All metal halide lamps produce light from an electrical discharge contained in an arc tube. An outer glass envelope or “bulb” surrounds the arc tube. If the arc tube ruptures, the debris, including halide compounds, relies on the outer glass of the bulb for containment.
When buying metal halide bulbs, you should see a specification called “luminaire requirement.” This specification addresses the issue of arc tube rupture.
For some bulbs, the luminaire requirement value will be enclosed, for others, open.
Enclosed-rated metal halide bulbs rely on the fixture lens or diffuser as another layer of protection. This layer is designed to contain the debris if an arc tube breaks and the outer glass of the bulb also shatters.
Open-rated metal halide bulbs are manufactured with an internal safety shroud around the arc tube. In addition, the outer glass jacket of the bulb meets ANSI test requirements for containment of arc tube rupture. These two features replace the need for the fixture to contain breakage and allow the bulb to be operated in an open fixture.
Another difference between enclosed and open rated metal halide bulbs is the mogul base. Open rated metal halide bulbs have a modified mogul, EX39 base. EX, or ‘exclusionary’ mogul base has an extended center contact designed to reach the deeper socket contact found in open fixtures such as high bay. A standard mogul base, found on enclosed-rated bulbs will not fit these sockets.
Why use the generally higher-cost open-rated metal halide bulbs? Fixtures without a lens or diffuser, using open-rated bulbs, produce at least 15% higher total useful illumination and lower maintenance compared to lensed fixtures operating the same wattage enclosed-rated metal halide bulbs.
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