Occupancy linking control
  
  
 

Occupancy-linked controls turn off the lights once they have failed to detect occupancy for a set time. When occupancy is detected, they usually switch the lighting on again. In a few cases (off-only systems), they leave the lighting off.

The occupancy linking controls can either:

  • Employ an independent controller and/or power supply. In this case, they may be mounted on the wall or in a corner as well as on the ceiling. They were the first types of motion detector to be used in lighting applications and remain the most popular type of sensor in use. Installation of these units requires opening the ceiling or wall, since they must be hardwired to the electrical distribution system. This result in a relatively high installation cost for retrofit applications.
  • Have all components in a single housing and are easily wired into existing switch boxes in the room. These are more recent products, primarily designed as retrofit replacements for common wall switches in smaller office. There is little design flexibility since the wall box location is fixed, generally at 110 cm above the floor. Another disadvantage is that room partition and furniture will limit coverage. However, in the appropriate spaces (small offices and open conference rooms that have wall switches) wallbox are very cost effective as the devices are fearly cheap and the installation cost is almost negligible.
Occupancy linking sensors fall into three categories:
  • Passive infrared (PIR) occupancy sensors operate by responding to the motion of infrared energy (or heat) produced by human bodies. They use one or more pyroelectric detectors located behind an infrared-transmitting, segmented lens. The detector’s field of view is typically divided into detection zones. Lights are turned on when the sensor detects the motion of a heat source across a detection zone boundary whitin a defined period. Passive infrared sensors are “line-of-sight” devices that need an unobstructed view of motion to operate effectively.
  • Ultrasonic occupancy sensors operate by responding to the change in reflected sound waves in a space caused by a moving object. Ultrasonic sensors operate at frequencies that are above human sensitivity (20 kHz), typical operating frequency are 25, 30 and 40 kHz. They do not require a direct line-of-sight to detect motion, unlike passive infrared sensors, and they will detect smaller motions.
  • Ultrasonic-infrared occupancy sensors, also called “dual” or “hybrid” sensors, combine passive infrared and ultrasonic technologies. They keep lights if either technology detects motion. These products reduce the likelihood of luminaires turning off the lights while the space is occupied.
Energy savings for any particular occupancy sensor will vary considerably depending on the size of the area covered and the occupancy pattern but typical values are 35% to 45%. It is important to have a time delay built into the system, since the occupant may remain still or quiet for short periods while remaining in the space, but would not wish the lighting to be extinguished before he has actually left. Care will need to be taken when choosing the type of presence detectors as differing types sense in different degrees of sensitivity.

Whether or not occupancy-linking control is suitable depends on the occupancy pattern of the space. The most appropriate applications for occupancy sensors are in spaces where occupancy patterns are intermittent or unpredictable such as cell offices, copy or storage rooms, hallways, classrooms, service corridors, toilet rooms, closet and warehouses.

Occupancy sensors should not be used with high intensity discharge lamps except in a few specific circumstances. This is because these lamps have extended long warm-up periods and can take several minutes to restrike after having been extinguished.

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