[en] In nuclear power plants, ungrounded low-voltage networks are frequently used. In this type of network it is also usual to have relays for insulation monitoring. These relays employ a saturable inductance of a high value in line with their measuring circuit, which could give rise to a ferro resonance phenomenon with the system capacitances. Ferro resonance could produce temporary overvoltage between each phase and earth which damages the insulation of the equipment connected to this network. This paper presents the theoretical bases of this phenomenon, the electrical resolution of the circuit and the actions which could be undertaken to eliminate or mitigate overvoltage. (Author)