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.
Types of emergency lights, and where each one belongs
The first split is behaviour. A maintained fitting is lit all the time and drops to its battery when the supply fails, which is what an exit sign in a public building needs. A non maintained fitting is dark until the supply fails, which suits a stairwell in a building that is empty at night. A sustained fitting carries both lamps in one body. The second split is where the power lives: self contained fittings have their own battery, which is cheaper to install and means a battery to replace in every fitting, while a central battery system keeps one bank of batteries in a plant room, which costs more to install and is easier to maintain and to test. The third split is the job the light does, and it is the one designs get wrong: escape route lighting along the route, open area or anti panic lighting in a room big enough for people to lose the way in the dark, high risk task area lighting where somebody has to shut something down safely before leaving, and standby lighting, which is not emergency lighting at all but a business decision to keep working.
Fire emergency lights, and why they are not on the fire alarm
Emergency lighting and fire detection are two systems that people assume are one. The lights come on when the NORMAL supply fails, not when the alarm sounds, because the hazard they answer is a dark escape route rather than a fire: a power cut with a full building is exactly the scenario they exist for. They have their own batteries, their own standard and their own test regime, a monthly short functional test and an annual full duration test with the results recorded. Some installations do interface the two so that the lights also come on when the alarm activates, and in a windowless building that is a sensible addition, but it is an addition rather than the design. The practical consequence is that a fire alarm service visit has not tested the emergency lighting, and a building with a certificate for one and nothing for the other is half tested.
Why are emergency lights provided, and by what law
Two pieces of law ask for them and they ask for different things. The fire safety order requires that emergency routes and exits needing illumination are provided with emergency lighting of adequate intensity in case the normal lighting fails, which is a duty on the responsible person for any non domestic premises and does not care how old the building is. The building regulations ask for it at the design stage of new work and point at the British Standard for the detail. So a building can comply with the regulations as built and still be in breach years later, because the duty is about the premises as used: a stock room turned into an office, a route blocked and a new one improvised, or a fitting that has been dead for a year all change the answer. The underlying reason is simple enough to test yourself: turn the lights off in your own stairwell at night and see how far you would get with a stranger following you.
Other questions people ask
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.
What does testing emergency lights involve?
Cutting the normal supply so the fitting runs on its own battery: a short function test monthly, and once a year a full duration test for the rated period with the result written down. A fitting that passes a flick test and fails at ninety minutes is the common finding.
Is an emergency lighting flick test enough?
Only as the monthly function check. It proves the fitting changes over and lights, and nothing about how long it will stay lit, which is why the annual full duration test for the rated period is the one that finds tired batteries.
What is the flick test emergency lights need, and is it enough?
The flick test is the monthly function check: the key switch or test facility is operated so the mains supply to the fittings is simulated as failed, somebody walks the route and confirms every luminaire illuminates, and the result goes in the logbook with the date and the fittings that failed. It takes minutes and it is not enough on its own, because it proves the lamp works and not that the battery holds. That is the annual full duration test, three hours for most systems, run at a time when the building can be left with the lights discharging, with a recharge period afterwards. Both go in the same logbook, and an inspector looks for the annual one.