More Than Convenience
Before the widespread use of electricity, high-maintenance lamps and expensive gas lit homes and cities after dark, but neither were reliable or economical enough to allow industry to operate around the clock. Not surprisingly, as activities and occupations brought more people together, accidents and disasters became more numerous; the Iroquois Theater Fire of 1903, Triangle Shirtwaist Factory Fire of 1911 and Our Lady of Angels School Fire of 1958 were three fires that had profound effects on fire and building codes, causing cities and states to set new standards for safety equipment. Emergency lighting was one such improvement; had it been present, perhaps some of the over 800 souls lost in these three tragedies alone could have found their ways through the dark smoke to safety. The technical problem with emergency lighting was twofold: it had to function without electricity and it had to turn on automatically without someone activating a switch. After the Our Lady of Angels fire, emergency lighting was one of the safety measures mandated for schools and other areas where large groups of people gathered.
The first emergency lights were simply large incandescent spotlights mounted on top of large lead-acid batteries (the type of batteries used in cars) wired into the building's circuitry. The units had transfer switches that switched on when the power went off, lighting up the spotlights, which could then illuminate an area as long as the battery charge lasted. Today, emergency lights are placed, according to individual cities' building and fire codes, along "egress" routes---paths that people would take to get out of a building. U.S. codes require that lights retain their charge for at least 90 minutes, and many states require that lighting plans or photometric schematics be filed showing area and egress lighting with illumination values and fixture specifications. Lights are frequently wired together in a circuit so that when power fails in one part of the building, the lights go on in all parts.
Modern emergency lights generally operate on a low-voltage charge to extend the life of lead-calcium batteries and use halogen incandescent with xenon filaments or light-emitting diodes (LEDs) in parabolic reflector (PAR) lamps that deliver maximum illumination for fewer watts. The heart of the system, however, remains the transfer switch that senses the absence of current and the inverter that begins changing direct current from the battery to usable alternating current for the lamps. Rather than the simple mechanical switches that the first lights used, modern systems, like other electronics, may be wired into an uninterruptible power supply (UPS) that contains an electronic transfer, transformer and inverter. These "hardwired" systems recharge the battery under normal conditions and transfer to battery power when the building system fails. Some emergency systems may be wired to emergency generators or include LED "pathways" about a foot and a half off the ground along egress routes to light the way in smoky fires.