While browsing our inventory for a new emergency light or exit sign, you have mostly likely stumbled upon the term"UL 924 listed" and the claim that this specification makes the fixture appropriate for use throughout the United States. Without a little more info (like this blog post, for example) that claim is meaningless. A sticker displaying UL compliance ensures that the emergency fixtures installed in your building meet the fire safety codes that must be observed by all commercial locations.
The Underwriters Laboratory (UL) is an independent testing organization that has been certifying various products' safety for over 100 years. There are UL listings for categories such as laboratory equipment, fire extinguishers, various types of wiring, and many other classifications. UL 924 specifically deals with the "Standard for Safety of Emergency Lighting and Power Equipment." Emergency lights and exit signs fall under this category, so they must be UL 924 listed for full compliance.
For an emergency light to be considered UL 924 compliant, it must provide at least 90 minutes of battery powered illumination. This gives the building's occupants ample time to evacuate without inciting hysteria. The internal battery is always hardwired to the main power supply so it is constantly charged in case of emergency. It is also imperative that the emergency light provides illumination automatically and immediately after power failure.
Exit signs follow a very similar set of rules for the UL 924 listing. The Underwriters Laboratory does not care if the exit sign's letters are red or green, but it is required that they are at least six inches tall with a stroke thickness of 3/4 inch and spaced out 3/8 inch from each other.UL 924 and NFPA 101 have set a minimum luminance of .06 ft. lamberts and a viewing distance of 100 ft. There are exceptions to this rule but the exit signs must be marked with the viewable distance of 50, 75, or 100 ft according to their completion of the test.
UL listings may not be the most exciting part of emergency lights and exit signs, but they are necessary. Just remember: your fixtures must be UL listed and all of our's are. You do the math.