Most of the exit sign models currently on the market are referred to as “battery backup” exit signs. This means that they have batteries to power the sign in the case of a power loss. According to the various building, safety and fire regulations that deal with exit signs, the sign must remain illuminated for a minimum of 90 minutes in a power loss situation.
In recent years, LEDs have replaced incandescent bulbs as the principal lighting source for exit signs. This change has been brought about by the reduced energy consumption of LEDs over incandescent lights and the associated savings in energy costs for facilities that use them.
Exit signs are required by regulations to remain lit whenever a building is occupied. This means that whatever power source is used must provide enough power for at least 12 hours and in some cases all 24 hours of the day, unless the exit sign is connected to the building’s power grid.
Edge-lit exit signs, the ones which typically have the lowest power consumption, use less than 3 watts of power. They are powered by nickel-cadmium batteries, which have a charge capacity of 700 mAH (milliamp hours). That means that the battery will provide enough power to keep the sign lit for slightly more than 2 hours in a power loss situation. While enough to meet regulatory requirements, it is not enough to insure that the exit is lit all day long.
Providing batteries large enough to run an exit sign, without any external electrical power would be cost prohibitive.
Battery powered exit signs do not exist, but there is an alternative. Self-luminous exit signs use a glowing form of hydrogen gas, called tritium, to brilliantly illuminate for up to 20 years. This type of self-powered exit sign is the perfect solution for applications where electricity is unavailable.
A battery powered exit sign might not be real, but a self luminous exit sign is the non-electrical code compliant alternative. Self-luminous exit signs are UL Listed and compliant throughout the United States. Self-powered exit signs like the self-luminous models, save time and money.
LEDs cannot run off of what we call “house current.” They are DC devices, whereas house current is AC. So, the current coming into the exit sign from the building has to be dropped in voltage and converted to DC current. The voltage drop is accomplished with a small transformer and the conversion to DC is accomplished by a half-wave bridge on the circuit card.
This electrical current both powers the LEDs that are providing light to the sign and maintain the batteries fully charged. In a sense, the LEDs are receiving their power off of the battery, which is constantly being recharged by the building’s electrical current.
In the case of an emergency, where electrical power is lost to the building, the battery takes over, powering the LEDs to keep the sign illuminated. Regulations require that the sign remain illuminated for a minimum of 90 minutes in a power loss situation, allowing all personnel time to exit the building.
While the idea of battery powered exit signs appears to be an attractive one on the surface, the need for maintaining exit signs illuminated at all times the facility is occupied makes it impractical in application. LED exit sign models that use batteries need connection to the building’s power in order to maintain the rechargeable battery at full charge, so that the battery can power the sign’s internal lighting in an emergency situation.
So, the solution is a self-luminous exit sign - a completely self-powered and self-illuminated exit sign.
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