Emergency lights should be installed by a qualified electrician and can usually be either ceiling or wall mounted.
The lights are connected to the buildings mains power circuits which keeps the backup battery charged. In the event of a power failure the emergency light will continue to operate under the power of the back-up battery.
Emergency lights are usually installed with a test switch (fish key switch) on their circuit which allows them to be tested by interrupting the mains power supply to the lights only.