In the UK, the general guidance for self-contained emergency lighting systems is that they should be supplied from the same final circuit that feeds the normal lighting in the area. However, there are exceptions and specific requirements detailed in UK legislation and supporting British Standards.
The Wiring Principle: Same Circuit for Self-Contained Luminaires
For self-contained emergency lighting luminaires, BS 5266-1 generally recommends that the un-switched permanent live supply, which keeps the battery charged, is connected to the same final circuit as the normal (mains) lighting in that area.
The reason is straightforward:
- Reliable Activation: If the normal lighting circuit fails (e.g., a circuit breaker trips), the loss of mains power to the emergency light on the same circuit will automatically trigger the emergency light to switch to its battery supply.
- Localised Failure Coverage: This ensures that a local fault, such as a tripped MCB for the lighting in a small area, brings on the emergency lights precisely where the normal lighting has failed, preventing a sudden localised blackout.
This connection is typically made using a 3-core or 4-core cable from the lighting circuit to the emergency fitting. Providing a switched live (for maintained lights) or simply a permanent live (for non-maintained lights) for battery charging, alongside the neutral and earth connections.
Exceptions: Dedicated Circuits and Central Battery Systems
While the “same circuit” rule is the default for typical self-contained systems, a dedicated circuit for emergency lighting is allowed and often necessary in certain situations:
1. Central Battery
- Centralised Supply: For systems supplied from a central battery, the emergency luminaires (called ‘slave’ units) are inherently on a separate, dedicated circuit from the normal lighting. This circuit is fed from the alternative power source.
- Cable Integrity: BS 5266-1 and BS 7671 have strict requirements for the wiring integrity of these dedicated circuits, requiring fire-resistant cable to ensure the supply remains intact for the required emergency duration, especially when the cable passes through other fire compartments.
2. Large or Open Areas
- In large, open-plan areas like supermarkets or large offices, a dedicated circuit supplying only the self-contained emergency lights may be considered acceptable.
- Assessment Required: The installation designer must ensure that a general mains failure still results in the activation of the emergency lights. A single dedicated circuit would likely be protected by its own MCB, so a fault on a separate normal lighting circuit wouldn’t trigger the emergency lights. This arrangement is typically justified only if a simultaneous failure of a single normal lighting circuit and the mains supply is considered a very low risk, or if sufficient normal lighting remains to illuminate the area.
Important Considerations
- Segregation: While emergency luminaires themselves might share a final circuit with normal lighting, the wiring of emergency lighting circuits must be seperated from other non-emergency circuits. This is to prevent mechanical damage or electrical faults on non-essential services from affecting the emergency system. This is a requirement under BS 5266-1 (Clause 8.2.6).
- Risk Assessment: All emergency lighting provisions in the UK are ultimately determined by a Fire Risk Assessment. The responsible person must ensure the system provides adequate illumination for safe evacuation, in compliance with the requirements of the standards.
- Testing Facility: A test facility (e.g., a local key switch) must be provided to simulate a mains failure for monthly and annual testing. This test switch “should not interrupt power to any other electrical equipment that could cause a hazard.” This is a critical factor when designing the circuit arrangements.
In summary, for most common installations using self-contained units, emergency lighting is designed to share the normal lighting circuit to ensure reliable activation. However, for central battery systems or specific site requirements, dedicated circuits are not only permissible but mandatory, subject to stringent standards on circuit integrity.