Specify the optimal occupancy sensor

Occupancy sensors are proven energysavers that meet code requirements, facilitate sustainable construction, and provide convenient lighting control. But with the breadth of products now available, how does one select the right sensor for a given application? Because sensor selection is key to proper operation and occupant satisfaction, this is an important question.

06/01/2008


Occupancy sensors are proven energysavers that meet code requirements, facilitate sustainable construction, and provide convenient lighting control. But with the breadth of products now available, how does one select the right sensor for a given application?

Because sensor selection is key to proper operation and occupant satisfaction, this is an important question. To choose the optimal sensor, a specifier must consider certain characteristics of the space that will be controlled, as well as how occupants will use the space.

Occupancy sensors use passive infrared (PIR) technology, ultrasonic technology, or both technologies to detect motion. Traditional occupancy sensors turn lighting on when a space becomes occupied and provide energy savings by turning lights off, following a time delay, after the space becomes unoccupied. Greater energy savings are achieved by providing a switch for the user and configuring the occupancy sensor for manual on-operation. This way, the lights are turned on only if they are needed.

Space evaluation

Identifying the ideal occupancy sensor for a particular application begins with examining the size and shape of the area to be sensed, including determining the location of the work area(s), furniture, partitions, suspended lighting fixtures, doors, windows, and HVAC ducts and fans. Ceiling height also is important, as is knowing what kind of activity will occur in the space.

Understanding how sensing technologies detect motion helps recognize why different sensors work best under different circumstances. PIR technology detects occupancy by sensing the differences in the heat emitted by humans in motion from that of the background space. PIR technology requires an unobstructed line of sight for proper sensing. This characteristic creates the opportunity to cut off coverage and prevent sensing of adjacent areas. PIR sensors are a good choice for individual offices, spaces with unobstructed views, and spaces with high ceilings, including lobbies and warehouses.

Ultrasonic technology uses the Doppler principle to detect occupancy; ultrasonic waves are emitted throughout a space, and shifts in the returning waves indicate motion. Because ultrasonic coverage does not have a clear cut-off and can see past partial obstructions, these sensors are ideal for areas with partitions and enclosed spaces with normal height ceilings. Use ultrasonic sensors for groups of office cubicles, multi-stall restrooms, and enclosed hallways and stairwells. High levels of airflow can disrupt ultrasonic sensing, so these sensors should be located at least 6 ft. from HVAC ducts.

Dual-technology sensors use both PIR and ultrasonic technologies and keep lighting on as long as one of the technologies detects occupancy. Dual technology sensors are recommended for hard-to-control spaces such as large offices, conference rooms, and classrooms where occupants may be relatively still.

Coverage patterns

Occupancy sensors are available in a wide variety of coverage patterns of various shapes and sizes. To select a pattern that is the best fit for the application, consider the shape and dimensions of the space, not simply the square footage. Additionally, select sensors that have been tested to confirm coverage of both major and minor motions as prescribed in NEMA Guide Publication WD 7-2000.

Sensor layout should facilitate detection of the type of motion anticipated in the space; in many workspaces small motions must be detected to ensure proper operation. Position the sensors above, or close to, the main areas of activity and then add sensors as needed to provide complete coverage.

In small offices and spaces up to 12x12 ft, wall switch sensors often work well if the activity can be sensed from the switchbox location. Beware of switchboxes behind doors or in vestibules where sensors might not be able to properly sense the controlled area.

In many cases, ceiling sensors will provide the best coverage for the space. When multiple sensors are used, place them so that the coverage patterns overlap slightly. Remember to consider the ceiling height, as this will affect the area that will be covered. Create zones controlled by different groups of sensors to manage lighting in large areas; check energy code requirements when determining zone sizes.

Power packs

Power packs are a key component of low-voltage sensor-based lighting control systems. They provide low-voltage power to occupancy sensors, including the ceiling sensors discussed above, and switches provided for manual control. Power packs respond to signals from the sensors and switches to switch a relay controlling the lighting. Low-voltage sensor systems are inexpensive to install and easy to relocate should the space be reconfigured.

Power packs also can provide advanced features such as switched inputs and inputs for scheduling, daylight sensors, and dimming control. These features, and many others, provide a finer degree of control over the different loads in a building and allow control strategies to be combined to achieve USGBC LEED credits or other goals. All the control needs should be considered before choosing the best power pack for the project.

Electrical considerations

The wall switch sensor or power pack must have a sufficient load rating for the combined current consumption of the fixtures that will be controlled, and also must work with the building's input voltage. Power devices also must be rated for the type of load that will be connected, and power packs that will be mounted on boxes in the plenum must be plenum rated.

If the design calls for bi-level control, or control of two independent loads, solutions include a dual-relay wall switch sensor. Alternately, one ceiling sensor and two low-voltage wall switches may be used to control individual power packs capable of manual-on operation.

Devices that use zero crossing technology provide reliability and long relay life by protecting the relay each time it is turned on. Zero crossing is important when switching electronic ballasts, which can strain relays due to high inrush current.

Commissioning and training

Some occupancy sensors are preset for typical applications and do not require commissioning. However, it is beneficial to have features such as an adjustable time delay, sensitivity setting, and controls for enabling advanced features in order to optimize sensor performance.

If needed, settings for sensitivity and time delay should be made according to the anticipated activity level. Lengthen time delay setting for spaces with lower activity levels; shorten time delay setting for high-activity areas and maximum energy savings. Finally, recommend that occupants be educated about the lighting controls.

Once the sensors are operational, no maintenance is required, and typically no readjustment is needed. Manufacturers are careful to make the controls for sensor settings inaccessible to the end-user in order to prevent unauthorized changes. Should the space be reconfigured, or the use changed, the sensor system may have to be adapted for the new application. Maintenance personnel or a contractor can move or reconfigure sensors as needed.

Proper sensor selection will prevent the nuisance problems such as false triggering (on or off) that distract occupants and can prevent energy saving goals from being achieved. When occupancy sensors are properly specified and installed, they will operate reliably for years to come.


Author Information

Fournier is product manager of commercial occupancy sensors and controls at Watt Stopper/Legrand.




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