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TJ-1 12.1.17
Size Matters
If you look at the three Hazardous Location Classes, you can see that this system is a categorization of materials that can create fires. These are described by a word unfamiliar to many - ignitables - taken from the word ignite, which means "to start to burn, or burst into flames."

These Classes focus on the materials (ignitable) that can become fuel for a fire. As seen previously in the Fire Triangle, with the requirements for a fire: Oxygen, Fuel and a Source of Ignition, the three Classes cover the Fuel side of the triangle.
And as you closely examine these Classes further, you can see that the ignitables have larger particles as the Class number increases. Class I has gases - very very small particles. With Class I gases there are some that naturally exist as gases, and others that become fumes as they evaporate from a liquid form - like gasoline.

With Dust, the NEC actually measures the size of the dust particles to determine Class II & III classifications. For simplicity one can say Class II is very fine dust, and Class III includes larger particles.

There isn't always a precise boundary between Class II and Class III categories, some dusts can include a mix of fine dust and larger particles.
If, for example, you are trying to determine the classification of a carpentry shop, you will find fine sawdust mixed with larger particles of wood.
What classification fits an area where most of the Fire hazard is larger wood fibers (Class III) and a far lesser percentage is fine sawdust (Class II)?
A good choice for the carpentry shop would be a classification that erred on the side of caution. It's not a matter of "What is the cheapest lighting classification?" but a matter of "What is the safest classification to eliminate the risk of ignition."

The location's Insurer will also need to approve the final classification, and if there is a reasonable chance of fine sawdust accumulation, they will likely want the higher classification for that location.

Once you certify an LED fixture for Haz Loc Dust Classification, even if it is totally dust-proof for fine particles, it cannot be used in Class I Locations.

A dust-proof light fixture will prevent dust from getting into the fixture, but it can't stop gases.

Next: Who Decides on a Class? Who Has the Power?
Bill Nagengast, President and Lighting Engineer, Solas Ray Lighting, holds over 20 patents in the lighting industry.
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