A device for electric insulation of an electrically conducting plane capable of having a first electrical potential relatively to a second electric potential, the device comprising: - an insulating substrate including two planar surfaces (32, 34) parallel to each other, - a first outer layer, electrically conducting, substantially planar and positioned on a first planar surface of the insulating substrate, the first outer layer forming the electrically conducting plane able to be connected to the first potential and having a peripheral edge, the second surface being able to be connected to the second electric potential, - reducing means for reducing the value of the electrostatic field in a point of the peripheral edge of the outer layer, the reducing means including at least one electrically conducting area distinct from the first outer layer, positioned in the insulating substrate, the reducing means being able to reduce the value of the electrostatic field in this point relatively to the value of the electrostatic field in this point in the absence of the conducting area, the conducting area being capable of having an electric potential (Vi) with a value strictly comprised between the values of the first and second electric potentials, the reducing means include N conducting areas, N being greater than or equal to 2, each conducting area being capable of having a distinct electric potential (Vi) of a value strictly comprised between the values of the first and second electric potentials, the values of the electric potentials (Vi) of the conducting areas decreasing from the edge of the first outer layer towards the second surface, each conducting area including an intermediate layer, electrically conducting and positioned in the insulating substrate parallel to the first outer layer, the intermediate layer extending at least up to the equivalent in the plane of the intermediate layer, of the peripheral edge of the first outer layer, translated along a direction normal to the plane of the first outer layer, characterized in that each conducting area includes a guard ring positioned in the plane of each conducting layer (36, 40, 62) except for a central intermediate layer, the guard ring surrounding the corresponding conducting layer (36, 40, 62), the spacing between the peripheral edge (38, 42, 68) of the corresponding conducting layer (36, 40, 62) and the guard ring being equal to a predetermined distance, each guard ring being electrically connected to an intermediate layer having a potential different than the one of the conducting layer (36, 40, 62) around which it is laid out. - an insulating substrate including two planar surfaces (32, 34) parallel to each other, - a first outer layer, electrically conducting, substantially planar and positioned on a first planar surface of the insulating substrate, the first outer layer forming the electrically conducting plane able to be connected to the first potential and having a peripheral edge, the second surface being able to be connected to the second electric potential, - reducing means for reducing the value of the electrostatic field in a point of the peripheral edge of the outer layer, the reducing means including at least one electrically conducting area distinct from the first outer layer, positioned in the insulating substrate, the reducing means being able to reduce the value of the electrostatic field in this point relatively to the value of the electrostatic field in this point in the absence of the conducting area, the conducting area being capable of having an electric potential (Vi) with a value strictly comprised between the values of the first and second electric potentials,