Distributed Control Systems Promise Greater Value

By Craig DiLouie, Lighting Controls Association

Automatic shut-off of building lighting is required by prevailing energy codes. ASHRAE/IES 90.1-1999, the national energy code standard, requires automatic shut-off of lighting systems in buildings larger than 5,000 sq.ft., with the exception of critical loads that must be operated continuously.

This strategy has been recognized by energy codes because it is proven to be effective in reducing energy consumption. For example, according to the California Energy Commission, scheduling a building’s lighting to automatically shut off at the end of the workday has been documented to generate 5-10 percent kWh savings and 0-5 percent peak kW savings. Scheduling the lighting to automatically shut off selected lighting during peak demand periods has been documented to generate 5-15 percent kWh savings and 5-15 percent peak kW savings.

To satisfy the energy code requirement, occupancy sensors, building automation systems and/or lighting control panels can be used. When occupancy is predictable, scheduling, using lighting control panels and building automation systems, can be most effective. When occupancy is not predictable, occupancy sensors can be most effective. These strategies can work together as layered control to optimize energy cost savings for the owner.

The ASHRAE/IES Standard also requires control for smaller areas than an entire building, requiring more granular control than a traditional scenario in which a control panel is scheduled to turn off the building’s lighting at the end of the workday.

In addition to this required level of granularity, owners can benefit from smaller control zones to provide greater flexibility in today’s buildings.

Distributed control is a lighting control panel strategy that enables flexible scheduling of multiple control zones while potentially reducing the system’s installed cost. Distributed intelligence is an option in distributed control that can increase reliability and flexibility in design of control systems.

In this special report from the Lighting Controls Association, we will present distributed control and distributed intelligence as options for scheduling/lighting control panel-based control systems. These options, which represent significant trends in lighting control, can present significant advantages for certain applications.

Traditional Control System

Control panels are typically used in larger buildings, providing a central platform for switching large numbers of loads. These panels typically contain relays with low-voltage inputs from control devices and line-voltage outputs to the load. The control panels are typically installed in the electrical room near the electrical panel.

Some lighting control panels include controllable breakers and can therefore replace the electrical panel, saving space. Some control panels also provide the capability to mingle switching and dimming modules.

For lighting automation to occur—that is, automatic shut-off of lighting based on a schedule—the control panel must be automated, or intelligent. In other words, it must include an internal time-clock that enables time-based functions.

This control system provides the backbone for the building’s lighting control, providing the basic function of scheduled shut-off. Additional layers of devices/strategies can be added to this system, such as photosensors, occupancy sensors, dimmers, etc.

The system is typically centralized, with all local switches and switch-legs (sub-circuits) connected to the control panel via line-voltage wiring, and accessory inputs such as photosensors connected to the panel via low-voltage control wiring. See Figure 1 for an example of a centralized control system.

The control panel polls connected control devices for input that is then filtered through its logic circuit to determine the output (ON or OFF). In a building requiring control of very large zones, this approach can be economical.

If the building requires greater granularity of control with smaller zones, then another approach—distributed control—should be considered, which in many applications can provide greater capabilities at a lower cost than centralized systems.

Figure 1. Centralized control system wiring requires switches and line-voltage switch-legs to be individually connected to the central panel. Graphic courtesy of Watt Stopper/Legrand.

Distributed Control

In a distributed control system, panel-based control functionality is distributed across the facility. In its simplest form, this approach is already quite common. An example is when a control panel is installed on each floor of a multistory building for networked control from a master panel, or convenient access and individual stand-alone control by each floor’s occupants.

This strategy can be taken further using small control panels, typically with only 2-4 control outputs each, that are installed in the immediate area of the loads they control, typically above the ceiling. (These distributed control panels may be called “automatic relay packs,” “remote relay packs” or some other term by manufacturers.) A completely distributed system would have no control panel in the electrical room, which can save space.

The distributed panels are typically networked via low-voltage cabling to share information and implement local and global commands based on a shared protocol. For centralized scheduling, these distributed panels can connect back to a system time-clock.

Advantages of this approach include:

Installation cost savings: A big advantage of this type of approach is installation cost savings due to a reduction in required input/output wiring. “The benefits and advantages are considerable,” says Pete Horton, VP of Market Development for Watt Stopper/Legrand. “There are lower labor costs since installation is often easier and less costly without the same complex line-voltage wiring requirements. When inputs and outputs are located within close proximity, the distributed architecture will reduce the total installed cost and provide more information to the user.”

He adds that the component cost of this type of approach has been declining to a level where, due to labor savings, the building owner can receive a greater value.

Scalability: Scalability is an important factor in designing control systems. Distributed control systems are more scalable than centralized control systems. “As devices are added to a centralized system, the central controller can become bogged down having to monitor an increasing number of devices,” says Scott Jordan, Marketing Manager for Square D Lighting Control, a division of Schneider Electric. “If the system becomes large enough it can surpass the capacity of the controller, resulting in system delays or missed events. With a distributed system, this bottleneck is removed.”

Greater flexibility, information: The electrical industry has long used distributed control without using communication systems between controls, and is now catching up to other industries by connecting these formerly stand-alone distributed devices to a monitored system. In a distributed lighting control system, the distributed panels can be networked via low-voltage wiring to share information and implement local and global commands. “An important fact about distributed control,” says Horton, “is that it’s founded on a communications protocol, rather than electrical wiring. This is the fact that makes its other qualities possible.”

Energy code compliance strategy: Perhaps the biggest advantage of distributed control, however, is the ability to meet prevailing energy codes. “Distributed control is increasing due to energy code requirements that mandate individual control for smaller spaces,” says Jordan. “If you’re looking for granularity in your control system, think distributed.”

Horton agrees. “Among the market trends that are influencing implementation of distributed controls, the most influential are mandatory energy codes that impose specific control requirements on square footage,” he says. “For many design professionals, it’s easiest to achieve compliance with a distributed controls approach.”

Disadvantages of distributed control include:

Component cost: The component cost of a distributed control system has been declining but may not be economical for most installations with few inputs and outputs controlling large control zones. Distributed control systems are typically most suitable for applications requiring a higher degree of granularity.

Maintainability: Jordan advises control system designers to ensure that if a distributed control strategy is adopted, that the panels are accessible and easy to find. “If not designed properly, a distributed system can be somewhat difficult to maintain, as there is no single central location,” he warns.

“Distributed controls tend to have more locations with smaller footprints,” says Horton. “Advantages include less equipment room spaces and the ability to gather more data about the input/outputs. Issues to consider include the serviceability of the devices when they are located above the ceiling. They can be more difficult to locate, and servicing such devices may be more intrusive to occupants.”

Figure 2. Distributed control units require only a twisted-pair communication cable to be run between devices and back to a system time-clock, which reduces wiring installation costs and increases information, flexibility and scalability. Graphic courtesy of Watt Stopper/Legrand.

Distributed Intelligence

In a traditional centralized control system, the control panel uses a processor to assign its switches to control zones, and polls connected devices for inputs that will result in responsive switching outputs. If the processor fails, the entire control system fails.

In a distributed-intelligence system, each control device has its own processor, which enables networking of devices using any configuration that the application may require. The devices communicate directly with each other instead of using a central processor as the intermediary. If a processor fails, that particular device fails but the rest of the system will not be affected, which increases reliability.

“One can think of distributed intelligence as a subset of distributed control,” says Horton. “The individual component in a distributed intelligence system has the ability to make decisions independently. Distributed intelligence builds upon distributed controls, moving decision-making capabilities to the distributed device. Advantages include faster response times because decisions can be made locally. Another advantage is independent operation in case of network communication errors.”

Other advantages include greater flexibility in wiring, future capability for reconfiguring a space (including greater scalability), and generation of more information for monitoring and decision-making. “The architect should consider how the owner will use the new information that will be available through a distributed intelligence based system,” says Dean Pournaras, VP of Marketing for Watt Stopper/Legrand. “Some owners may not have resources available to monitor their control systems and may want to outsource the management; other owners may not be interested in the data at all. An architect can help the owner decide how they will take advantage of these new control features and information, and assess first cost and ongoing costs to confirm that such a system will have benefits outweighing these cost considerations.”

Horton concludes: “As a trend, distributed intelligence is emerging. Technologies such as DALI-based digital systems are attracting attention. Wireless RF and Zigbee are additional examples of technologies being explored by manufacturers. While the market demand is not significant currently, interest in the benefits appears to be high. The transition from control devices such as switches becoming signaling devices rather than power switching devices, as they most often are used today, will have a tremendous impact.”

Final Word

“The owner is seeing the need for better lighting control coming from two areas,” says Jordan. “The first is an environment that has created new regulations regarding energy efficiency. Consequently, new building energy codes must be satisfied. The second is an internal push to find new ways to conserve energy. The energy costs associated with lighting can be a significant piece of the overall energy bill. Whether you’re talking about building codes, adding value through greater control capabilities, or simple energy savings, lighting control panels are an excellent way to satisfy these needs.”