hold-on or push-to-test? Some sensors require a separate override switch
7. Photocell integration: Specify whether lux inhibit is required and at what threshold (typically 300–500 lux for office environments)

Related Topics

• Domestic lighting circuits — adding a light, loop-in wiring, and LED compatibility
• Fire-rated LED downlights — zone requirements and installation
• Wiring accessories — sockets, switches, dimmers, and USB outlets
• UK electrical earthing systems
• Electrical installation testing — continuity, insulation resistance, and loop impedance

Recommended Products from APM

APM stocks PIR detectors, photocell sensors, and PIR-integrated lighting for domestic and commercial applications:

Visit us at APM Electricals, 31 Inglis Road, London, W5 3RL. Call 020 8819 5852.

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