The insert includes at least three contacts having essentially linear parts and at least one three-pole capacitance between three of the contacts. One of the contacts of each three-pole capacitance is connected to a central armature. A first dimension of the central armature, in the direction perpendicular to the substantially linear parts, is greater than a second dimension, in a direction parallel to the substantially linear parts, the second dimension defining the widths of the zones of the central armature. The mean width of the central armature, between the zones where it faces other armatures, connected to the other contacts of the three-pole capacitance, is greater than one third of the mean length of the central armature in these regions. Preferably, in at least one three-pole capacitance, the mean width of the central armature between the regions where it faces the lateral armatures is greater than one third of the distance between the lateral armatures.