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The Control Zone

The next step is to develop the basis of design or design intent, which can be expressed using two best practice tools. The first tool is the lighting control narrative, a document that describes how the intended control system will satisfy the owner project requirements. It includes a detailed sequence of operations or description of system outputs performed in response to various inputs (see “The Control Narrative,” in the December 2011 issue or on www.ECmag.com).

The second tool is the control zone plan, describing which lighting loads are operated by which controllers and/or control strategies. As control requirements become more complex with increasing demand for flexibility and layered control strategies, control zoning becomes more important. The control zone plan may be developed after the design intent is established, as it bridges the conceptual and finished design of the system.

The control zone plan

Control zones may be expressed as text (see Table 1, page 32) or graphics (see Figures 1 and 2 on next page). The written approach is simple to produce, and multiple overlapping zones are more easily expressed but may require a lot of reading. The graphic approach is easily understood at a glance. However, it must be printed in color unless patterning or gray shading is used to distinguish between zones. Additionally, layered control strategies with overlapping zones, and very dense zoning, can be difficult to express.

Control zoning in action

In a control zone, one or more lamps are turned on and off by a single controller or allocated to a given control strategy.

Suppose we have a lighting system in an open office that is turned off automatically every night by a time-based controller. In this case, all lighting can be grouped in a single zone for automatic shutoff. However, suppose a single occupant decides to work after hours and activates an override of the automatic shutoff function; we don’t want just one person to keep all of the lights on when he or she only needs a small area illuminated, as it would waste energy. Ideally, the lighting included in the override would be limited to a small area. Such a capability requires the lighting to be divided into smaller zones, each assigned to a different conveniently located override switch.

Now, suppose one wall of the open office space is windowed with high, consistent daylight falling on the space under the first row of lighting fixtures mounted parallel to the wall. The row of fixtures could be assigned to a daylight-harvesting control zone; a controller could dim the lights once light levels pass a certain threshold.

Additionally, wallwash and accent lighting is used to illuminate the other three walls and highlight artwork. These fixtures could be turned off at the end of working hours, while the rest of the general lighting isn’t turned off until the end of full operating hours. This would require additional zoning.

Going further, say we want to maximize energy savings and flexibility by installing an individually controllable suspended direct/indirect fixture over each workstation. Each fixture contains an integral occupancy sensor and dimmable ballast.

In this example, the downlight component of each fixture is individually zoned to turn on and raise to full output and dim to off based on occupancy in the workstation below. The downlight component also is individually zoned for personal dimming control by the occupant using a slider icon on their PC. The uplight component of groups of eight fixtures is zoned to dim to off if all workstations in the group are unoccupied and turn on and raise to full output when a single workstation becomes occupied. The perimeter fixtures are zoned for daylight harvesting. Finally, all fixtures in the space are combined into a single zone for emergency demand response control.

In this example, separate zoning for automatic shutoff, override, personal dimming, daylight harvesting and demand response control are all layered in the same space.

Control zone sizing

The general trend in control zoning is toward higher density of smaller zones in spaces. Control zones can be set as small as single ballasts or lamps. Smaller zoning makes the lighting control system more responsive, flexible and potentially energy-saving, but it also increases complexity and cost.

The first determinant is energy codes, which establish maximum control zone sizes for space controls. Codes based on ASHRAE/IES 90.1 require space control zones to be no larger than 2,500 square feet if the space is 10,000 square feet or smaller. Zones can be 10,000 square feet if the space is larger than 10,000 square feet. Codes based on IECC establish a maximum of 5,000 square feet, except for single-tenant retail, industrial, arena, mall, arcade and auditorium spaces, where the maximum is 20,000 square feet.

Additional code requirements establish other control zoning. For example, IECC and the 2010 version of ASHRAE/IES 90.1 require bilevel switching in certain spaces, requiring the lighting to be zoned appropriately. And IECC 2009 and 2012 and ASHRAE/IES 90.1 2010 establish maximum zones for separate control of general lighting in spaces receiving consistent daylight.

Energy savings, worker satisfaction and space characteristics may necessitate smaller zones than even these maximums prescribed by code.

Energy savings using controls are maximized by ensuring only the right amount of light is produced only where and when it is needed, eliminating waste. For example, by allowing lighting on an entire floor to be turned on, but limiting the override to a small area for after-hours use, more energy waste is eliminated, resulting in greater cost savings.

Worker satisfaction may also be increased in some applications by designing smaller control zones, which can increase flexibility. For example, in an open office, users could be given individual dimming control of overhead fixtures. A classroom teacher could have the ability to separately control lighting at the front of the room from other general lighting, which in turn could be zoned for bilevel control. This would enable the teacher to set optimal conditions for multimedia education.

Finally, space characteristics may necessitate smaller zones. Zoning should be appropriately matched to common lighting equipment (e.g., fluorescent versus LED), space characteristics (e.g., furnishings and finishes), task characteristics, lighting schedules and daylight availability. For example, if daylight availability covers two rows of lighting fixtures, but the daylight level falling on the row closest to the window is much higher than the second row, each row should be zoned separately.

Daylight versus control zones

The latest generation of energy codes—California Title 24 2008 and codes based on IECC 2009 and ASHRAE/IES 90.1 2010—contain requirements for identifying areas of consistent daylight availability and controlling the general lighting in these areas separately from other general lighting in the space. In some cases, these requirements use the term “daylight zone.” These are not control zones but, instead, define areas of predictable high, consistent levels of daylight adjacent to windows and around toplighting. Once the daylight zone is determined for a given space, the designer must then decide how to zone the general lighting controls.

Zoning and control equipment

The level of precision and flexibility required from the lighting control system as reflected in its zoning, in turn driven by owner project requirements, will reveal appropriate equipment solutions.

Traditionally, control zoning has been based on the lighting circuit or subcircuit (or switchleg), with zone size limited to the current-carrying capacity of the circuit or switch/dimmer. That made granular (smaller) zoning expensive and difficult to change after installation.

The digital revolution sweeping the lighting control industry makes control zoning independent of circuiting, with a number of benefits. In a digital lighting control system, control devices are connected using a single pair of low-voltage wires. If the system is distributed, controllers are located in the controlled space rather than in a central location such as an electrical room, eliminating wiring homeruns and potentially eliminating the need for a lighting control panelboard. (The controller may be a micro-panel or digital dimming ballast.) Multiple input devices can be connected to the controllers, allowing multiple control strategies to be enacted in the same space. Control zoning and rezoning, in addition to calibration and programming, can be performed using software, without tools.

This type of system supports simple and economical development of highly granular zoning schemes involving multiple, overlapping zones created by multiple control strategies. The result is availability of control solutions that optimize flexibility and energy savings.

Get in the zone

Control zoning is a critical design decision because it enacts the functionality of the lighting control system. As such, the control zone plan is an important design document that helps ensure the proposed control solution satisfies the owner project requirements and the constructed control solution is properly installed and performs as intended. The general trend—driven by owner interest in flexibility and energy savings, the sustainable design movement and energy codes—is toward smaller control zones. This, in turn, is driving demand for lighting control equipment that can deliver these zones simply and economically, with digital lighting control systems being optimal for highly granular systems.

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