There is some confusion here between the requirement for internally illuminated exit signs in which the illumination is maintained at all times and those lighting units that are provided to ensure that there is a minimum level of light along the escape route for safe movement, and at specific points of emphasis- eg changes in direction, changes in level, exit doors, fire points or signs that are not internally illuminated so that they can be seen. Often this general escape lighting for safe movement is not maintained on all the time but operates in the event of a failure of the primary lighting.
The need for illuminated exit signs will depend on the use of the building- eg if the premises are likely at any time to have subdued primary lighting- entertainments or for sleeping accommodation where the lights in escape routes will normally be off.
It is permissible to count include the light emitted from an illuminated exit sign as contributing to the minimum level of escape lighting and some illuminated exit signs are specifically designed to do this- having for example a lens in their base to illuminate an exit door below.
If you have included the light from the illuminated exit signs in your general lighting provision then care needs to be taken when stickers on illuminated signs are updated and light level measurements should be taken to ensure that the minimum 1 Lux throughout the exit route (can be 0.2 lux in exit corridors guaranteed to be clear of obstructions), and average 0.5l ux across the general floor area are maintained.For some high risk premises higher levels are required, for some uses lower levels are permitted.
Look out for BS5266 part 1 2005 and BSEN 1838 1999 for Emergency lighting standards, and BS5499 - 3 1990 (I think still current) for internally illuminated safety signs.
So care is needed in the choice and selection and stickers should not be placed willy nilly on lighting units over exits- the unit may just have been provided to ensure sufficient light for safe movement.
