Understanding Requirements for Hazardous Locations, based on the 2017 NEC

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This thread requirement ensures that if an explosion occurs within a raceway or enclosure, the expanding gas will sufficiently cool as it dissipates through the threads. This prevents hot flaming gases from igniting the surrounding atmosphere of a hazardous (classified) location.
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I once repeated this comment at a class and the students ate me alive. Does a threaded raceway inhibit all gas flow outside the conduit or does the explosion pass through as described?
 
It’s accurate. The flame path through threaded joints and/or machined faces of enclosures cools the gases.

For conduit runs it also acts as a relief to reduce pressure piling. This is among the reasons “wrenchtight” and five or more threads engaged is specified in Section 500.8(E). It increases the length of the flame path without actually closing it.

Several major manufacturers have literature and films or videos describing these features. At least one can provide onsite demonstrations.
 
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