Shedding Some Light on Dimming


Lighting systems are one of the largest consumers of energy in a building, by some estimates using approximately 38% of a building's total energy. However, choosing the appropriate dimming fixture for a project can be tough. Dimming light fixtures can be expensive, and the available choices for dimmers are numerous.

Most of the lighting being specified today is fluorescent, whether it is for offices, retail or even residential kitchens. With the price of energy continuing its upward trend, the return on investment for the installation of dimming lighting controls is now short enough to warrant the upfront installation cost—good news for owners.

One consideration is whether the system will be used in a new or retrofit installation. Some dimming systems are capable of using existing wiring, which can reduce costs.

But a greater consideration for a light dimming system is deciding on the lowest level to which the lights should be capable of dimming. This is because the pupil of the human eye automatically dilates as ambient light is reduced, allowing more light to strike the retina. This causes a condition where the perceived light in a space is greater than the actual light being provided.

In fact, it is possible for some dimming systems to control light output down to 1%, which is often referred to as architectural dimming. Although this may seem excessive at first, it can become very important in spaces where lighting will play a key role in illuminating critical tasks for prolonged periods of time. Architectural dimming can also be helpful in spaces where lighting is required to provide illumination with no distractions or glare—areas like a theater, restaurant, patient exam room or continuous-use computer workstations. In areas occupied for smaller time durations, a similar but less costly dimming system may be employed called high-performance dimming . These systems will often dim light output to approximately 5% and are best used in small meeting rooms and lobbies.

The most popular level of light output control is general dimming and can dim light output to around 10%. This level is utilized in most areas of load shedding through daylight harvesting and occupant detection. Both large and small offices, as well as restrooms, corridors and large-volume public spaces, can benefit from providing this level of dimming.

A good rule-of-thumb application for the three different dimming controls mentioned is: 1% architectural dimming for critical areas, 10% general dimming for load shedding and daylight harvesting, and 5% high-performance dimming for the spaces in between.

Types of dimming ballasts

Once the type of application is known and the necessary level of light output is determined, one is able to properly specify a dimming ballast. There are three main types: zero to 10-volt DC, two-wire and three-wire. Although these three types are not interchangeable, it is possible to see variations that include elements from the other types. For example, a three-wire dimming ballast might have some control inputs for low-voltage DC wiring.

Zero to 10-volt DC ballasts use four wires between the control and the ballast; one switched phase conductor (hot wire), one neutral, one positive DC control wire and one negative DC control wire. Power is fully supplied to the ballast with the switched phase conductor and the two control wires provide DC voltage signals to dim. Typical installations that use this type of scheme involve low-voltage wiring that is routed from the control, usually a wall switch, to the ballast. Other forms of controls have since entered the market, including handheld infrared devices and daylight sensors. In fact, anything that can provide a DC signal is a candidate for lighting control. The cost to provide this type of dimming is economical and makes it a popular method for daylight harvesting and load shedding. The disadvantages are that the lights can only be dimmed to 10% output, and the DC voltage signal strength will diminish over longer runs, causing differences in light output between fixtures.

Two-wire ballasts use two wires; one is the dimmed and switched phase conductor and the other the neutral. Only the phase conductor is routed through the control. This system will dim the lights to 5% light output making it a great selection for retrofit applications because there is no need to pull extra wires to the ballast or switch. Simply replace the original ballast with a dimming version and the existing toggle switch with a fluorescent two-wire dimming switch, and connect to the existing wiring. As with the zero to 10-volt DC ballast, a disadvantage is that through voltage drop and line losses, it is possible on larger switching groups to notice a difference in the light output between fixtures.

The final type of dimming ballast, the three-wire ballast , uses three wires between the control and the ballast. The wires are a switched-phase conductor, a dimmed-phase conductor and a neutral. The switched-phase conductor supplies a 100% light output baseline reference to all the ballasts on the switch group and thereby eliminates the light output difference between fixtures in large switch groups. Also, this system will dim the lights to 1% light output, an important consideration in architectural and critical areas. This system is stable and provides the best level of control of the three ballast types, but is typically the most expensive option.

In the specs

Most lighting designs will not require the provision of wiring diagrams for the type of dimming ballast used. The main thing to remember is to select and call out the proper dimming ballast on the drawings and light fixture schedule. It is also important to place the type of dimming ballast used in the associated specifications. In this way manufacturers and contractors will be able to provide the proper dimming system that will meet the owner's needs with little or no surprises.

Switches and sensors

Once a preferred light level and dimming ballast are selected, controls for occupants and users to adjust the lighting should be considered. For fluorescent dimming, common slider and dial switches are available that look and operate like standard incandescent dimmers. These dimmers are a standard affair and are determined more by the type of dimming ballast and the manufacturer specified than anything else, although various aesthetic options are available.

One of the fastest growing segments of lighting is load shedding, or daylight harvesting. Because lighting calculations typically assume no light contribution from any other source, electric light fixtures are arranged to provide all the light needed throughout a space. With the advent of daylight-sensing photocells, it is now possible to accurately gauge real-time in an area the amount of light being provided from non-electric sources, and thereby reduce the light needed from the electric lights. This reduction in electric light reduces the amount of heat generated, resulting in a diminished cooling demand and further energy savings.

Most other sensors available are an extension of some form of low-voltage wall switch, daylight sensor or occupancy sensor. The possibilities are endless and are best covered in a discussion on lighting control panels.

An enlightened guide

With the proper information and strategy it is possible to select and specify appropriate dimming controls that are not only economical, but also effective and energy-efficient. Two tangible drivers that make dimming ballasts a sound owner investment are rising energy costs and more quantifiable evidence that productivity is lost due to user discomfort. Finally, dimming continues to be a viable option for owners as the initial cost to install a dimming system becomes more affordable and quicker to recuperate.

That said, perhaps the best way to enhance the effectiveness of fluorescent dimming is to gain an understanding of the current technology being offered and to seek the services of a lighting professional to act as a guide throughout the process.

Saranac Hotel Renovation

An unoccupied hotel being renovated for mixed use in Spokane, Wash., including offices, a movie theater, a restaurant and loft-style apartments, proved an excellent opportunity for the use of extensive dimming.

Energy savings was the primary reason for using daylighting and dimming controls as a part of the overall lighting control system, but user comfort and control of systems were also considerations. Smaller office areas occupied by one or two individuals received independent dimming controls, while larger areas utilized automatic dimming controls based on available daylight along with occupancy-sensing capabilities. Occupants were provided a local switch, handheld infrared remote or computer-based software that is able to provide temporary override of local set points to either turn on or off the electric lighting or dim the lighting to the preferred level.

A three-wire dimming ballast, capable of dimming the lights to 10% light output and able to accept low-voltage DC control wiring provided by the sensors, was selected. In this case, general dimming was the appropriate and economical selection, because the primary reason for dimming was energy reduction and occupant comfort. The lighting control system was designed to maximize energy savings (almost 50% over a standard similar building) and individual user control and comfort, as well as to provide control points through relay contact closures for the building's automation system, based on occupancy and light level data from the sensors for increased energy savings.

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