Dimming System and Occupancy Sensor Myths

By James Piper, P.E.  
OTHER PARTS OF THIS ARTICLEPt. 1: Fluorescent Lights and Controls MythsPt. 2: This Page

MYTH #3: Dimming systems don't work with fluorescent lamps

One of the long-standing advantages of incandescent lamps is their ability to be dimmed smoothly and evenly from full brightness to zero. Incandescent dimming systems range from small, low wattage units, to ones capable of dimming several kilowatts of lights.

But problems arose when people tried to use incandescent dimmers with fluorescent lighting systems. While some dimming resulted, it was inconsistent, resulted in lamp flicker, reduced lamp life, and could not be used over a very wide range of illumination.

Fluorescent lamps cannot be dimmed using incandescent lamp dimmers. They require special dimming ballasts. Dimming a fluorescent lamp requires a reduction in the lamp arc current while the electrode heater voltage is maintained. Maintaining the electrode heater voltage while reducing light output levels means that the overall efficiency of the lamp will be reduced when it is operated at dimmed levels, but this reduction in efficiency is more than offset by the total reduction in energy use.

The range of dimming that can be achieved in the latest generation of dimming ballasts for fluorescent lamps depends on the particular ballast design and the type of lamp used. Practically all fluorescent lamps can be dimmed to 5 percent of the lamp's rated light output. This range of dimming makes the systems particularly suitable in a wide range of applications that currently use incandescent lamps.

MYTH #4: Occupancy sensors leave you in the dark

Perhaps the most widely used lighting control system is the occupancy sensor. They have been applied in spaces ranging from individual offices to restrooms, storage rooms, conference rooms and library stack areas. Depending on how frequently the area is used and how diligent people have been in turning off lights when they're not needed, occupancy sensors can reduce lighting use in these areas by up to 90 percent.

One of the biggest complaints concerning the occupancy sensor is that it can leave occupants in the dark if it falsely believes that the space is unoccupied. Most occupancy sensors work by detecting motion. Once detected, the sensor turns the lights on for a preset amount of time. Each time that motion is detected, the sensor's timer is reset. If no motion is sensed and the timer reaches its preset interval, the lights are turned off. If the space is still in use, the occupants can be left in the dark.

In most cases, the failure of the occupancy sensor to detect occupants is the result of installation or application errors, not the fault of the occupancy sensor. To be effective, the sensor needs to be able to see all or most of the space. Sensors have a limited viewing range and angle. Objects within the space or unusual room configurations can partially block the view of sensors resulting in false readings. By selecting the right type of occupancy control and by properly placing that control, most false readings can be eliminated. If there still are concerns about leaving occupants in the dark, a single, low wattage fixture can be left switched on at all times to provide backup lighting.

A second part of this myth is that, as a result of their frequent on-off cycles, occupancy sensors kill lamp life. While there is no question that this frequent cycling does reduce lamp life as measured by total operating hours, it can actually extend the calendar life of the lamps, particularly in those applications where light is only needed a small fraction of the time.

James Piper, PhD, PE, is a writer and consultant who has more than 35 years of experience in facilities management. He is a contributing editor for Building Operating Management.



Where to Apply Lighting Controls

Properly designed and installed lighting controls can produce savings in practically all applications. Some controls, however, will provide greater savings and a quicker payback than others. Start by conducting an after- hours walk-through of the building. Identify areas where lights are left on when there are no occupants or activity. If the building is cleaned after hours, the system installed will have to take that into consideration.

Conduct a similar walk-through during occupied hours, identifying areas where the lights are operating in unoccupied spaces. Similarly, identify areas that have sufficient windows to allow lighting systems to use daylight harvesting.

It is important to remember that the objective of installing lighting system controls is not to minimize lighting energy use. Rather, it is to provide the most appropriate level of lighting where it is needed, when it is needed.

— James Piper

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  posted on 3/31/2011   Article Use Policy

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