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These types of luminaire contain one or more lamps, which operate if the normal mains supply fails. An unswitched* (see not below) emergency light is "off" during normal operation. A live feed is provided to the unit that feeds an inverter which trickle charges the integral battery pack keeping these fully charged. In the event of a power failure a relay inside the unit switches the DC battery supply to the lamps on until either the batteries are completely discharged or the mains power is reinstated. See below for more information.
In the UK emergency lighting normally operates for three hours. This is based upon pre-existing design standards (before EU legislation that requires only one hour) and continues to be the norm for UK installations. Three hour battery packs are bigger than one hour battery packs which can cause location problems.
*A test of the emergency lighting should be carried out periodically to ensure the system operates (and lamps have not failed) It is also suggested that the emergency lighting is fully discharged to reduce "memory fade" within the battery packs. To make testing easier a "key switch" is incorporated to allow the supply to the emergency pack to be switched off and trigger the emergency lighting during working hours.