Recessed box for wall mounting made of special steel (Magnelis) for mounting of VD 500 in walls of concrete or in other hard surfaces. The wall box is intended for mounting the VD 500 into a wall or ceiling and consists of a polystyrene block which is mounted on a metal mounting plate. The mounting plate has threaded mounting holes for the VD 500. The wall box is mounted in connection with the casting of the walls and ceilings that the VD 500 is intended to protect. The polystyrene block keeps the concrete away when casting so that space is given to the VD 500. Conduit pipes are drawn before casting so that the wiring is hidden and protected. The mounting plate is attached to the reinforcement bars to best capture the signals formed during an attack. When the casting is complete, the polystyrene block is removed and the VD 500 can be mounted. The hole after the polystyrene block is then covered with a cover that is screwed into two holders which are located on the mounting plate. The cover is included in the mounting kit to provide a nice and discreet installation.