Exit sign
From Wikipedia, the free encyclopedia
An exit sign is a device in a public facility (such as a building, airplane or boat) that displays where the emergency exit is.
Most exit signs around the world are in pictogram form, with or without text supplement. There has been a shift towards the adoption of such exit signs in the recent decade. A small minority of nations adhered to the exit signs that show the word "EXIT" (or similar in another language). Its purpose is guiding people to the closest exit in case of fire or other emergency. The English word "exit" comes directly from the Latin word meaning "to go out." Most fire codes require exit signs to be permanently lit.
Since visibility may be reduced in a fire, due to smoke or failure of electric lighting, the sign is often permanently illuminated, usually by one of:
- Radioluminescence, ('traser'), self-luminous, where a phosphor coating inside a glass tube glows due to the beta decay of radioactive tritium in the tube.
- Phosphorescence ('glow in the dark'), Photoluminescent, where light is absorbed from the surroundings and slowly re-emitted.
- Electric light, with a local rechargeable power source.
- Electric light, with the building's emergency lighting circuits providing back-up power from a UPS and/or a generator in case normal power is lost.
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[edit] History
Early exit signs were generally either made of metal and lit by a nearby incandescent light bulb or were a glass cover that fit directly over a single-bulb light fixture. The inherent flaws with these designs were that, in a fire, the power to the light often went out. In addition, the fixtures were small and dim, making them hard to spot in a fire where smoke often reduced visibility. Better signs were soon developed that more resembled today's modern exit sign, with an incandescent bulb inside a rectangular-shaped box that backlit the word "EXIT" on both sides. Being larger than its predecessors, this version of the exit sign solved some of the visibility problem. Again, however, the sign was only useful in a fire situation as long as main power remained on.
As battery-backup systems became smaller and more efficient, some exit signs began to use a dual-power system. Under normal conditions, the exit sign was lit by main power and the battery was in a charge state. In the event of a power outage, the battery would discharge and the sign would remain lit. It continued to discharge until main power returned to the unit or the battery was no longer able to provide sufficient power to light the sign. Early battery-backup systems were big, heavy, and costly; modern systems are lightweight, can be installed virtually anywhere, and are integrated as a part of the fixture rather than requiring a separate box. In addition, as battery technology improved, so did the amount of time that a fixture could stay lit on batteries.
While exit signs were more visible by virtue of having large letters, even a 60-watt bulb shown through a plastic or glass cover (see image), appeared somewhat dim. With the development of fluorescent lamp and light-emitting diode technology, exit signs could be made even brighter to show up in the limited visibility of a fire situation, and/or use less electricity. LED signs work by combining a large number of bright light-emitting diodes to illuminate the sign from the inside. Fluorescent bulbs work in the same way as incandescent bulbs, backlighting both sides of an exit fixture from the inside. Because of the constantly-lit nature of an exit sign, fluorescent bulbs need to be changed more often than LEDs. As a rule, LEDs have a very long life, and may last for 10 years or more of continuous use, although the brightness may diminish. Incandescent bulbs are still in use, because they are cheap and common, even though they use more electricity and require frequent replacement. In recent years, radioluminescent and phosphorescent signs have also been developed which require no electricity to run. Radioluminescence uses the radioactive decay of tritium to naturally light the sign, while phosphorescence uses light-emitting phosphors to glow in the dark. While both of these signs meet California State Fire Marshall standards, where it is practical electricity is still used in the vast majority of signs.
[edit] Modern installations
Most exit signs in the world, except in countries such as the United States, Australia, and Singapore are of pictogram type. Australia and Singapore have made changes to their respective life safety codes in the direction of pictogram use . In the United States, New York City has endorsed a new code requiring high rises to have glow in the dark exit signs with a pictogram (ISO).
Modern exit signs in the United States can be seen over every path to an exit in commercial and large residential buildings that are properly up to fire code. Certain circumstances, such as the year a building was built, may leave it exempt from some of these codes. However, in most situations, the owner of the building complies regarding exit signs. This is especially true in older buildings that serve as multiple residences, such as apartment buildings, hotels, and campus dormitories. Modern fixtures are usually in a rugged plastic or metal housing securely bolted to the wall or ceiling. The signs have the word EXIT, or a picture representing exit, on both sides. Single-sided signs are also available for wall-mount installations. The signs often have metal or plastic knock-outs which can be removed so that an arrow is also lit pointing left or right. Many modern exit signs are combined with other safety devices, such as emergency floodlighting, as shown above. Exit signs draw a relatively small amount of power, and as a result can generally be tacked onto the end of any existing electrical circuit without adverse effects. Modern exit signs are also, to some degree, flame retardant.
[edit] Battery backup
Battery backup systems are the standard way of complying with new building code requiring that exit signs be permanently lit. Early systems were bulky and expensive, and generally unsuited for use with exit signs. Part of the reason for this was that they used lead acid batteries to supply the necessary 120 volts to light an incandescent bulb in the event of a power outage. For comparison, most automobiles use a single lead acid battery in the ignition system. This made early incandescent battery backup systems very large, extremely heavy, and cost-prohibitive.
Modern exit signs are designed to run on much less power. LED, fluorescent, and new incandescent signs only require 3-12 volts to be fully lit, depending on the sign. All battery backed signs include a transformer, which steps down the main current to charge the battery and operate the sign. In the case of a fluorescent light, the transformer only charges the battery; the bulb requires standard 120-volt current to supply the ballast.
LEDs are electronic diodes, and as such the entire sign can be run directly off of the low voltage power supplied by the battery. Modern incandescent bulbs resemble flashlight bulbs, and are extremely bright while only using a few watts of power. They can also be lit directly from the battery. The primary bulbs in fluorescent signs are high voltage, but the signs include a second set of incandescent bulbs. In the event of a power outage, the sign automatically switches to the lower-voltage bulbs. Most battery backup systems use NiCd batteries, which last a few years before needing replacement. Cadmium is a heavy metal and requires proper disposal, so other battery types such as NiMH might be environmentally preferable.
Some manufacters have replaced batteries with capacitors. This makes the product more environmentally friendly, as it containes only conventional eletronics. Capacitor's lifetime is equal to LED's lifetime, eliminating the need for frequent battery change. Typically these types of exit signs use the latest super- or ultracapacitors together with high power LEDs.
[edit] Self luminous exit signs
Self Luminous Exit Signs do not require an external power source or battery backup. They use a mechanism similar to Fluorescent lighting, but powered by Tritium instead of an electrical current, and are available in models that are rated to last up to 20 years. The half-life of tritium is 12.5 years. The Tritium is hermetically sealed in a glass tube and poses little risk for radiation exposure. (One manufacturer claims that if a person were enclosed in a 10' X 10' room and all the tubes on the sign were to break, they would be exposed to an amount of radiation similar to that used in a dental x-ray.)[citation needed] They are prohibited in some areas. Proper disposal of the radioactive material is required, and may be expensive.
[edit] Red or green
- In most regions around the globe, including the European Union, Australia and New Zealand, Japan, Korea and China, exit signs are green, because they give directions which way to go in an emergency. (In this color scheme, red is used to show prohibited activities).
- In the US exit signs are both red and green, both colors being legal. This is due to states enacting the building code laws which specify what color sign can be used. For example, in Virginia all exit signs should be red, whereas just north in Maryland, they should be green.
New York City requires that exit signs be red in letter color, as does Chicago.
In the Senate Chamber of New Parliament House in Australia the exit signs are red with white writing. This is to fit in with the colour scheme and is legal because of a special law passed for that particular purpose.
