Abstract: An explosion proof vault for housing electrical equipment is isolated from ambient air by the provision of an auxiliary expansion chamber. The auxiliary expansion chamber is coupled to the explosion proof casing and an inelastic bladder is located in the auxiliary expansion chamber in fluid communication with the explosion proof casing. An airtight seal around the mouth of the bladder prevents the passage of air between the explosion proof casing and the auxiliary expansion chamber except within the bladder.

Abstract: A double lid assembly is provided for a pre-fabricated pit designed for installation below the surface of an aircraft docking and refueling area. A first, heavier lid is mounted by hinges on the pre-fabricated pit for rotation about a horizontal axis. A second, lighter lid is seated within the confines of the first lid and is rotatable about a second axis parallel to the first. When the second, lighter lid is opened, it reduces the force necessary to open the first, heavier lid by reducing the torsional moment holding the heavier lid shut. As the heavier lid is opened, the lighter lid serves as a counter-balance to facilitate opening the heavier lid.

Abstract: A servo power system is provided for the heavy lid of a pit designed for installation below the surface of an aircraft docking and refueling area. For one embodiment, a counterbalancing system is shown. The lid is mounted for rotation at the top of a pit wall about lid hinges lying along a horizontal axis. A counterweight is secured for rotation about a horizontal counterweight axis which is located below and parallel to the lid axis. Meshed gear sections on the lid and on the counterweight couple the lid and counterweight to move together in counter-rotation relative to each other. The weight of the lid is largely balanced by the weight of the counterweight, thereby facilitating opening and closing of the pit lid.