What is the main requirement for exit signs posted above doors in a building?

The main requirement for exit signs posted above doors in a building is that they must be illuminated and in good condition. This is essential to ensure that in times of emergency, individuals can easily locate the exits, even in low-light or smoke-filled environments. Illuminated signs are crucial for visibility, providing clear guidance for occupants to evacuate safely and efficiently. Maintaining the signs in good condition further enhances their effectiveness, as any damage or lack of functionality could hinder safe egress during an emergency. Other methods, such as battery operation and visibility in daylight, are secondary considerations that cannot substitute for the fundamental requirement of illumination and overall condition.