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Example:
Consider a room rated as Class I, Div I for Group D, which includes gasoline vapor. Gasoline vapor will ignite at a relatively low SIT (Spontaneous Ignition Temperature) of 232 C˚ (or 495 F˚). So, if a light fixture has a hot spot on the outside of the aluminum fixture housing, near the Driver’s location, that measures 260 C˚ (or 500 F˚) after being lighted for 12 hours…you have a big problem. A lighting fixture with a hot spot measuring 260 C˚ would be given a temperature rating a ‘T’ rating of T2B (Refer to Chart).

If you used this fixture in the room with gasoline vapor you will get a BOOM! The hot spot on the fixture (260 C˚) will ignite the gasoline vapor, which ignites at 232 C˚. But - if you used a T5 rated light fixture, it will only have an exterior hot spot no higher than 100C, and it will provide a margin of safety - since it will be 132 degrees C below the SIT of gasoline. The T5 rated fixture does not have a hot spot that will ignite the gasoline vapor.
Click the animation on the left to view or download the reference chart.
While the flammability of gases and vapors is more easily understood, the hazards associated with explosive dusts and hot surfaces are just as risky.

Dust clouds from any source shown in the chart below may explode including a layer or accumulation of dust. Dusts layers may ignite due to hot surfaces and cause a fire hazard. A wide variety of workplaces may contain activities that produce explosive or potentially explosive dust atmospheres. Dust explosion hazards are common in industries like coal mining, storage and processing of agricultural products (starch, flour, sugar, cocoa) and organic dusts (drugs, dyestuffs, plastics), and the manufacture of metal powders (aluminium, magnesium). Machining operations producing combustible dusts, such as grinding, drying and dust collection are always exposed to explosion risk.
Bill Nagengast, President and Lighting Engineer, Solas Ray Lighting, holds over 20 patents in the lighting industry.
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TJ-14 2.23.18