When a high voltage transient appears across the fieldbus lines the TVS diode turns on very quickly (in nanoseconds) and limits voltage seen by control cards to the nominal clamp voltage. The gas discharge tube may take up to 5us to turn on and during this time the TVS diode dissipates a high power pulse. Once the gas discharge tube turns on it clamps voltage across the lines to between 20-70V and steers the high current transient back to the field. The gas discharge tube is rated to handle very high current levels – up to 20kA for 20us. After the transient has expired the gas tube returns to its high impedance state (>10Gohm from each line to earth).