Emergency Lighting Inverter Systems and Central Battery Units — BS 5266 Compliance for Commercial Electricians

Emergency lighting is a legal requirement in all non-domestic buildings and many HMOs in the UK. While self-contained luminaires with integral batteries handle most residential and small commercial applications, larger or more complex buildings require a centralised approach: either an inverter system fed from a central battery set, or a distributed wiring system powered by a dedicated mains supply with battery backup at the sub-distribution level. This article covers the design, specification, and compliance requirements for central battery and inverter-type emergency lighting systems to BS 5266-1.

Regulatory Framework

BS 5266-1:2016 + A1:2018

This is the primary standard governing the design, installation, and maintenance of emergency lighting in the UK. BS 5266-1 references BS EN 1838 for luminance requirements and BS EN 62034 for automatic test systems. Compliance with BS 5266-1 is deemed to satisfy the requirements of the Regulatory Reform (Fire Safety) Order 2005 and the relevant Building Regulations (Part B for new builds, Part M for accessibility).

Regulatory Reform (Fire Safety) Order 2005

The RRO requires the Responsible Person (typically the building owner or employer) to ensure adequate means of escape are illuminated at all times. Emergency lighting is explicitly required as part of the fire risk assessment process. Failure to provide, test, and maintain emergency lighting can result in enforcement action including improvement notices, prohibition notices, and prosecution.

Building Regulations Approved Document B

Part B sets out emergency lighting requirements for escape routes in new and altered buildings: minimum 1 lux on the floor at the centre-line of escape routes, and 0.5 lux over the central band (width measured to BS EN 1838). For open areas (anti-panic lighting), a minimum of 0.5 lux is required over the core area.

Self-Contained vs Central Battery Systems

Self-Contained Emergency Luminaires

The most common approach: each luminaire contains its own rechargeable battery (typically sealed lead-acid or NiCd) that is kept charged by the mains supply. When the mains fails, the battery powers the luminaire for the rated duration (1 hour or 3 hours to BS EN 60598-2-22). Self-contained units are simple to install and test but have limitations:

• Each luminaire requires individual testing and maintenance
• Battery replacement is required across many units, adding maintenance cost
• If a mains supply is lost in a specific zone (e.g., circuit fault), only that zone loses charge maintenance — other zones remain available
• Automatic test systems (ATS) can be retrofitted but each luminaire needs to be individually addressable

Central Battery Systems (CBS)

A central battery system uses a single battery bank located in a plant room or dedicated battery room to power all emergency luminaires throughout the building. The luminaires themselves are standard (non-battery) fittings connected to the central battery wiring circuit. When the mains fails, the CBS detects the failure and switches the emergency circuits to battery supply within 0.5 seconds (or 5 seconds for maintained systems under BS 5266-1).

Advantages of CBS:

• All battery maintenance concentrated at one location
• Consistent battery replacement schedule (typically 5-year life for VRLA, 10-year for NiCd)
• Easier load monitoring and capacity management
• Address each luminaire individually through the wiring loop for automatic testing
• More reliable for high luminaire-count buildings (hotels, hospitals, large retail)
• No individual luminaire battery failure risk

Disadvantages of CBS:

• Higher capital cost for the battery bank and distribution wiring
• Dedicated plant space required with ventilation (hydrogen evolution from lead-acid batteries)
• Single point of failure risk for the entire system (mitigated by automatic monitoring and alarms)
• Complex wiring design — two-wire or three-wire maintained/non-maintained circuits

Inverter-Based Emergency Lighting Systems

Inverter systems are a variant of the central battery approach, specifically designed for applications where the emergency luminaires are the same fittings used for normal lighting. Rather than switching to a separate emergency circuit, the inverter maintains a continuous AC supply from its battery when the mains fails. This allows standard luminaires (fluorescent, LED) to operate from battery without any switching artefacts.

How Inverters Work

An emergency lighting inverter is essentially a UPS (Uninterruptible Power Supply) optimised for lighting loads. The key components are:

• Rectifier/charger — converts mains AC to DC to charge the battery and supply the inverter
• Battery bank — VRLA (Valve Regulated Lead Acid) or NiCd, sized for the required load and autonomy
• Inverter — converts DC back to AC (230V 50Hz in the UK) to supply the luminaires
• Bypass switch — allows the load to transfer back to mains when supply is restored
• Monitoring system — provides status indication, fault alarms, and often Modbus/BACnet communication

System Configurations

Emergency lighting inverters are available in several configurations:

• Maintained — luminaires are on at all times; the inverter provides uninterrupted power during a mains failure. Typical for hotel corridors, hospitals, and buildings where absence of normal lighting is undesirable.
• Non-maintained — luminaires are only powered during a mains failure. Less common in inverter configurations as inverters are normally chosen specifically for their uninterrupted supply.
• Sustained — a combination where some luminaires are maintained and others (normally off) are switched on by a mains failure relay.

Rating and Sizing

Inverter sizing requires calculating the total connected load of all emergency luminaires to be supplied, allowing for power factor correction at low loads, plus a derating factor of approximately 20% for battery aging and temperature effects. Autonomy is specified in hours: 1 hour is the minimum for most premises, 3 hours is required for high-risk areas and buildings where evacuation takes longer (hospitals, multi-storey car parks, high-rise).

Example sizing calculation:

• 50 LED emergency luminaires at 8W each = 400W
• Add 20% aging factor = 480W
• Select a 500VA inverter (noting that for LED loads with power factor 0.9, actual wattage capacity = 500 × 0.9 = 450W — check manufacturer's LED load rating)
• For 3-hour autonomy, battery sizing follows manufacturer's capacity tables

Wiring Requirements

Cable Selection

Wiring from a central battery or inverter to emergency luminaires must maintain circuit integrity during a fire — this is the critical distinction from normal lighting wiring. BS 5266-1 requires that cables supplying emergency lighting on escape routes maintain circuit integrity for at least the rated duration of the emergency lighting system.

Acceptable cable types for emergency lighting circuits:

• Fire-resistant cable to BS 7629 or BS EN 50200 — the most common choice; typically armoured or unarmoured LSOH (Low Smoke Zero Halogen) cable with mineral or intumescent fire protection
• MICC (Mineral Insulated Copper Clad) cable — the gold standard for fire integrity; survives up to 950°C for 3 hours. Suitable for the most demanding applications (hospitals, nuclear).
• Standard T&E behind fire-protected construction — acceptable where the cable is in conduit within a solid masonry wall or behind plasterboard with 30-minute fire protection, but not on the surface of escape routes

Cable segregation: Emergency lighting circuits must be segregated from normal mains wiring to prevent a fault on the normal circuit compromising the emergency circuit. Where co-routing is unavoidable, maintain a minimum 300mm separation or use metallic containment for one of the systems.

Two-Wire vs Three-Wire Systems

For maintained systems (where normal and emergency are the same luminaire), a two-wire system is the simplest: the inverter output powers the luminaires continuously, so no additional switching is needed. Non-maintained central battery systems typically use a three-wire loop: live (normal), live (emergency), and neutral, with a changeover relay in or near each luminaire switching between the two live supplies.

Automatic Test Systems (ATS)

BS 5266-8 and BS EN 62034 cover automatic test systems for emergency lighting. These systems connect to each emergency luminaire (or battery module in a central system) and automatically perform the regular tests required by BS 5266-1:

• Daily function test — simulates a mains failure for a few seconds to confirm the luminaire switches on
• Monthly 25% capacity test — powers the luminaires from battery for the equivalent of 25% of rated duration
• Annual full-duration test — simulates a mains failure for the full rated duration (1 or 3 hours)

Results are logged automatically and any failures are immediately flagged. Modern systems communicate via addressable two-wire loops (similar to fire alarm systems) or wireless protocols. Automatic test systems significantly reduce the maintenance burden in large buildings and provide a verifiable test record for compliance purposes.

Testing and Maintenance

Mandatory Routine Testing (BS 5266-1)

The Responsible Person must ensure the following testing schedule is carried out and documented:

• Daily — visual check that charging indicator lights are illuminated (for maintained self-contained units)
• Monthly — functional test: briefly disconnect mains supply to confirm luminaires operate. Duration: short enough to avoid significant battery discharge (typically 5–10 seconds), then restore mains and confirm luminaires extinguish and charging resumes.
• Annually — full-duration test: simulate mains failure for the rated duration (1 or 3 hours). Confirm all luminaires maintain adequate illuminance for the full period. For a 3-hour system, this test requires the building to be unoccupied or alternative lighting to be provided.

Logbook Requirements

BS 5266-1 requires a log book to be maintained recording: date of each test, duration of test, any defects found, defects rectified and date, and the name/signature of the responsible person. The log book must be available for inspection by the fire authority.

Battery Replacement

VRLA (sealed lead-acid) batteries in central battery systems have a typical design life of 5 years. NiCd batteries last longer (up to 10–12 years) but contain cadmium and require specialist disposal under the Waste Batteries and Accumulators Regulations 2009. Battery replacement should be planned into the maintenance contract and carried out before battery capacity falls below 80% of rated capacity.

Specification and Design Tips

• Lighting design before product selection — use photometric software (Dialux, Relux) to confirm the required maintained illuminance on escape routes before specifying the number and type of luminaires
• Zone the system logically — group luminaires by floor or fire compartment so a fault in one zone doesn't affect the whole building
• Provide clear discrimination — use MCBs or fuses at the central battery board to ensure a fault on one circuit doesn't take down the whole system
• Label emergency circuits clearly — all distribution boards, cables, and luminaires should be marked as emergency lighting to prevent accidental disconnection
• Commission with the fire alarm system — confirm that the fire alarm does not switch off the emergency lighting supply in any mode
• Consider LED load compatibility — not all inverters and central battery units handle modern LED driver electronics well; check manufacturer compatibility data before specifying

Common Installation Faults

• Inadequate cable fire protection — using standard T&E cable on exposed escape routes without protection
• Insufficient illuminance — too few luminaires or incorrect spacing giving below 1 lux on the floor of escape routes
• Damaged or flat batteries — systems not tested since commissioning, batteries long past design life
• Emergency luminaires wired on switched circuits — luminaires lose charge maintenance when the local light switch is off
• No testing records — fire authority finds a logbook with no entries and deems system non-compliant

Key Standards and References

• BS 5266-1:2016+A1:2018 — Emergency lighting. Code of practice for the emergency escape lighting of premises
• BS EN 1838:2013 — Applied lighting. Emergency lighting
• BS EN 60598-2-22 — Luminaires. Particular requirements. Luminaires for emergency lighting
• BS EN 62034:2012 — Automatic test systems for battery-powered emergency escape lighting
• BS 7629 — Fire-resistant electric cables
• Regulatory Reform (Fire Safety) Order 2005
• Building Regulations Approved Document B (2019 edition)

Related Articles from APM

• Emergency Lighting for UK Electricians — BS 5266 Overview
• Building Regulations Part P — Notifiable Electrical Work
• Electrical Installation Testing — Continuity, IR, and Loop Impedance
• Consumer Units, MCBs, and RCBOs — BS 7671

Products from APM Plumbing & Electrical

APM Plumbing & Electrical does not currently stock central battery units or emergency lighting inverter systems. Self-contained emergency luminaires and fire-rated cable may be available — CM to identify published products before the 2028-04-16 target publish date.

Get it from APM Electricals, Acton

We stock what UK trades need — same-day collection from our trade counter in Acton. No minimum order. Trade accounts welcome.

Browse our lighting range, or visit us at 24 Western Avenue, Acton, London W3 7TZ. Call 020 8702 8080 to check stock.