Abstract: An electronic device placed in series with the probe ground-to-chassis connection. The device comprises a current-detecting element such as a resistor or RC network and may employ frequency-discriminating circuitry to limit only ground current signals below a certain frequency. The detector element triggers either a current limiter or a latching switch to prevent currents exceeding about 20 .mu.A rms from flowing between the probe and the patient. This is achieved by limiting such currents dynamically or by latching the transducer-probe-ground/system-chassis connection in an "open" state. In normal operation, the limiting/latching function is inactive, i.e. it is transparent to the system and the device shunts noise signals to the system chassis to prevent image degradation. The resistance of the device in normal operation may be in the range of about 0.1 to 10 ohms for effective noise reduction (shielding) at operating frequencies at and above 100 kHz.