There are two different types of emergency lighting, these are; Maintained Emergency Lighting and Non-Maintained Emergency Lighting.
Maintained Emergency Lighting will remain illuminated and operate as a normal light fitting. These fittings are controlled with all the other lights in the area, however as and when the power fails the maintained emergency fitting will continue to operate at a lower light level for a minimum of 3 hours.
Non-Maintained Emergency Lighting is ordinarily powered off so that the batteries can continuously be charged via the mains, however, should the power fail, the fitting switches on using its battery supply. Non-maintained fittings are not a general light fitting but are fitting such as emergency exit signs.