The illuminated sign over the door tells you the building has power. Emergency exit lighting testing exists because that is the one condition under which the unit is never needed. Whether the internal battery can actually run the lamps through an outage is a different question, and only a test that removes normal power answers it.
What the unit is supposed to do
An emergency lighting unit or a combination exit sign contains a battery, a charger and a transfer circuit. Under normal conditions the charger maintains the battery and the lamps run from building power. When the supply fails, the transfer circuit switches the lamps to the battery and they run for a specified duration, commonly ninety minutes, at a stated output. The small indicator lamp on the housing generally shows that the charger has power, which is useful information and is not the same as confirming the battery holds a charge.
Why batteries fail silently
Sealed lead-acid and nickel-cadmium cells lose capacity with age and with heat, and the loss is gradual. A battery at half its original capacity charges normally, shows a healthy indicator, and runs the lamps for a fraction of the required duration. Nothing about the unit’s appearance changes. Units mounted high on walls in stairwells sit in the warmest air in the building, which accelerates the process. Most failures found during inspections are batteries that would have run the lamps for a few minutes rather than the full period.
The test regime and what each part proves
Standard practice involves a brief functional test at monthly intervals and a full-duration test annually. The monthly test confirms the transfer circuit works and the lamps illuminate on battery. The annual full-duration test confirms the battery actually holds enough capacity to last the required period, which is the part that catches degraded cells. Both are usually initiated with a test button on the unit that simulates loss of supply. Results are recorded, and in commercial buildings that record is what an inspector asks to see.
Self-testing units and what they change
Many modern units include self-diagnostic circuitry that runs the required tests automatically and signals a fault with an indicator or an audible alarm. These reduce the labor of a manual program substantially and are worth specifying in new installations. They do not eliminate the need to look: a self-testing unit reporting a fault is only useful if somebody notices the flashing indicator during their rounds. Nor do they eliminate record keeping where the jurisdiction requires documented results.
Residential applications of the same idea
Homes rarely have code-required emergency lighting, but battery-backed fixtures, plug-in emergency lights and combination smoke alarm units with backup all rely on the same principle. Those batteries age the same way, and the test button serves the same purpose. A plug-in emergency light that has sat in a hallway outlet for five years is very likely to have a dead cell, which is worth knowing before the night it matters. Test them on the same schedule as smoke alarms so the habit sticks.
What to check before you act
- Press the test button rather than relying on the indicator lamp.
- Run a full-duration test annually, not just a brief functional one.
- Record results where the jurisdiction requires documentation.
- Replace batteries on a schedule rather than waiting for a failed test.
Common questions
What does the small green light on the unit mean?
Generally that the charger has power. It is not a statement about the battery’s remaining capacity.
How long should emergency lighting run on battery?
Commonly ninety minutes for code-required installations, but the applicable requirement depends on the occupancy and the adopted code, so confirm locally.
The practical takeaway
Test by removing power, not by looking at an indicator. Run brief functional tests monthly and a full-duration test annually, keep the records your jurisdiction expects, and treat battery replacement as scheduled maintenance rather than a response to failure.
References and further reading
- NFPA: electrical and life safety resources covering emergency systems
- OSHA Subpart S: electrical safety requirements in the workplace
- NFPA: battery safety guidance including storage and charging
Numerical scenarios are illustrative unless identified otherwise. Follow the exact product instructions; component ratings and local installation requirements can differ.
