Methods and apparatus for monitoring and actively reducing leakage currents flowing on patient applied parts used in ablation therapy. In an example, a signal-processing circuit connected between a toroidal-coil sensor and a sleeve-capacitor coupler, both AC-coupled to the catheter cable, applies a Fourier transform and an energy minimization algorithm to the output of the toroidal-coil sensor to determine amplitudes and phases for frequency components of the signal applied to the sleeve-capacitor coupler. A corresponding current coupled through the sleeve-capacitor coupler into the catheter cable counteracts the leakage current to force the total non-therapy electrical current flowing on the patient applied parts to a level that is lower than a fixed threshold value, e.g., selected in accordance with an applicable standard.