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Bi-level Switching
May 12 2010
Bi-level switching offers manual and automatic control of a lighting system in groups of fixtures or lamps, allowing two levels of light in a room when full illumination is not required. This form of control is simple to operate and easy to understand.
Some people prefer lower overhead lighting levels, especially if daylight is available. Lower light levels are often preferred for computer use, meetings or tasks that are not visually demanding. With the flip of a switch, you can switch one ballast off, so 50 percent of the lamps are lighting the space. Continuous dimming can accomplish the same result, but can be more expensive using dimming ballasts and controls.
Bi-level switching is commonly used in offices, conference rooms, classrooms, warehouses and manufacturing facilities. The most typical bi-level situation is a high bay lighting application such as a 4-lamp T5HO or 6-lamp T8. If you add controls, a combination of the lamps could be switched on by a photocell, daylight sensor or occupancy sensor and the other two lamps would be on all the time for low ambient lighting. Or the lamps can be switched independently with a manual control. Ballasts can be switched separately to create different light levels. Some ballasts can do what is called step-dimming, which creates two or three levels of light. The first step turns on at 33 percent, the next at 66 percent, and the third at 100 percent. Step-dimming and bi-level situations operated by automatic controls is not recommended for classrooms or offices where it could disrupt occupant comfort.
While bi-level and step dimming applications and products have been available for several years, some new products are coming on the market. New bi-level ballasts take lamps from 50 to 100 percent power. When turned on, lamps come on at 50 percent and with another switch or automatic control they come on at 100 percent.
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