In any dimming system, the ballasts and controllers must be able to speak the same language. In the case of digital dimming systems, this language is either proprietary — unique to a particular manufacturer (and other adopters, if allowed) — or an open standard, such as the Digital Addressable Lighting Interface (DALI) protocol.
The advantage of a proprietary protocol is that a single manufacturer, which has tested all of the individual components to ensure interoperability and support of the entire system, can furnish the complete control system. The disadvantage is that the owner is then tied to a single manufacturer, which limits choice and potentially sacrifices economy.
DALI is a royalty-free, non-proprietary, two-way, open and interoperable digital protocol. Currently, DALI consists of a set of commands to and from ballasts within a defined data structure and specified electrical characteristics. As an open standard (NEMA 243-2004), DALI is supported by five major U.S. electronic ballast manufacturers and a growing number of controls manufacturers, which offer DALI-compatible products. Advantages of DALI include:
- True interchangeability across ballasts and controls
This results in lower costs, ensures future availability, and enables the system designer to select product functions from one manufacturer and combine them with products from other vendors.
- Standardized ballast performance
DALI ensures consistent dimming performance across all dimming ballasts regardless of type or manufacturer, currently not achievable with analog dimming methods such as 0-10VDC.
When examining whether to use DALI for a fluorescent dimming installation, you must first assess the advantages and disadvantages of digital control, and then weigh the pros and cons of DALI as the communication protocol that enables the digital components to talk to each other. As a starting point, it's important to determine what degree of flexibility you're looking for in your lighting control strategy.
The ultimate in flexibility
When configuring a fluorescent dimming system, the designer must specify control zones — that is, a fixture or group of fixtures that are controlled simultaneously by a single controller. For example, in a daylight-harvesting scenario with windows (Photo), the designer may place lighting circuits parallel to the window, and set up each circuit as a separate control zone. The smaller the control zones, the more granularity, or flexibility, you can achieve, along with higher energy savings. However, installation cost also increases.
Digital lighting control provides complete flexibility. When using analog dimming systems, the smallest zone is often a branch circuit. With a digital system, zoning is implemented using software, independent of circuits, using individual ballast addresses stored. Because each ballast is individually addressable, control zones can be established that are as small as a single ballast or light fixture. Ballasts or fixtures can also be grouped to provide up to 16 layers of controls/scenes, enabling highly granular and responsive control, the ability to generate a wide variety of zones and scenes in any controlled space, and the power to adapt to changes over time — even be completely reconfigured without rewiring. For large installations, you can network individual buses for the control of hundreds or even thousands of ballasts.
DALI-based digital control systems can include all controls that would normally be used for multi-scene fluorescent dimming, such as preset controls, as long as they are compatible with DALI. They can be used to operate with a DALI-compatible digital ballast for fluorescent lamp dimming or a DALI-compatible solid-state transformer for precise incandescent lamp dimming in both 120V and 12V versions. Digital controls are also compatible with switching devices, such as occupancy sensors and photo sensors.
With a digital control scheme, the designer can implement automation strategies, such as scheduled dimming from a central PC for centralized control, while enabling occupant control and override via local interfaces, such as preset controllers or occupant PCs.
The digital lighting network can be connected to a lighting automation or building automation system (BAS) for integration with other lighting or building systems. For a DALI-based digital control system to communicate with a non-DALI lighting control system, or a BAS operating using another protocol such as BACnet or LonWorks, a translator device (called a gateway) is required to enable these systems to communicate with each other.
In a digital dimming system, a single set of control wires form a low-voltage control bus — sometimes inappropriately called a loop. Compatible ballasts and controls (up to a total of 64 devices — with each ballast having its own unique stored address) connect to this bus to provide control signal interaction (click here to see Fig. 1). For larger installations, multiple buses can be networked to proper scale (click here to see Fig. 2). The control bus provides two-way communication; ballasts can both respond to commands and reply to queries.
The digital ballast includes a microprocessor that functions as storage (ballast address, intensity settings, fade rate), receiver (control signals), and sender (intensity, lamp/ballast status) of digital information. DALI instructions, such as GoToScene and SetMax, are sent to the ballasts, using the data stored in its microchip memory. The ballast is not only “smart,” but it can also “talk back.” As the control bus enables two-way communication, the digital ballast receives commands and also responds with maintenance and energy information, such as the status of the ballast and lamps. This enables the lighting system operator to query ballasts for energy usage (using feedback such as Intensity-Setting), which can be used for a variety of purposes from energy savings verification to benchmarking to billing internal departments or tenants individually for their lighting usage. It also allows the operator to query ballasts for lamp and ballast failure (querying for a response such as BadLamp), which can improve the efficiency of lighting maintenance and improve customer service from the facilities department.
Control options include centralized systems (a personal computer or building automation system) as well as local controls, such as manual dimmers, occupancy sensors, and photo sensors.
A single pair of control wires, which forms the bus, connect the ballasts and controls directly (Fig. 3), which simplifies wiring in spaces with multiple control zones by reducing the number of homeruns. The level of skilled labor is reduced because there is no need to pull wire according to a zoning schedule. Each ballast on the circuit is wired the same. There are no switch legs or three-way travelers. In addition, dimming control panels/modules are not needed to control light output. Instead, digital systems use a small power supply connected to the bus.
Digital ballasts can be wired into the lighting system using Class 1 or Class 2 wiring methods according to the National Electrical Code. Digital ballasts may use a Class 1-rated 5-conductor cable that uses one hot (live), one neutral, one ground, and two polarity-insensitive control wires, all routed together in the same conduit. It is also possible to install the ballasts and controls as a Class 2 installation, in which case the control wires must be routed separate from the power wires. Check with the ballast and controls manufacturers on whether their products are rated for Class 1 installation.
With DALI-based digital lighting, any ballast or group of ballasts on the network can be given an ON/OFF or DIM command without rewiring the circuits. This enables compliance with prevailing energy code requirements for automatic shutoff in controlled spaces without the need for a control panel with a scheduling function.
The higher level of capabilities from digital control systems often entails trade-offs in cost and complexity.
Digital ballasts and controls typically present a higher component cost largely due to power supply/router requirements, but they can present a lower installed cost due to a reduction in wiring labor for group and scene control, and the removal of the need for dimmer modules/control panels in larger installations.
Digital control also presents more sophisticated programming when centralized systems are used, and requires on-site field commissioning, which should be factored into the design specification. During the start-up phase, a database of ballast addresses, with physical locations and the control device that operates them, needs to be created and then maintained when the layout of the space changes. It may be advisable to involve the client's IT staff in the creation and maintenance of the database.
Digital dimming systems can be configured as large networked systems requiring commissioning and training, or as simple stand-alone room preset dimming controls requiring no special tools or PCs. As an open standard, DALI enables true interchangeability among vendor products and standardized performance across manufacturers. Although digital systems can present a higher component cost, labor savings resulting from simplified wiring can result in a lower installed cost compared to 0-10VDC dimming systems.
Miller is president of RNM Engineering, Inc., San Francisco. DiLouie is communications director for the Lighting Controls Association, Rosslyn, Va., and principal of ZING Communications, Calgary, Alberta, Canada.
Sidebar: DALI-Based Ballasts
DALI-compatible fluorescent dimmable electronic ballasts are currently available in:
- Universal input voltage (120VAC to 277VAC).
- One-, two-, three-, and four-lamp models for T8 lamps.
- One- and two-lamp models that operate CFL, T5, and T5HO lamps.
Digital ballasts are available that provide a dimming range of 100% to 1%, using a logarithmic dimming curve. The inverse-square dimming curve is used for better control of the lighting intensity in response to the human perception of brightness.
Digital ballasts use programmed-start technology to maximize lamp service life, which makes them highly suitable for frequently switched applications, such as installations with occupancy sensors.
Sidebar: Application Notes
If fluorescent dimming is desirable for a given application, digital dimming can offer distinct advantages related to intelligence, flexibility, and two-way communication. It is particularly well suited for:
Energy management applications such as scheduled automatic shutoff to meet energy codes, daylight harvesting (due to the ability to more economically set up very small control zones around the daylight aperture), and scheduling via a central computer, which allows load shedding, demand savings, and potential utility incentives.
Supermarkets, some retail spaces, and similar applications with frequent merchandise or layout changes.
Small and open offices where users are given dimming control over their own lighting as part of a strategy to increase worker satisfaction.
Conference rooms, classrooms, training rooms, and similar spaces that require different lighting scenes for multiple types of use.
Larger installations with multiple buildings, where feedback on lighting component status can facilitate more efficient lighting maintenance.