If you ever feel annoyed when the power failure while a working or a business. and in important area. Such as: the way up the ladder and in the bathroom could be dangerous, or shops at night you might be case it gets stolen the products. Emergency lighting is necessary to replace the main light during power failure.
Internal emergency lights will have battery backup. As usual This circuit will charge to stored in batteries. But when the electricity failure, it does the opposite is the battery will discharge into a lamp to provide light immediately.