Many inspector red tags are written when metal halide luminaires are installed as the main source of exterior lighting on a building.

High intensity discharge (HID) lighting such as high- and low-pressure sodium, mercury vapor, and metal halide is a type of lighting that takes a few minutes to warm up to full brightness once switched on. A short duration power outage or power failure can cause the arc in a HID luminaire to extinguish. Once extinguished, the HID luminaire will need to cool and restrike.

When power comes back on after an outage, HID lighting might take a few minutes to warm up to full illumination levels. During the warm-up time, areas of egress may not have sufficient illumination levels required by the Building Code or Life Safety Code.

For this reason, a time delay feature is required on the emergency luminaires to ensure that once power is restored, the emergency lighting system is still fully illuminated long enough to give the normal area HID lighting time to provide the full required level of illumination.

There are many HID luminaires equipped with a quartz restrike which is a small second lamp that will energize immediately when power is restored. Emergency luminaires serving an area where the normal lighting is provided by an HID luminaire equipped with a quartz restrike feature, does not need to have a time delay feature.

The main intent of the code section is to ensure that whether power is on, or has failed, one way or another, there should always be sufficient lighting to allow for building safety and egress.

For complete code sections, refer to the actual NEC^® text at NFPA.ORG. Once there, click on their link to free access to the latest edition of NFPA 70.

NEC 700.16 Emergency Illumination.

(A) General. Emergency illumination shall include means of egress lighting, illuminated exit signs, and all other luminaires specified as necessary to provide required illumination.

(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...

(C) Discharge Lighting. Where high-intensity discharge lighting such as high- and low-pressure sodium, mercury vapor, and metal halide is used as the sole source of normal illumination, the emergency lighting system shall be required to operate until normal illumination has been restored.

(D) Disconnecting Means. Where an emergency system is installed, emergency illumination shall be provided in the area of the disconnecting means required by 225.31 and 230.70, as applicable, where the disconnecting means are installed indoors.

Exception: Alternative means that ensure that the emergency lighting illumination level is maintained shall be permitted.