How emergency lighting works
Self-contained fittings charge a battery in normal use and light automatically when the supply fails.
The principle
Emergency lighting keeps escape routes and exits visible when the normal supply fails. Self-contained fittings have their own battery that's kept charged from the mains during normal use, and which powers the light automatically the instant the supply is lost.
Maintained vs non-maintained
A maintained fitting is lit all the time (like an exit sign) and stays lit on battery if the supply fails. A non-maintained fitting is off in normal use and only lights on a supply failure. Each needs the right supply arrangement — typically a permanent (unswitched) feed to keep the battery charged, plus the correct switched feed for maintained types.
Because they're life-safety fittings, they must be tested on a schedule: a discharge test proves the battery sustains the light for the required duration, and the results are recorded.
Why it matters
A fitting that fails its discharge test, won't charge, or is stuck in emergency won't protect occupants when it counts. These are never left out of service, and faults are rectified and re-tested by a licensed person.
Safety first
Emergency/exit lighting is life-safety — never leave a fitting out of service. Testing and rectification are licensed, with record-keeping.
Isolate, lock out / tag out, and prove dead before working unless a live test is specifically required, authorised, and carried out under proper supervision. Always follow local regulations, your site procedures, and the equipment manufacturer's documentation.
Related faults
Exit sign not illuminated
An illuminated exit sign is dark — it may have lost supply, failed its lamp/LED, or have a faulty driver. A life-safety fitting that must be kept working.
Emergency light fails its discharge test
An emergency fitting works on mains but won't stay lit for the required duration when tested (mains removed) — usually a failed/aged battery, but can be charging or lamp issues.
Emergency light stays on / won't go back to normal
An emergency fitting stays in emergency (battery) mode or its charge/fault LED stays wrong even with normal supply present — pointing at the supply sensing, charging, or the fitting.
Related definitions
Surge protective device (SPD)
Diverts transient over-voltages (from lightning or switching) to earth to protect equipment.
Control vs power circuits
Low-power control logic decides what happens; the power circuit carries the load — kept separate for safety and clarity.
Isolator / disconnector
A switch whose job is safe isolation — visibly and securely disconnecting a circuit for work.