Abstract: A system including a coupling device and a data processing device designed to supply an excitation signal (SE) so that an internal signal (?) occurs in the coupling device, this internal signal (?) having a deformation resulting from the current to be measured (I0), obtain a measured signal (Sm) for a variable (Vm) sensitive to the deformation, supply a compensation signal (Sc) in order to cancel out the deformation, and evaluate the current to be measured (I0) based on the compensation signal (Sc). The data processing device is furthermore designed to analyse the measured signal (Sm) and to detect a fault on the trans-mission line based on this analysis.