How to Determine the Optimum Group Relamping Schedule
It is generally understood that the largest cost of a lighting system to a commercial user is not the initial cost of the fixtures and installation, but of energy and maintenance costs. For most applications the largest lighting maintenance cost is relamping, (i.e., changing the burned out light bulbs). Usually its not the cost of the lamp itself that is expensive (although that does need to be factored), but the labor involved, and the disruption to business. One of the best ways to minimize these costs is to use a practice called “Group Relamping.”
With the use of Group Relamping procedures, all of the lamps in a given area are installed at once and then at a predetermined time in the future all of the lamps are replaced, before they start burning out on a regular basis. This allows a trained maintenance crew to schedule access to an area, bring in all the necessary equipment and supplies, work in an efficient manner (possibly at night, on the weekend or some other scheduled downtime), and then leave that area until the next scheduled Group Relamping. Some of the lamps that are not burned out are retained for future spot relamping until the next group relamping cycle.
Determining the Group Relamping schedule is based on a number of variables that contribute to what are called Light Loss Factors (LLF). LLFs are conditions that degrade the light output of a lighting system, and maintaining a proper light level is important to the operation of any facility.
Lamp life is the most obvious LLF and this data can be obtained from the manufacturer’s literature. The way that a manufacturer determines lamp life (LL), is by taking a large sample of the lamp to be tested, (maybe 100 or so), and cycling them on for 3 hours and off for 15-20 minutes, over and over until 50 percent of them burn out. That time is stated as the lamp life. So by definition, half of the lamps will last longer than the rated LL and half less. This is why we don’t see all the lamps in an area burning out at the same time on the same day. This variation in LL makes it difficult to predict when to change the lamps on a scheduled basis. Additionally, LL for most lamp types varies depending on burn cycle (i.e., the time between on and off periods). The longer the burn cycle the longer the LL. (This does not mean that a lamp(s) should be left on when a space will be unoccupied for a long period).
Related to Lamp Life is annual use. Annual use is simply how many hours a given lamp is on per day, (or ultimately per year). A fixture that gets very little use will not need to be serviced as frequently as one that is used every day. For most commercial operations annual use is easy to predict with a reasonable degree of accuracy.
Another of the Light Loss Factors is lamp lumen depreciation (LLD). All common light sources get dimmer as they age. The question is, how much dimmer and how quickly. Some types loose a lot of light rather rapidly (8’ T12/VHO Fluorescent looses 25 percent light by 40 percent lamp life and 400 watt Metal Halide looses 33 percent by 40 percent lamp life). Others maintain their light output very well over their life (4’ T5/HO looses less than 7 percent by 40 percent life). Some lamp types, such as Mercury Vapor and old style circular fluorescent will even continue to burn long after their rated life, but deliver far less light than when they were new. This can create a real maintenance problem because it is hard to identify a fixture whose lamps are still glowing, but are no longer delivering the proper light level.
So, the trick to determining the Group Relamping interval, is to find that point when the lamps in an area will start burning out on a regular enough basis to make it cost effective to just replace them all even if they aren’t burned out yet. This usually works out to 70 percent to 80 percent of rated lamp life (after allowing for burn cycle changes). There will be some lamps that will burn out prior to this time and should be spot relamped as needed. This percentage can be adjusted earlier or later than, 70 percent to 80 percent depending on the following factors:
- How critical it is to maintain design light levels, knowing that Lamp Lumen Depreciation and early burn out will lower light levels in the area. If there are critical tasks in the area, waiting too long could introduce safety or productivity concerns.
- How important is the appearance of the space, and will burned out lamps make the space appear shoddy. For example, if this is a retail store or corporate meeting room, burned out lamps may reflect poorly to customers and clients.
- How difficult will spot relamping early lamp burnout be. If the lamps are easy to access, then maybe spot relamping the area until closer to 80 percent LL (when the lamps will start to burn out much more frequently) could be cost effective. For instance, if all that is required is a short stepladder in an aisleway that has open lamp fixtures (like in a low ceiling warehouse), then more frequent spot relamping may be viable. Remember to use the old lamps from the last group relamping cycle. Group relamping will still need to be done to maintain consistent and proper lighting and to minimize workplace disruption, but not until closer to the end of rated lamp life.
Ideally, the lighting system in a facility should provide the optimal light levels as inexpensively as possible. Maintaining that system diligently ensures optimal lighting, but maintaining the system intelligently ensures that it is done with maximum savings. Group Relamping is one of those intelligent steps.
This article has been adapted from an article titled "The Logic of Group Relamping" by Eric Strandberg, Lighting Design Lab, Seattle, WA.