An apparatus includes a controllable signal source anda processor. The controllable signal source is configuredto apply an Alternating Current (AC) signal to multipleelectrodes of a multi-electrode catheter immersed in anaquatic solution. The processor is configured to,responsively to the applied AC signal, estimate arespective surface impedance or a respective electricalnoise level of each of the electrodes. The processor isfurther configured to disconnect each electrode,independently of other electrodes, when the estimatedsurface impedance or electrical noise level of theelectrode drops below a preset value.
