The prime use of Emergency Lighting is to help ensure a safe escape in a fire when mains lighting is lost. They may be used to light the escape route, and/or as a beacon of light at the exit that people can aim for in the darkness.
They may also be used as stairwell backup lighting, and as automated convenience lighting for living areas, consumer unit, and so on.
Why Have One?
No-one expects they'll have a fire, but the fact is a lot of people die in house fires every year. Electrical faults, irresponsible tenants, stupid kids, smokers, neighbouring fires, these and more are causes of house fires.
A big problem with fire is smoke. Smoke can totally disorient people, even when they know their own property well. Even trained firefighters quickly become disoriented in dense smoke.
Smoke inhalation is a major killer, more so than burning by fire.
Smoke produced in a fire rapidly and heavily decreases light levels and visibility.
With all these factors, escape from a property, while normally elementary, can fail to happen in a fire, with disastrous results.
For under £20 you can give yourself and your family more chance of staying alive.
Maintained & non-maintained
Non-maintained fittings only light when the mains supply fails.
Maintained fittings light when mains lighting fails, but also can be switched on by a light switch for regular daily use.
Sustained fittings behave similarly to maintained fittings, but the internal setup is different. They use 2 separate sets of lights, one for mains use and one for battery use.
Filament & fluorescent
Fluorescent lighting is much more efficient than filament, so fluorescent fittings give much more backup time and/or light output.
Fittings should be tested every so often, as batteries & bulbs don't last forever. Testing should check they work and that they illuminate for the required time period.
To light an escape exit point, lighting high up is needed, as low lights are liable to be obscured from direct view by various objects.
Smoke tends to rise, with clearer air toward the ground. The recommended method of escape in dense smoke is on hands and knees. Smoke may be too dense for light to carry from high up to the ground. For these reasons, mounting lighting low down is more effective when used to light an escape route.
These are applied to a light fitting to give an emergency escape symbol. They block most of the light output, making the emergency lighting much less effective.
People who are genuinely unable to work out that the only remaining light in a fire is at the exit are also likely to not understand the symbol either.