Shedding light

Until recently it has not been possible to dim high wattage, metal halide lamps. Consequently, industrial premises using high bay fittings have had to forego the benefits of lighting control techniques such as dimming light output in relation to natural daylight levels.

Holophane Europe has designed a system to tackle this issue. Its system will dim metal halide lamps rated up to 400 W, so contractors may soon find themselves installing lighting control systems for warehouses and shed-type retail outlets.

These types of buildings often have a partly-glazed roof, so are prime candidates for saving energy by making better use of daylight. Now that the Building Regulations encourage the use of lighting controls and a certain amount of daylighting, it is hardly surprising that many building operators are keen to introduce these systems.

From the contractor's point of view, it is important that the controls are easy to install and commission, otherwise they may need to bring in specialist subcontractors. Ideally, the control system will be installed at the same time as the mains cabling, saving considerable time.

Light distribution
In a recent extension to Schneider Electric's warehouse and distribution centre in Telford, the controls system and mains cabling were included in the same installation scheme. Here, 138 high bay luminaires using 400 W ceramic metal halide lamps are installed. This was one of the first installations of a new system that combines Holophane's Prismatron system with a Schneider Electric Canalis busbar carrying 230 V mains power and integral low voltage control signals.

What makes the project different is the ability to dim the lamps using the Prismatron ballast. This is the first system capable of dimming such high wattage metal halide light sources. "We opted for Prismatron because it enables us to make savings by using dimmable lighting in areas that are not used very often and at times of day when the lighting does not need to be at full output," explains Schneider's Mike Biggs.

The warehouse is divided into four quadrants for the purposes of lighting control. A lux sensor in each quadrant is used to maintain an illuminance of 280-300 lux. The assembly area for control panels is divided into two sections, each with a lux sensor maintaining illuminance levels at 400 lux.

"Each of the lux sensors produces a 0-10 V control signal in direct proportion to the lux levels, sending the signal to a Schneider plc building controller, which in turn communicates with the Prismatron dimmable ballasts," says Biggs. "In this way, the lamps are dimmed or ramped up in direct relation to the lighting requirements of the space."

The system was installed by Kendall Electric on a design and build basis. It uses 138 Prismatron luminaires connected in pairs to Prismatron remote-dimmable ballasts. The luminaires are suspended directly from a Schneider Electric Canalis busbar carrying 230 V mains power and integral low voltage control signals.

"Light levels in the warehouse area are set to 280-300 lux in the aisles and 400 lux in the low voltage assembly area," recalls Kendall's Bruce Gregory. "The fittings were very easy to install, hanging directly from proprietary clips direct from the busbar. Connections between the ballasts and the control wiring use RJ45 connectors, while the light fittings themselves connect to the 230 V mains supply with a plug and socket connection. These features made a significant contribution to the duration of the project and enabled us to install the lighting and associated controls in just three weeks," he adds.

In the aisles between racking, occupancy detectors are used in conjunction with the dimming system, reducing the lamps to 25% of their full output when no movement has been detected in the area for ten minutes. When someone walks into the aisle the lamps are increased to 100% output immediately. This is achieved through the Schneider building controller.

With conventional on/off switching of metal halide lamps this rapid response to changes in daylight levels or occupancy would not be possible because of the warm up time required by high intensity discharge lamps. As Prismatron dims the lamps, rather than switching them off, they are able to change the light output within seconds of receiving the appropriate control signal.

The big experience
Similar principles were applied to the new Woolworths Big W store in Aberdeen, where Woolworths was keen to save energy by making optimum use of daylight. "I said that I wanted prismatic glass, high bay fittings with metal halide lamps that could be dimmed in relation to the amount of daylight entering the space," states Woolworths' David Gray. "Most companies said that it couldn't be done with higher wattage discharge lamps but Holophane took a more positive approach," he adds.

The lighting solution uses 314 Prismatron luminaires with 320 W metal halide lamps in the store area. These are directly suspended from the multi-pole busbar system. They are spaced on a 4·5 m grid at a height of 5·7 m to the underside of the fitting.

Each pair of luminaires is connected to a remote Prismatron dimmable ballast, which is linked to a Canalis busbar system supplying mains power and an integral low voltage control wire. "The ballasts are connected to the control wiring using RJ45 connectors and the fittings use a plug and socket connection to the 230 V mains supply," explains electrical contractor Bruce McFarlane, then of Albion Electrical and now with PPE Electrical. "This arrangement helped to ensure a fast and simple installation, so the extra control element could be introduced at the same installation cost as a conventional Woolworths Big W store," he continues.

The 0-10 V analogue control system is supplied from the building's plc control installation, which is a special system developed by Schneider Electric and provides an interface to the Prismatron system. The 230 V supplies to the luminaires are provided via the multi-pole contactors located within the main switch panel. The trading area is divided into four zones of luminaires, each connected to a photocell mounted at the same 5·7 m height as the undersides of the light fittings.

During the trading day the system maintains a horizontal illuminance of 900 lux at 800 mm and a vertical illuminance of 450 lux up to a height of 1·8 m. This is achieved without using the full light output of the lamps to ensure that it can be increased to maintain the illuminance levels as they age.

Outside trading periods the illumination level is set to fall to 225 lux, providing enough light for cleaning and restocking of merchandise and generating a 50% energy saving during non-trading hours. Earlier stores had a similar arrangement, but this was achieved by turning off half of the luminaires. "In the older stores we needed two control circuits to switch off half the lighting, but at Aberdeen all of the lighting can be dimmed from one supply, so there was less installation of control cabling," notes McFarlane.

These projects illustrate the benefits to shed-type buildings of the dimming of high bay luminaires. As well as the cost of ownership savings to the end-user, the contractor benefits from faster, easier installation and straightforward commissioning.