Fluorescents and HID

Group relamping is a concept that applies to fluorescent and high intensity discharge. These lamps tend to have predictable end of life where incandescent lamps are subject to random failure.

The logic of group relamping reflects the total cost of ownership concept where labor is a much greater cost than the lamp itself or any remaining life it may have. Fluorescent and HID lamps are typically used for ambient lighting and typically mounted very high from the floor. It may be necessary to get a man lift to replace these lamps because of mounting height. By replacing lamps in an area based on schedule rather than failure labor savings can be as much as 50%.

Group relamping makes sense for discharge lightingLamp Service Life

Lamp service life is a point stated in hours when 50% of a large group of bulbs will still be illuminated. Looking at it the other way, 50% have failed. Service life is the average life of a bulb not a guarantee. Service life can be reflected by a normal distribution as shown above.

Failures of incandescent lamps, because of they use filaments to generate light, can occur in less than 1000 hours. It is possible a lamps could last twice as long as the service life. The reality is failures are likely to be grouped very closely around the average.

Fluorescent and HID lamps do not have a filament. The parts and processes used to generate light are fairly predictable and are more likely than incandescents to be tightly grouped around the average. The service life for fluorescents and HID lamps are typically longer than 10,000 hours and could be as long as 40,000 hours.

HID Lamp Lumen Depreciation

All lamps have some lumen depreciation but certain types of high intensity discharge have a very high light loss. Lumen depreciation means the bulbs puts out less light as it ages. That could mean that depreciation of 33% could be experience by the time the bulb uses 40% of its life. The depreciation continues so it is unlikely an HID bulb would reach 100% of its service life. It should be replaced at 60 to 80% depending on the tasks performed in the area. Note: Lumen depreciation is not a problem for fluorescents that maintain 95% of their output to service life.

Special Considerations

Service life of the lamps is a good starting point for scheduling group relamping but adjustments need to be made for certain fixtures. Interior night lights, for example, are typically left on 24 x 7 and will fail before lamps used only when the business is open.

Occupancy sensors will also spread the burn hours over a longer calendar time. Occupancy sensors need to be set so there is a minimum burn time to maximize the service life of the lamp. Flipping a fluorescent lamp on and off will significantly shorten the life of the lamp. Allowing the lamp to remain on for 15 minutes achieves an appropriate balance.

Schedules for different areas of the building may also be a factor. Some distribution centers may only receive loads for an 8 hour period but will prep and load on a 24 hour schedule. The receiving fixtures will be on a different group relamping schedule because of the shorter schedule.


The primary benefit of group relamping is lower maintenance cost. Properly maintaining the lighting system will lead to better light levels and therefore higher productivity.