Metal halide lamps occasionally fail "non-passively" at end of life, which may cause hot quartz elements to be ejected from the light fixture, presenting a fire risk.

Although more than 100 million metal halide lamps have been used over the past 10 years in the United States, there have been few cases of ruptured lamps producing property damage claims, according to the National Electrical Manufacturers Association (NEMA).

To help further reduce risk, NEMA recently revised LSD25-2008, “Best Practices for Metal Halide Lighting Systems Plus Q&A About Lamp Ruptures in Metal Halide Lighting Systems,” a detailed white paper about how to choose, operate and maintain metal halide systems with special emphasis on minimizing risk of lamp rupture.

Reducing the fire risk associated with lamp rupture means using enclosed light fixtures, Type-O lamps or both. The 2005 National Electrical Code (NEC) requires that all fixtures using metal halide lamps—except for thick-glass PAR lamps—either be enclosed (and use Type-E, Type-S or Type-O lamps) or, if open, have some type of mechanism ensuring that only Type-O lamps can be used in them.

Enclosed fixtures feature some type of protective lens or shielding that can completely contain the effects of a non-passive lamp failure. Type-O lamps are protected lamps, typically for 175–1,500W sizes, that have additional containment around the arc tube. They feature a special base (EX39), so they can only operate in compatible special sockets.

As more states adopt newer versions of the NEC, and as existing installations are gradually replaced or renovated, these solutions are slowly becoming the norm.

Type-E lamps are limited to use in enclosed fixtures, Type-S lamps may be used in either open or enclosed fixtures. To determine the type of lamp, check the ANSI code printed on it—e.g., M59/E. If the fixture is listed, lamping and relamping information—specific lamp wattage and lamp type—is provided on a fixture label and may be included on an instruction sheet that comes with the fixture.

In new construction projects (requiring an electrical permit) where the 2005 or 2008 NEC is not in effect (about 11 states as of June 2008), or in existing installations anywhere, Type-S lamps in open fixtures are likely to be encountered due to a lower lamp initial cost, higher fixture efficiency and shorter restrike times. These are more common in high-bay applications, as low-bay metal halide fixtures typically feature a lens to reduce direct glare. Limited to the vertical operating position and 360–1,000W sizes, these lamps are rated for use in open fixtures provided the manufacturer instructions are followed to the letter.

In such applications, NEMA offers guidelines for minimizing rupture risk.

Fixture placement: Avoid placing open fixtures with Type-S lamps over flammable materials or vice versa if the fixtures are already installed. For example, fixtures in warehouses should be located in the center of the aisles.

Maintenance: Users should pay attention and follow lamp and fixture manufacturer warnings and instructions related to safety and maintenance. For example, group relamping is recommended and should occur at the time recommended by the lamp maker. If the lamps in an installation run 24/7, they should be “cycled,” meaning they should be turned off for at least 15 minutes at least once per week. Special cycling controllers (such as time clocks) can be installed in both new and existing installations to provide cycling. Note that standard metal halide lamps take 12–20 minutes to hot-restrike, while pulse-start metal halide lamps take 1–2 minutes.

Fixtures: The fixture should be listed to UL 1598 or CSA C22.2 No. 250.0 by an accredited third-party testing lab. It should not be used in applications where the ambient temperature exceeds the fixture’s temperature rating (usually 25°C or 77°F, unless the fixture is designed and rated for higher temperatures). Ensure all components are installed properly, particularly the lens if the fixture is an enclosed fixture. If an existing fixture has a damaged lens, the lens should be replaced immediately. If the owner wants to add a protective lens to an existing open fixture, consult the manufacturer to ensure proper containment of a lamp rupture and maintenance of components within approved temperature limits.

Ballasts: The ballast should comply with UL 1029 or CSA 22.2 No. 74.0. Ensure the ballast is designed to provide the required lamp wattage. If dimming is required, NEMA recommends the lamp not be dimmed below 50 percent of rated power to avoid degradation of service life and performance as well as possibly voiding the lamp warranty. NEMA also suggests the lamps be operated at full power for at least 15 minutes before dimming (unless a voltage interruption extinguishes the lamp and then the lamp is restarted, in which case 30 minutes is recommended).

Download the revised LSD25-2008 Best Practices for Metal Halide Lighting Systems for free at www.nema.org.

Thanks to Edward Yandek, manager, Industry Standards for GE Consumer & Industrial Lighting for his assistance in developing this article.

DILOUIE, a lighting industry journalist, analyst and marketing consultant, is principal of ZING Communications. He can be reached at www.zinginc.com.