The intent of 700.16(B) is to ensure that a space requiring emergency illumination is not left in the dark due to something like a lamp failure. Having an emergency luminaire with at least two lamps or having two emergency luminaires in the same vicinity (each with one lamp) ensures that the failure of one will not leave the area in total darkness.

One thing that experienced plans examiners look for is the number of lamps shown on the luminaire schedule in a set of plans. If a single emergency luminaire is shown at an exit door on the lighting layout page, then the plans examiner will be looking at the luminaire schedule to see how many lamps the luminaire type is equipped with to get some sort of a feeling of code compliance with NEC^® 700.16(B).

At final inspection before a certificate of occupancy is to be issued, the inspector often finds emergency luminaires, such as in the image, that have two lamps that provide normal area illumination when power is present, but upon the absence of normal power, only one of the lamps is connected to the backup battery. This is a violation of 700.16(B) since the failure of the only lamp connected to the backup battery will cause the area requiring the emergency illumination to be in total darkness. If these types of fixtures are to be used in an area requiring emergency lighting, it is best to install two fixtures side by side to end up with a total of two lamps functioning when normal power goes out.

Below is a preview of the NEC^® (NFPA 70). See the actual NEC^® text at NFPA.ORG for the complete code section. Once there, click on their link to free access to the edition of the NEC^® being enforced.

NEC 700.16(B) System Reliability. Emergency lighting systems shall be designed and installed so that the failure of any illumination source cannot leave in total darkness any space that requires emergency illumination. Control devices in the emergency lighting system shall be listed for use in emergency systems. Listed unit equipment in accordance with 700.12(I) shall be considered as meeting the provisions of this section.