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    Updated: 10 May 2020


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Making sense of detectors

As building operators seek to gain enhanced control of energy costs, the use of movement detectors linked to lighting and other services is on the increase.

Here we explain the importance of knowing your detectors and how this can significantly reduce your costs and provide payback within a year.

In the building industry there has always been a tendency to specify a lighting system that provides the client with all they require, only to be cut back to the bare essentials or, at best, a lower specification when the rest of the project has exceeded the budget.

Itís a well known saying in the lighting industry that lighting is the first budget to be cut and the last thing to be ordered; and nearly always at the last minute!

Therefore, leaving lighting and other services running when they are not needed is still one of the most common causes of energy wastage in buildings. Whilst any system is cheaper to install at the time of initial building, the retro-fit costs neednít be as high as some are today. Ironically, once the building is completed and the handover has occurred, the tendency to leave the lighting system alone for a few months, or even years, is a common one.

Nevertheless, occupancy detectors, using presence or absence detection, are now being deployed more widely in buildings both at initial build and in refurbishment. The installation of even the lowest performing types of these items can reduce ongoing energy costs.

However, just because they perform the same basic function, this doesnít mean that all detectors are the same. They can vary greatly in both their functionality and controllability. Choosing the right detector for each space can make all the difference to achieving the maximum return on investment; after all, the additional costs of installation, wiring, commissioning and maintenance can be severely affected when using the wrong product.

There are two types of detector in common use Ė passive infra-red (PIR) and microwave. PIR detectors work by passively detecting a rapid change in the infra-red spectrum within their field of view. Microwave detectors actively transmit energy into the space and measure the reflected energy from the background. When a person, or item, enters or moves in the space, the pattern of the reflected energy changes and this is detected as a change in the Doppler pattern.

In all cases, itís important that the detector can distinguish between human movement and other movements, so that false responses are reduced, at worst, or eliminated. This requires in-built programming to analyze the size and pattern of the movement Ė something that is lacking from lower quality detectors.

The choice of detector should also be dictated by the space itís controlling Ė a decision that should be based on performance rather than price. In particular, range and direction make a significant difference to the performance of the system and making the right choices can also minimize the number of detectors required for effective control.

Corridors are a case in point. People will often install a basic detector with 360o coverage in a corridor and, while this will work in most cases, itís far from being the best solution. By using an opposing radial detector with a range of eight metres in each direction, provided more accurate coverage and also reduced the number of detectors needed to cover the space. In fact, with the right choice of detector, many corridors can be served perfectly adequately by a single detector.

Similarly, in open plan spaces there are considerable benefits from being able to control lighting in zones to respond to changing occupancy patterns. To achieve this effectively it may be necessary to use a close cell detector with very precise adjustment capabilities, or a detector with wide coverage which is narrowed down to enhance the local sensitivity. Better still, use a PIR which detects Ďtrue presenceí like the IDW or IDS office detector and not just merely claim to have presence detection. The IDW or IDS can detect a person sitting still at a computer desk up to three metres away from the detector and walking up to 18 metres away in any direction around a 360o plain.

Where detectors are being retrofitted to an existing lighting system, they are most likely to be operating on a stand alone basis rather than simply sending signals to a lighting management system. In these cases, the detectors need to have the capabilities to deliver the required control functions themselves so a basic detector that just switches lighting on and off will not fit the bill. For example, to achieve maximum energy savings it is generally desirable to use an integrated movement detector and light detector, so that lighting is only turned on when ambient light levels need to be supplemented.

However, there are some installations where linking to a full Building Management System is required. By using potential free detectors, the BMS can be triggered by the detector in one of several ways but the BMS is responsible for the power control. For example, IDW or IDS detectors to link in with a full BMS where the detectors are just set to send a pulse trigger to the BMS, even though they could have been set to any number of seconds between 1 and 900, or any number of minutes from 1 to 15. By using intelligent, easy to install, potential free detectors the client saves money on excessive adaptation of the BMS, fewer ramping cards, etc because of the flexibility of the detector.

Sometimes, where the level of activations equate to the lights to be on during darkness for 80 - 90% of the time, the saving created by turning the lights off for 10 - 20% of darkness may be less of a saving than may be originally thought and a simple photocell will ensure long lamp and control gear life.

These are just a few examples of how detectors can vary in the levels of performance and functionality they can offer and why itís so important to consider what is required of each space. Taking the time to understand the lighting control needs of the space and choosing detectors accordingly will make all the difference to the energy savings that are achieved through the life of the installation. Careful and correct consideration of the type of detector and its specific location will also lead to other savings:

  • Time (preparation, installation, commissioning and ongoing maintenance)

  • Materials (cable, connectors and interface cards for BMS)

  • Improved lamp and control gear life

  • Reduced carbon footprint!

In summary, installing fewer, but better quality, detectors will ensure you save more money than you think. Typical payback is under two years but if you use the correct advice and products you can get this down to as low as six to 12 months!

Motion detector, surface room - IDS









Motion detector IDS c/w light detector



Perspective view

Recommended mounting height beween
1 and 1.8 metres.








Motion Detector, room - IDR









IDS c/w light detector


Motion Detector, ceiling - ICD








Several PIR occupancy detectors

In open plan areas.
For best coverage the PIR occupancy Detectors should be spaced every 5 meters in either direction.


Lighting equipment is set to be controlled in small zones instead of individually, which gives a more pleasant working environment.

Lighting equipment in corridors is controlled by the presence in other areas and is turned off when all spaces are unmanned.

The environment in a building feels safer when the lights are on in the corridors.

The time delay after the last observed movement decides when the light is to be switched off.

An adjustable delay option enables the user to select the most suitable times for each situation.

The light is turned off when the time delay has elapsed.


E-mail: lars@fjellcom.se     

Phone: +46(0)31-287202

Mobile: +46(0)706-755561



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