Lighting Control Systems: Occupancy Sensors, Daylight Linking, DALI and Emergency Override

Quick Answer: Modern commercial lighting controls are required by Building Regulations Approved Document L 2021 to include occupancy sensing with auto-off and daylight-linked dimming in most new and refurbished non-domestic buildings. DALI (Digital Addressable Lighting Interface) to IEC 62386 is the dominant wired protocol; wireless alternatives include Casambi, Zigbee, and Bluetooth Mesh. Emergency lighting must comply with BS 5266-1:2016 — non-maintained luminaires must illuminate automatically on mains failure regardless of control system state. Manual-on / auto-off operation is the recommended default for occupancy controls to balance energy saving with user acceptance.

Summary

Lighting controls have evolved from simple wall switches to integrated networks managing energy use, occupant comfort, and emergency response across entire buildings. Building Regulations Part L 2021 substantially raised the bar for commercial lighting — occupancy sensing, daylight linking, and zoning are now mandatory features in most office, retail, and educational projects. CIBSE TM65 and LG14 provide practical design guidance. The result is that virtually every commercial lighting installation now involves at least a basic control layer, and many involve sophisticated networks with hundreds of addressable devices.

The control protocols fall into two camps. Wired protocols — DALI, KNX, LonWorks — use dedicated low-voltage data cables between control devices and luminaires; they are reliable, deterministic, and well-suited to new builds where the cabling can be planned. Wireless protocols — Casambi (Bluetooth Mesh), Zigbee, EnOcean — eliminate the data wiring at the cost of network complexity and ongoing battery / firmware maintenance; they are well-suited to retrofit projects where running new cable is impractical.

The third critical element is emergency lighting — non-maintained luminaires that illuminate on mains failure to provide escape route illumination. Emergency lighting under BS 5266-1:2016 must operate independently of the main lighting control system; a power loss must illuminate the emergency luminaires regardless of whether the master controller is functioning. Integrating emergency lighting with general lighting controls requires care to maintain this independence.

This article covers occupancy sensing technologies (PIR, microwave, dual-tech), daylight harvesting strategies (closed-loop and open-loop), the DALI and DALI-2 protocols including emergency commands, wired vs wireless trade-offs, Part L compliance requirements, emergency lighting integration per BS 5266-1, override switching, scene control, and retrofit options for existing buildings.

Key Facts

Quick Reference Table

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Control Technology Wiring Max Devices Typical Application Strengths
Standalone PIR Mains + load 1 luminaire group Toilets, small rooms Cheapest, simplest
1–10V analogue 2-wire control Multiple per channel Small office, single zone Simple dimming, no addressing
DALI / DALI-2 2-wire control 64 per universe Office, retail, education Per-luminaire addressing
DALI-2 emergency 2-wire control Per universe Combined general + EM Full DALI emergency reporting
KNX Twisted pair 256 per line Whole-building integration Multi-trade (HVAC, blinds)
Casambi (Bluetooth Mesh) Mains only 127 in basic network Retrofit, small commercial No data wiring needed
Zigbee Mains only 100+ per gateway Hospitality, retrofit Mature ecosystem
Bluetooth Mesh Mains only Theoretically 32,000+ Large commercial wireless Scalable, open standard
EnOcean Energy-harvesting Per gateway Retrofit, heritage Battery-free switches

Detailed Guidance

Occupancy Sensors — PIR, Microwave, and Dual-Tech

PIR (Passive Infrared)

Detects infrared radiation from moving warm objects. The sensor "sees" the room through a Fresnel lens that creates a pattern of detection zones. As a warm body moves between zones, the sensor detects the change in infrared signature.

Microwave (Doppler radar)

Emits low-power microwave signal and detects frequency shift in the reflection when objects move within the field. Coverage typically 10–15m radius and penetrates plasterboard partitions, glass, and thin doors. Sensitive to very small movements (typing, slight shifts) but can detect motion through walls or windows — false positives from corridor traffic, vehicles, or mechanical ventilation movement. Best for stairwells, plant rooms, and areas where line of sight is poor.

Dual-tech (combined PIR + microwave)

Both technologies must trigger together to turn on. Either alone is sufficient to keep on once activated. Eliminates the false-on problems of each individual technology.

Sensitivity and timeout: set sensitivity just high enough to detect normal occupancy without false triggers from outside the controlled space. Timeout defaults: 15 minutes for offices, 5–10 minutes for toilets, 1–2 minutes for corridors — too short frustrates sedentary users, too long wastes energy. Many sensors include an integrated photocell to prevent activation when daylight is sufficient.

Daylight Linking — Closed-Loop vs Open-Loop

Closed-loop daylight harvesting: a photocell at the workplane (often on the luminaire) measures actual illuminance at the working surface, and the controller dims to maintain a target setpoint (typically 300–500 lux for offices). Self-correcting — it adjusts artificial light as daylight changes — and preferred where precise illuminance matters.

Open-loop daylight harvesting: the photocell measures external or window-position illuminance and the controller applies a pre-calibrated dimming relationship. Simpler to install but requires careful commissioning; suitable for perimeter zones with consistent daylight aperture.

Zoning for daylight linking: a typical office is zoned in bands parallel to the window wall — daylight zone (window edge to ~4m, strong daylight contribution, significant dimming), mid zone (4–7m, moderate daylight, partial dimming), and core zone (>7m, minimal daylight, full output during occupied hours). Each zone has its own dimming control for gradient operation.

DALI and DALI-2 — The Industry Standard

DALI basics: two-wire control bus (polarity-independent for most devices), individual addresses 0–63 per device, up to 16 groups and 16 scenes per universe stored in each ballast, bus power 16V DC at 250mA standard, half-duplex 1200-baud asynchronous communication.

DALI universe layout:

DALI universe — example wiring
                        ┌──────────────┐
                        │ DALI Controller│
                        │ + Bus PSU     │
                        └───────┬──────┘
                                │ 2-wire bus
                  ┌─────────────┼─────────────┐
                  │             │             │
            ┌─────▼─────┐ ┌────▼─────┐ ┌────▼─────┐
            │ Luminaire │ │ Luminaire│ │ Sensor   │
            │ Address 0 │ │ Address 1│ │ Address  │
            │ Group 0   │ │ Group 0  │ │ 60-63    │
            └───────────┘ └──────────┘ └──────────┘
                  │             │             │
            ┌─────▼─────┐ ┌────▼─────┐ ┌────▼─────┐
            │ Luminaire │ │ Luminaire│ │ Wall     │
            │ Address 2 │ │ Address 3│ │ panel    │
            │ Group 1   │ │ Group 1  │ │ input    │
            └───────────┘ └──────────┘ └──────────┘

Cable: 2-core 1.5mm² (or LSZH equivalent); max 300m bus length

DALI-2 improvements: backward compatible with original DALI, adds standard sensor and input-device protocols (Parts 303/304), improves cross-manufacturer interoperability through DALI-2 certification testing, and enhances emergency lighting commands (Part 202).

DALI emergency commands: Part 202 defines a standard set — function test (short), duration test (full discharge), status reporting (battery, lamp, last result), and identification (locate a luminaire by flashing). This enables centralised testing and logging through the same bus that controls general lighting, while the emergency luminaire still operates independently on mains failure.

Wired vs Wireless — Choosing the Architecture

Wired (DALI, KNX, LonWorks) — deterministic delivery with no RF interference, no batteries to maintain, decades-long cable life, easily scalable via gateways, and (with KNX) wider building integration including HVAC and blinds. Downsides: 2-wire data cable to every controlled device, disruptive to retrofit in occupied buildings, and commissioning requires DALI/KNX-trained engineer.

Wireless (Casambi, Zigbee, Bluetooth Mesh, EnOcean) — no new data wiring needed, easy re-zoning by software, and battery or energy-harvesting sensors eliminate sensor wiring. Downsides: battery replacement cycle for some sensors, RF attenuation through metal partitions, careful credential management for security, and standards drift between manufacturers' "open" implementations.

Selection rule:

Building Regulations Part L 2021 — What's Required

Approved Document L Volume 2 (Buildings other than dwellings) sets out minimum standards for non-domestic lighting:

Efficacy: new luminaires must meet minimum efficacy targets (typically 95 lm/W for general lighting in the 2021 update).

Controls: occupancy sensing for most office, education, and similar spaces; daylight linking on perimeter zones with daylight access; time-of-day scheduling for predictable-occupancy spaces; manual override always available; zoning by function and daylight availability.

Specific requirements: toilets, store rooms, kitchens, corridors get occupancy sensing with auto-off; open-plan offices need combined occupancy + daylight linking; cellular offices need manual-on/auto-off with daylight linking near windows; stairwells use occupancy sensing with low-level background (typically 10%) during occupied hours.

Manual-on/auto-off (recommended) matches the energy saving of fully automatic but with much higher user acceptance — no surprise activation walking past — and achieves Part L compliance. Auto-on/auto-off is occasionally specified for long corridors where wall switching is impractical, but user complaints in offices are common.

Emergency Lighting Integration — BS 5266-1:2016

Fundamental principle:

Emergency luminaires must operate on mains failure regardless of the control system state. A power failure to the local distribution circuit must trigger immediate illumination of all emergency luminaires on that circuit. The control system cannot prevent this — the emergency function is hard-wired into the luminaire ballast.

Categories: non-maintained (off in normal use, illuminates only on mains failure — most escape route luminaires); maintained (on in normal use, stays on during failure — exit signs, retail signage); combined (two lamps, one mains and one battery, independent operation — theatres); sustained (lamp on mains normally, switches to battery on failure — older category, less common).

BS 5266-1 escape route illuminance: 1 lux minimum on the centreline of an escape route, 0.5 lux average across the route, maximum illuminance variation 40:1 along the route. Open areas (anti-panic) — 0.5 lux minimum. High-risk task areas — 10% of normal lighting.

Battery duration: 3 hours standard; 1 hour for small premises with rapid evacuation; longer for high-risk industrial per fire risk assessment.

Testing: monthly function test (brief switch to battery) and annual full duration test, with a log book of all tests including failures and remedial action. Self-test luminaires schedule both tests internally with status LED; DALI-2 emergency and other addressable systems provide centralised reporting.

Override Switching and Scene Control

Override switching: boost/extended-on extends the auto-off timeout; manual-off overrides automatic switch-on (useful for meeting-room presentations); a cleaning override gives cleaners full lighting out of hours.

Scene control: pre-programmed combinations of dim levels and colour temperatures (where tunable white is installed) — e.g., a presentation scene with low front-of-room and dim audience, a cleaning scene at 100%, a reception scene at 2700K and 60%, a concentration scene at 5000K and 100%. DALI and KNX both store up to 16 scenes per group natively; activation via wall panels, switches, BMS schedule, or app.

Retrofit Options for Existing Buildings

Adding controls to an existing lit space without disrupting cabling:

Frequently Asked Questions

Does Part L 2021 apply to single-room refurbishments?

Yes — Part L applies to any refurbishment that involves replacement of more than 50% of the lighting installation in a controlled service. A single-office refit where existing luminaires are being replaced triggers Part L compliance for that space. Small repairs or like-for-like lamp changes do not trigger Part L. Always check the project scope against the threshold tests in the Approved Document.

Can I run DALI and 230V mains in the same conduit?

DALI is classified as SELV (Separated Extra-Low Voltage) when bus voltage is below 30V AC / 60V DC. BS 7671 Section 528 requires separation between mains and SELV unless either: the SELV cable has insulation rated for the mains voltage (e.g., 300/500V insulation on the data cable), or there is a physical partition / separate conduit. For DALI, use cable rated for the local mains voltage (typically 300/500V cable, or 0.6/1kV for industrial) and you may run in the same conduit as mains. Verify cable spec before installation.

How does emergency lighting work in a DALI system if the DALI bus loses power?

DALI emergency luminaires have their own integrated battery and inverter. On mains failure to the luminaire, the lamp illuminates from battery, irrespective of the DALI bus state. The DALI bus is used only for testing and status reporting in normal conditions; it has no role in the emergency operation itself. Verify this principle by removing DALI bus power on a commissioned luminaire — emergency operation must still trigger on mains failure.

Should I use manual-on or auto-on for a meeting room?

Manual-on with auto-off is the strongly recommended default. Meeting rooms are often entered briefly without intent to use the space, and auto-on activation in this scenario wastes energy and frustrates users. A manual-on switch ensures the lights only come on when someone deliberately uses the room. Auto-off after 15–20 minutes of vacancy provides the energy benefit without the user-experience downside.

What happens if a Casambi node goes offline — does the whole network fail?

Casambi uses Bluetooth Mesh, where each node is also a relay. Loss of a single node degrades the mesh slightly but does not break the network — messages route through alternative nodes. Loss of multiple adjacent nodes can fragment the network. Resilience is one of the strengths of mesh protocols compared to star topology. However, the network does require sufficient density (typically nodes within 10m of each other) to maintain reliable mesh.

Regulations & Standards