The correct answer isD. Forexplosionproof threaded conduit joints,API RP 540requires a minimum offive fully engaged threads. This requirement is critical because explosionproof joints are intended tocontain an internal explosionand prevent flame propagation into the surrounding hazardous atmosphere. The threaded path acts as a controlled flame path, cooling and quenching hot gases before they can exit the enclosure or conduit system at an ignition-capable temperature.
If too few threads are engaged, the flame path may be inadequate, reducing the joint's mechanical strength and compromising its ability to contain pressure and hot gases generated by an internal fault or ignition event.
In hazardous locations, this becomes a major safety issue, especially for conduit-connected equipment such asjunction boxes, motors, control panels, and other Class I installations. During source inspection and quality surveillance, the inspector should verify not only that the correct explosionproof fittings are used, but also that the conduit joints are properly assembled with the required thread engagement and in accordance with the governing hazardous-area standard and approved drawings. Therefore, the minimum required number of fully engaged threads isfive, makingoption Dthe verified answer.