The power grid is a particular case for electromagnetic compatibility and interferences risk management because it presents a significant coupling area. The profusion of an electronic equipment connected to the network requires a control of the couplings to prevent any incident induced by a weak immunity of electronic devices to natural electromagnetic interferences in the environment or induced by EW-, HPEM-sources. It is of fundamental interest to estimate the maximal voltage (or current) magnitude induced on the power socket. The influence of loads and the transfer of energy between the power grid and the appliances require the characterisation of the electric and electronic devices. A new approach to achieve this task is proposed.