Prevent and Treat Premature Lamp Burnout

Why it Happens and How to Avoid it.

Lamp burnout is the number one headache for both installers and homeowners. The CAST office gets many calls from both installers and homeowners complaining about lamp burn-out and all we can do is point out the most common causes - voltage that's too high or low, skin oil on the lamps and water splashing on a hot lamp.

Qaurtz-halogen 12-volt lamps are very sensitive to conditions of over and under-voltage. Voltages over 12v will overheat the filament, accelerating loss of its tungstem atoms, leading to a filament thinning and rapid breakage. Voltages under about 10v cause premature burnout for the same reason.

Lamp burn-out should not be a problem if you:

  1. Use good quality lamps. Some points to keep in mind:
    • use only  glass-covered MR-16 lamps. Open-faced MR-16's suffer rapid internal reflector damage.
    • MR-16 lamps have two main types of internal reflectors, dichroic and aluminum. Dichroic reflectors are designed to allow heat to pass through the back side of the lamp (good for display cases, bad for outdoor fixtures). Excess heat build-up inside a fixture can damage sockets and pins. For this reason, CAST supplies the Service Saver line of MR-16's with aluminum relfectors. We also use a heat shield behind the lamp inside all our MR-16 fixtures.
  2. Never touch the lamp envelope of a quartz-halogen lamp with bare fingers. Oils from your skin create hot spots that can burst the lamp. (Note: MR-type, PAR, and non-quartz-halogen lamps can be touched, but to be on the safe side, wear gloves when replacing all lamps.)
  3. Avoid situations where the irrigation system may spray water on unprotected lamps in Path Lights. If there's no way to avoid this, then work with the homeowner to set timers so the irrigation system turns on after the lights go off or reposition the fixtures.
  4. Aim for 11.0v (operating voltage) at each fixture. The ideal range is between 10.5v and 11.3v
  5. Only test voltages with all system lamps on (including the lamp you are testing).
  6. Make sure your meter is accurate at low voltages. Most meters in the $50 to $150 range are calibrated to 120v with an accuracy of +/- 2%, These inexpensive meters maintain this accuracy only at 120v and may be off by as much as 0.8 volts at 12v.
  7. Measure voltage at the fixture socket with the lamp in place. (Note: use our CTESTMR16 pigtail for MR-16 lamps)
  8. If you cannot measure the voltage at the socket with the lamp in place, test the voltage at the Spider Splice and subtract the following amounts from the meter readings:
    • If the lowest fixture wattage on the run is 20W, subtract 0.3 volts.
    • If the lowest fixture wattage on the run is 35W, subtract 0.5 volts.
    • If the lowest fixture watage on the run is 50W, subtract 0.8 volts.
  9. Record the voltage at the GFI outlet as a reference for future testing. A difference in 10v on the 120v side translates to a difference in 1.0v on the low voltage side. Household voltages can vary according to time-of-day, season, and the presence of other loads (especially air conditioners) on the system. Always check for these variations when burn-outs inexplicably occur.
  10. Replace burned-out lamps as soon as possible. When a lamp burns out, all other lamps on that run will receive a voltage jump. The amount of the jump depends on the voltage loss on the run and the number of fixtures. A worst case scenario would be a very long run (250 ft.) with (2) 50W fixtures. One lamp burns out and the voltage jumps 2.4v, enough to burn out the other lamp in very short order. An easy way to estimate the risk of lamp brunout on a run is to use the formula:
    Voltage loss/ number of fixtures = voltage jump when one lamp burns out
    A voltage jump of over one volt is a risky situation. It would be a wise strategy to increase wire gauge to minimize voltage loss on these high-risk wire runs. (Try out different wiring scenarios with our Online Calculator.)
  11. Replace all lamps once every 18 months. Many installers will schedule maintenance visits every Spring and Fall. With this schedule, you can do a complete lamp replacement in Spring of one year, the Fall of the next year, then skip a year and do another complete replacement the following Spring, and so on.
  12. Provide the homeowner with replacement lamps with instructions to replace them asap after burn-out. Be sure to mark all Fixture Record Tags with the correct replacement lamps.
  13. Make sure that lamps are securely seated in sockets, and that socket contacts are in good condition. It oftem happens that lamps fail to function not because of lamp burn outs, but rather that the socket and/or lamp contacts are making poor contact. Both situation lead to increased electrical resistance creating excess heat that damages the contacts and eventually causes them to fail.

Created on: 07/11/08