SIDEBAR: The Rules for Threaded Conduit [500.8(E)]
This 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 location.
Keep in mind — it’s assumed that the flammable atmosphere outside the raceway will seep into the raceway system over time. The goal of the Code is to contain any explosion that occurs inside the raceway so the event won’t ignite the flammable mixture outside the raceway system.