Lighting control ranges from simple wall switches to complex dimming systems networked with other systems.
In some industries, lighting accounts for more than 60% of a facility's electrical bill and 40% of the total energy bill. Add indirect costs, such as increased loads on cooling systems and increased luminaire maintenance, and the total can be even higher. As a means of offsetting high energy costs, many codes and standards, such as California Title 24 and ANSI/ASHRAE/IESNA 90.1, Energy Efficient Design of New Buildings Except Low-Rise Residential Buildings, require some type of automatic lighting control system for all new construction and major renovations. Even when not required by Code, designers often include automatic lighting control as a financial benefit for their clients.
Lighting control can range from simple wall switches to complex dimming systems networked with other building systems. Each lighting control system has a unique set of capabilities and price points. It's usually up to you to decide which system will perform best for the building owner.
Because lighting needs vary with the intended use (for example, lighting offices, corridors, cubicles, and training rooms) and characteristics of the area (such as room size and shape, ceiling height, and availability of natural light), most buildings contain more than one type of lighting control system. Mixing the available technologies often results in the most cost-effective approach. The Table, on page 42, summarizes the commonly available lighting control techniques.
By combining control methods that include manual, scheduled, and occupancy with the on/off and dimming actions they perform, you can design an effective and economical lighting control system. Let's look at each method and action separately and then see how they can work together.
It may seem simple, but on/off operation is an area where many designers create an unworkable lighting scheme. For example, consider the metal-halide lighting system in the Photo, on page 40. Restrike time, which refers to the time it takes a lamp to begin giving off light after being turned on, is crucial for this type of system. Once metal-halides are shut off, they take several minutes to begin giving off light again after being turned back on. If all of your lamps are metal-halide and you shut them off at night, you'll wait 15 min for a reasonable level of light when you turn them on the next day. By adding other types of light, as well as dedicating certain fixtures to an “always on” configuration, you can reduce the effect of the restrike time. In planning the layout of your lighting controls, make it obvious which lights should not be shut off, and pay special attention to exit path lighting.
When you plan dimming, consider how long it takes for a lamp to go from its floor dimming level to 80% output. The effective “floor” of dimming for fluorescent lamps is 20% — you won't see any energy savings below that level. The effective floor of dimming for metal-halide lights is about 50%, because you are effectively restriking the lamp below that level. Be careful where you place your sensors and how you aim them. You want the lights to come on whether a person or a lift truck enters the area, but you don't want adjacent traffic to cause the lights to dim up and down all day. When you dim lights based on ambient lighting, a time delay on the dim-down will eliminate nuisance dimming.
Manual lighting controls
Manual lighting controls range from a single switch to a bank of switches and dimmers, that are actuated by toggles, rotary knobs, push buttons, remote control, and other means. Manual controls are the most cost-effective options for small-scale situations. However, as the size of the lighting system grows, manual controls lose their cost-effectiveness. But they can still be an important part of a larger plan, as evidenced by the effectiveness of task lighting with manual controls.
Scheduled lighting controls
When you have a predictable occupancy pattern, scheduled lighting controls are often your best option. You can add special manual overrides to make this work when the area needs light outside the normal hours. Manual controls typically work in conjunction with the scheduled controls to override them for a preset time. You should always leave an exit path lit, regardless of the occupancy schedule. If you are unsure whether such a configuration is necessary for your lighting situation, refer to the Life Safety Code, NFPA 101, as well as state and local regulations and fire codes.
The most important thing to consider with occupancy controls is the zone concept. Imagine you have the lighting controls tied into your building's access card reader. When Bob cards in on a Sunday afternoon, you don't want the whole facility to light up. Instead, you want the lights leading to, and inside of, his office to turn on. The copy machine near Bob's office and the water fountain will also power up. Suppose he needs to visit another part of the building. Motion sensors can track his progress and light up the area ahead of him. As he passes into the next zone, the sensors could turn off the lights behind him or leave them on for a preset time (perhaps an hour). However, you don't want the lights to shut off while Bob is sitting at his desk without moving or while he is working behind a partition and beyond the range of the sensors. Occupancy controls, when applied correctly, improve the usability, security, and efficiency of a building. If applied improperly, however, they force the owner to bypass them or remove them altogether.
Good electrical design
Regardless of the system you choose, it's important to remember lighting control systems are electrical switching systems with lighting loads. As with any electrical system, you must observe the same Code rules and design practices relating to overload, short-circuit protection, and grounding. However, misapplications of lighting control devices with limited short-circuit current ratings are common occurrences. These underrated devices may remain in service for many years without incident.
Combining control schemes
Many office, retail, or industrial buildings have been successful in using schedule-based systems as the backbone, supplemented by occupancy sensors and manual switches for smaller offices and special-use areas. The backbone system:
- More easily handles the large amounts of power needed for larger areas.
- Switches HID lamps.
- Ties into building automation systems where desirable.
Schedules can accommodate the large number of people who share open areas, while allowing people to override the system for special circumstances or emergencies. However, the schedule system does not work as well for small areas where the variable work schedule of one person may drive the need for lighting. In those cases, an occupancy sensor or manual switch works well. If you are switching exterior lights, you'll probably need a more robust device than what you are using inside.
Building the backbone
When you lay out the lighting control system, you are building what most designers refer to as “the backbone system.” Planning at this stage is crucial to success. Do the electrical design before working out the details of the control scheme. To do this right, you need to address the following key considerations:
Electrical switching capability. Be sure your lighting control system can handle the steady-state current, lamp inrush, ballast harmonics, and available fault currents. You'll often have trade-offs among these factors. For example, a “low-harmonic” ballast will result in a higher inrush — which your system might not be able to handle without significant modification.
Mounting location. The “brains” of lighting control systems should be mounted near the lighting panelboards in the electrical closets. In most cases, however, the owner and the installer typically leave insufficient room for this installation. To paraphrase a rule of carpentry, “Measure twice, install once.”
Schedules and override. Changing schedules should be easy. Create a flexible design that allows for different schedules for areas of the building with different needs and alternate schedules for weekends and holidays. Be sure to include overrides by wall switch, telephone, or network interface for unusual circumstances.
Sufficient circuits and zones. To maximize savings, zones must be sufficiently small; you don't need to light up an entire floor to accommodate one person who works late. On the other hand, zones that are too small result in extra circuits and installation expense.
It's easy to see why some lighting-controls projects render mediocre results and why others result in systems that owners show off to visitors. By choosing the right combination of controls, you'll have a system that falls into that second category, and by basing that system on a solid electrical plan, you'll provide a reliable system with a low total cost of ownership.
Jordan is a Power Link marketing manager with Square D, Palatine, Ill.