The objectives of this research were to develop, design, fabricate, and demonstrate two permissible prototype quick-entry explosion- proof electrical enclosures, one large and one small, for use in coal mines. The decision was made to proceed with a conventional enclosure using a ground explosion-proof joint between cover and body, but with a circular access employing a threaded joint installed in the cover. Using this concept, the circular access would be used for routine maintenance, whereas the entire cover would be separated at the ground joint for initial installation or major equipment replacement. Several different threads were investigated for the circular access. A multiple start thread was the one selected to best suit the mining environment. This thread had the advantage of being comparatively easy to machine, inherently safe as any thread, and self cleaning. This design permitted the elimination of a special inspection device since the locking device designed for the installation would not operate unless the circular cover was properly installed.