ln all fields, but more especially in electricity, everybody is more and more worried about the security of users as weil as goods. This requires equipment with automatic switch off power supply as soon as an electric defect in a installation occurs. This protection is made by differential devices, where the main component consists in a high sensitivity polarized relay. The improvement of this protection requires a complete control of the characteristics of the relay. However the various parts the component present tolerances, during assembly those will involve drift of its electric characteristics in proportions that remains to be determined. The control of these drifts, requires new tools to quantify the parameters influent but also to improve electric properties of the relay. The first tool is an analytical modelling techniques which enables us ta apprehend the electromagnetic phenomena like saturation, the eddy currents, polarisation points of the magnet, as weil as the influence contact points between the fixed part and the moving one. The second is a numerical model using the finite elements techniques representation, with an original method which uses a two-dimensional formulation associating .. two cuts coupled Il to analyse three-dimensional devices.
