An apparatus includes a controllable signal source and a processor. The controllable signal source is configured to apply an Alternating Current (AC) signal to multiple electrodes of a multi-electrode catheter immersed in an aquatic solution. The processor is configured to, responsively to the applied AC signal, estimate a respective surface impedance or a respective electrical noise level of each of the electrodes. The processor is further configured to disconnect each electrode, independently of other electrodes, when the estimated surface impedance or electrical noise level of the electrode drops below a preset value.
