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Occupancy Sensors A-Z

The principle of operation and benefits of occupancy sensors are well understood. By turning off the lights when a space is unoccupied, energy savings can result by eliminating waste. Occupancy sensors can also enhance security and reduce light pollution.

According to the U.S. Environmental Protection Agency, energy savings can range from 40 to 46 percent in classrooms, 13 to 50 percent in private offices, 30 to 90 percent in restrooms, 22 to 65 percent in conference rooms, 30 to 80 percent in corridors, and 45 to 80 percent in storage areas.

Due to their relative simplicity and high energy savings, coupled with the requirement in prevailing energy codes for automatic lighting shut-off, occupancy sensors are rapidly becoming a standard feature in new buildings and retrofits.

State energy codes in the United States must be at least as stringent as the ASHRAE/IES 90.1-1999 standard, which requires automatic shut-off of lighting in commercial buildings greater than 5,000 sq.ft. in size, with few exceptions. Automatic shut-off can be provided by occupancy sensors or programmable time scheduling devices. When occupancy in a space is intermittent or not predictable, occupancy sensors are economical.

While today’s occupancy sensors offer robust features, proven utility and reliability, they remain application-sensitive devices, which requires a properly educated designer and installer for the controls to be effective. The right occupancy sensor must be selected, it must be properly located and installed, and it must be field-calibrated.

In this article, we will examine the seven steps of the effective application of occupancy sensors.

Step 1: Understand the application. Define the characteristics of the space and determine applicable energy code requirements.

occupancy sensorsStep 2: Choose the right sensor technology. Determine whether passive-infrared (PIR), ultrasonic or dual-technology sensors are best matched to the application requirements.

PIR sensors sense the difference in heat emitted by humans in motion from that of the background space. These sensors detect motion within a field of view that requires a line of sight; they cannot “see” through obstacles and have limited sensitivity to minor (hand) movement at distances typically greater than 15 ft. The sensor is most sensitive to movement laterally across the sensor’s field of view. The sensor’s field of view can be adjusted.

PIR sensors are most suitable for smaller, enclosed spaces (wall switch sensors), spaces where the sensor has a view of the activity (ceiling and wall-mounted sensors), and outdoor areas and warehouse aisles. Incompatible application characteristics include low motion levels by occupants, obstacles blocking the sensor’s view, mounting on sources of vibration, or mounting within 6-8 ft. of HVAC air diffusers.

Ultrasonic sensors utilize the Doppler principle to detect occupancy through emitting an ultrasonic high-frequency signal throughout a space, sense the frequency of the reflected signal, and interpret change in frequency as motion in the space. These sensors do not require a direct line of sight and instead can “see” around corners and objects, although they may need a direct line of sight if fabric partition walls are prevalent. In addition, ceiling-mounted sensor effective range declines proportionally to partition height. They are more effective for low motion activity, with high sensitivity to minor (hand) movement, typically up to 25 ft. The sensor is most sensitive to movement to and from the sensor. Ultrasonic sensors typically have a larger coverage area than PIR sensors. The sensor’s view cannot be adjusted.

Ultrasonic sensors are most suitable for open spaces, spaces with obstacles, restrooms and spaces with hard surfaces. Incompatible application characteristics include high ceilings (>14 ft.), high levels of vibration or air flow (which can cause nuisance switching), and open spaces that require selective coverage (such as control of individual warehouse aisles).

Dual-technology sensors employ both PIR and ultrasonic technologies, activating the lights only when both technologies detect the presence of people, which virtually eliminates the possibility of false-on, and requiring either one of the two technologies to hold the lights on, significantly reducing the possibility of false-off. Appropriate applications include classrooms, conference rooms and other spaces where a higher degree of detection may be desirable.

Regardless of sensor type, it should activate the lights as soon as the person enters the room, but should not monitor the area outside the door to avoid nuisance switching. The door swing should not obstruct the view of the sensor.

Table 1. Typical characteristics of common occupancy sensors. Adapted from 2001 Advanced Lighting Guidelines, New Buildings Institute.

Mounting Location Sensor Technology Angle of Coverage Typical Effective Range* Optimum Mounting Height
Ceiling US 360º 500-2000 sq.ft. 8-12 ft.
Ceiling PIR 360º 300-1000 sq.ft. 8-30+ ft.
Ceiling DT 360º 300-2000 sq.ft. 8-12 ft.
Wall switch US 180º 275-300 sq.ft. 40-48 in.
Wall Switch PIR 170-180º 300-1000 sq.ft. 40-48 in.
Corner wide view PIR/DT 110-120º To 40 ft. 8-15 ft.
Corner narrow view PIR 12º To 130 ft. 8-15 ft.
Corridor US 360º To 100 ft. 8-14 ft.
High mount PIR 12-120º To 100 ft. To 30 ft.
High mount corner DT 110-120º 500-1000 ft. 8-12 ft.
High mount ceiling DT 360º 500-1000 ft. 8-12 ft.
*Sensitivity to minor motion may be substantially less than noted above, depending on environmental factors.
PIR = passive infrared, US = ultrasonic, DT = dual-technology

Step 3: Select coverage pattern. Determine the range and coverage area for the sensor based on the desired level of sensitivity. Improper coordination between required coverage area and required sensitivity is a leading cause of application problems.

Manufacturers publish range (ft.) and coverage area (sq.ft.) for their sensors in their product literature, which may be different for minor (hand) motion and major (full-body) motion. Many different coverage sizes and shapes are available for each sensor technology.

In a small space, one sensor may easily provide sufficient coverage. In a large space, it is recommended to partition the lighting load into zones, with each zone controlled by one sensor; the sensors are networked together by low-voltage wiring. It is recommended that when creating zones, to ensure that sensor coverage overlaps by 20 percent.

Step 4: Select mounting configuration. Determine whether sensor should be installed at the wall switch, wall/corner, ceiling or task.

Ceiling-mounted sensors are appropriate for large areas that feature obstacles such as partitions, in addition to narrow spaces such as corridors and warehouse aisles. Units can be networked for control of areas that are larger than what can be controlled by a single sensor. Typically 2-3 times higher installed cost than wall switch sensors, but can be very economical if controlling large zones. High wall- and corner-mounted sensors are similarly appropriate for coverage of large areas that feature obstacles.

Wall switch (wall-box) sensors, relatively inexpensive and easy to install, are appropriate for smaller, enclosed spaces such as private offices with clear line of sight between sensor and task area.

Workstation sensors are appropriate for individual cubicles and workstations. The sensor is connected to a power strip for simultaneous control of lighting and plug-in loads such as computer monitors, task lights, radios and space heaters.

Step 5: Layout. Sensors should be located so that they have the least possibility of nuisance switching, activate the lights as soon as the person enters the space, and have a permanent unobstructed line of sight to the task areas. Another aspect of location is orientation. For example, ultrasonic sensors should be oriented toward the area of greatest traffic in a space.

Step 6: Specify the Sensor. Determine the specific feature set for the sensor and the power-pack, and whether the sensor must be integrated with other control devices.

First, if a common sensor is used, the power-pack must be specified. For extended life, it is recommended to use relays with zero-crossing circuitry. While most occupancy sensors are low-voltage, however, some are line-voltage and do not use a power-pack. They are suitable for applications where there is no plenum or junction boxes are hard to access. Be aware that line-voltage ceiling sensors may only have the ability to switch about one-third the load of a ceiling sensor that uses a power pack.

The sensor may be available with optional special features, such as self-calibration, manual-on operation, manual-off override, masking labels to adjust coverage area, bi-level switching (using two relays in the power-pack), daylight switching (works with photosensor), combination dimmer/occupancy sensor, isolated relay (separate low-voltage switch for interfacing with other loads such as HVAC), and connectivity to a digital network. Some occupancy sensors are small units integrated with light fixtures.

Step 7: Installation and Commissioning. Install the occupancy sensors according to manufacturer instructions and wiring diagrams. Commission the sensor by tuning its adjustable features. Commissioning is a critical, but often neglected, final step in a successful lighting controls project.

The first step is to verify proper placement and, if applicable, orientation of the sensors, so that they match the specifications and construction drawings.

The second step is to calibrate the sensor by adjusting its sensitivity and time delay settings. This should be coordinated with furniture placement, as occasionally furniture or equipment may be moved or relocated, which can affect sensor placement and/or orientation.

The occupancy sensor’s sensitivity level indicates how much movement will cause the sensor to activate the lights. Too high a level of sensitivity can increase the possibility of false-on triggering. Too low a level can increase the possibility of false-off triggering. Note that changing the sensitivity can result in changes to the coverage pattern.

Occupancy sensors are shipped with a factory setting for sensitivity, which can be adjusted in the field so that they respond appropriately to primary tasks in the space at the designed distance, while addressing possible sources of nuisance switching such as airflow.

The occupancy sensor’s time delay indicates how much time it will take to shut off the lights after the last motion has been detected. Longer time delays avoid continual on/off cycles of the lighting as people may go in out of the space frequently. Time delays also help to overcome brief periods of time when activity levels are low, as the sensor may not detect the occupant and therefore inadvertently switch the lights off. Typical factory settings are 5-10 minutes, which can be adjusted in the field. The shorter the time delay, the greater the energy savings but also the shorter the lamp life. In many applications, time delays of no less than 15 minutes are recommended.

After commissioning is completed, tell the users about the intent and functionality of the controls, especially the overrides. This is critical because if users do not understand the controls, they will complain and attempt to override or bypass them. Give all documentation and instructions to the owner’s maintenance personnel so that they can maintain and re-tune the system as needed.

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