An emergency luminaire is a lamp with its own power supply and the electronics to switch between the two. Most are self contained, meaning the battery is in the fitting. The alternative, a central battery system, puts one large battery in a plant room and feeds every luminaire from it. Both achieve the same outcome and they fail in different ways.
Inside a self contained luminaire
A charger keeps a small battery topped up from the mains. A control circuit monitors the supply. When the supply fails, the circuit switches the lamp onto the battery within a fraction of a second. A charging indicator, usually a small green light, shows the charger is working, which is the only visible sign that the fitting is healthy while the mains is on. When the mains returns, the charger begins recovering the battery.
What the changeover watches
The supply to the local lighting circuit, not just the incoming mains. That matters, because the most common real failure is a tripped breaker or a fault on one circuit rather than a whole building outage. A luminaire wired to sense its own local circuit will operate when that circuit fails, which is the behaviour the design requires.
Central battery systems
One battery installation in a protected location feeds every luminaire over fire rated cabling. The advantage is a single set of batteries to maintain and monitor rather than hundreds of small ones, which suits large buildings. The disadvantages are the cost of the fire rated distribution and the fact that a single fault can affect a large area, which is why the distribution is designed with that in mind.
Why batteries decide everything
The battery is the component that ages, and it ages whether or not it is ever used. Its capacity falls gradually, so a luminaire that lights instantly on test may only last a fraction of its rated duration. That is exactly what the annual duration test is for, and it is why a monthly flick test alone leaves a building with no idea how long its lighting would actually last.
Questions people ask about how do emergency lights work
How long do emergency lights stay on?
For their rated duration if the battery is healthy, commonly three hours in buildings used by the public. The annual duration test is what proves it.
What does the green light on the fitting mean?
That the charger is receiving power and charging. It says nothing about the state of the battery, which is why testing is still required.
Do LED fittings still need batteries?
Yes. The lamp technology changed and the stored energy requirement did not, though a more efficient lamp needs a smaller battery for the same duration.