Making sense of sensors

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.
It’s 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 needn’t 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 sensors, 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 doesn’t mean that all sensors are the same. They can vary greatly in both their functionality and controllability. Choosing the right sensor 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 – 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, it’s important that the sensor can distinguish between human movement and other movements, so that false responses are reduced, at worst, or eliminated. This requires in-built programming to analyse the size and pattern of the movement – something that is lacking from lower quality sensors.

The choice of detector should also be dictated by the space it’s controlling – 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 minimise the number of sensors required for effective control.

Corridors are a case in point. People will often install a basic sensor with 360o coverage in a corridor and, while this will work in most cases, it’s far from being the best solution. The Park Plaza Hotel Group were told they would require almost 40 360o recessed PIR’s for the corridor on a single accommodation floor at the Riverbank Park Plaza. By using an opposing radial detector with a range of eight metres in each direction, the IS345, Steinel provided more accurate coverage and also reduced the number of detectors needed to cover the space. The installation now only required 11 sensors per floor. In fact, with the right choice of detector, many corridors can be served perfectly adequately by a single detector. The Park Plaza Hotels now specify Steinel sensors throughout their hotels in Europe, 24 in total, and all hotels are designed in Relux, prior to any work being carried out; ensuring that the sensors are installed in the right location, for best detection, first time!

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 sensor 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 ‘true presence’ like the BLS office sensor and not just merely claim to have presence detection. The BLS can detect a person sitting still at a computer desk up to three metres away from the sensor and walking up to 12 metres away in any direction around a 360o plain.

Where sensors 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 sensors 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 sensor, 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 sensors, the BMS can be triggered by the sensor in one of several ways but the BMS is responsible for the power control. For example, the Addington Street Hotel, being built by Galliard Construction Ltd, will utilise the Steinel 3rd generation range of sensors to link in with a full BMS where the sensors 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 sensors the client saves money on excessive adaptation of the BMS, fewer ramping cards, etc because of the flexibility of the sensor.

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 – as was the case when BAA chose the NM5000 for the control tower at Heathrow’s Terminal 5.

These are just a few examples of how sensors can vary in the levels of performance and functionality they can offer and why it’s so important to consider what is required of each space. Taking the time to understand the lighting control needs of the space and choosing sensors 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 sensor 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, sensors 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!