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Internet protocol (IP), the hottest trend in integrated building systems, is attracting interest because any computer with an Internet connection can talk to any building system that runs a digital addressable IP network.
How can that be applied to building systems? Since the 1990s, a digital addressable lighting interface (DALI) protocol has allowed manufacturers to produce a lighting system in which every fluorescent ballast has a unique digital address.
What could be more natural than to hook up DALI systems to an IP-based building control system based on the same type of architecture? Not surprisingly, manufacturers are producing hardware and software to make just such a match.
ZVEI, the German Electrical and Electronic Manufacturers’ Association, established the DALI-AG trade association to draft and publicize the protocol. In 2004, the National Electrical Manufacturers Association (NEMA) created the 243-2004 DALI Standard for the U.S. market. Manufacturers who adopt DALI agree that their ballasts will be usable in any DALI control system regardless of who manufactured the ballast.
The beauty is that the grouping of luminaires doesn’t need to be hardwired at installation but can be established through software after the luminaires have been installed. Each DALI ballast requires five wires, three for power and a pair of low-voltage signal wires. The polarity of the signal doesn’t matter.
According to the DALI-AG Handbook, ballasts can be connected in linear, star or tree configurations. Also, since DALI is designed only for lighting, it uses a low data transmission rate, so there is little chance for electrical interference. So the signal wires can be run in the same conduit as the power supply cables without fear of losing signal quality. This is acceptable according to the National Electrical Code (NEC) as it meets the requirements for a Class 1 circuit, as defined in Section 725.41(A) of the 2007 edition. According to Section 725.48(B)(1), “Class 1 circuits and power-supply circuits shall be permitted to occupy the same cable, enclosure, or raceway only where the equipment powered is functionally associated.”
Running the signal and power supply cables in the same conduit simplifies installation. The five wires can be pulled together and attached identically to every DALI ballast, without worrying about the polarity of the signal.
Rick Miller, a DALI system designer based in San Francisco, said although electrical contractors tend to resist running the signal and power wires in the same conduit, it makes pulling the wires much easier. It’s worth the effort to urge them to go to the NEC and check it out.
It’s important to use a DALI power supply for the signal source, since it will limit the current to a maximum of 250 mA, as required by the standard. (Each DALI device draws 2 mA at a nominal 16 volts.)
Features, advantages and drawbacks
Each of the 64 devices can be programmed with up to 16 different scenes by being assigned to one of 16 different groups, extending the flexibility of the system. One of the most important features of a DALI ballast, Miller said, is that it can be queried to respond and send back signals, indicating if the lamps or ballasts are functioning correctly; the lights are on or off; and if the lights are at, above or below their dimming set point. Although there is a limit of 64 devices per system, multiple systems can communicate through gateways connecting DALI systems to building management systems based on standards, such as LonWorks.
There are some important advantages to DALI in addition to the ease of wiring and setting up scenes. These are a few:
• The ballasts can turn off the lamp, eliminating the need for a separate on/off switch.
• The dimming range is from 1 to 100 percent and follows a logarithmic curve; that’s the way the human eye responds to light.
• Lamps are gradually ramped on with a “soft start” to prolong their life, which is especially important when they are subjected to frequent on and off cycles if, for example, controlled by an occupancy sensor.
• Switches and sensors can be added to the system by connecting to the DALI signal leads anywhere. They need not be physically wired to the device they are controlling; the connection is programmed rather than wired.
• If the system is in a building where tenants often change, it can be reprogrammed to suit the needs of the new tenant without any rewiring.
A major drawback is that only manufacturers of DALI ballasts are required to make their devices interoperable with those of other companies. To put together a system including all of the controllers and peripherals, the designer is forced to go to a single manufacturer for everything, aside from the ballasts. Miller said NEMA is working on an enhanced standard, which promises interchangeability of all required system elements. Some predict that will spark exponential growth for this system.
There is a wealth of practical information, including a list of sources for the whole range of DALI devices, on Rick Miller’s Web site: www.dalibydesign.us.
BROWN is an electrical engineer, technical writer and editor. He serves as managing editor for SECURITY + LIFE SAFETY SYSTEMS magazine. For many years, he designed high-power electronics systems for industry, research laboratories and government. Reach him at [email protected].