The system consists of fixing the printed circuit board (3) to the bottom (1) of the body of the box with the cooperation of a perimetral and spacing flap (6) which creates a chamber (7) under the board (3), in which the components (4) associated to the lower side of said board are housed, subsequently filling the outer perimetral space of said flap (6) with a resin mass (8), which is applied in liquid state and solidifies immediately afterwards, such that said resin (8) perfectly seals the chamber (7), while at the same time provides a thick protective layer on the upper side of the printed circuit board (3), also protecting the latter, which allows for the cover (2) of the box to have openings for ventilation of certain components (4') which generate a remarkable amount of heat.