Patent ID: 7129711

Claim:
A device for controlling the manufacture of a medium or high voltage cable having an inner conductor, a sheath of insulating material, an inner field controlling layer between the conductor and the insulating sheath, and an outer semi-conducting layer on the insulating sheath, the individual layers being co-extruded on the conductor, the device comprising a relatively short, cylindrical measuring electrode encircling the cable, a high frequency voltage source connected to the measuring electrode to establish a high frequency voltage between the measuring electrode and the inner field controlling layer and the conductor, respectively, cylindrical equipotential electrodes at both ends of the measuring electrode encircling the cable and connected to the voltage source, the equipotential electrodes having a length which is a multiple of the length of the measuring electrode, the equipotential electrodes and the measuring electrode are dimensioned such and/or the potential of the equipotential electrodes is selected such that in the area of the measuring electrode a substantially homogeneous radial electric field or an electric field with concentrated flux lines between the measuring electrode and the inner field controlling layer or the conductor is generated, the measuring electrode, the equipotential electrodes and the cable being immersed in water, and a measuring device which measures the current between the measuring electrode and the inner field controlling layer or the capacitance of the sheath, and comparing the measured values with a predetermined threshold value and generates a failure signal if owing to an interruption of the inner field controlling layer or the outer semi-conducting layer the measured value for the current or the capacity deviates from the predetermined value a predetermined extent, wherein the proportion of the length of the equipotential electrodes to the length of the measuring electrode is adapted to the proportion of the specific resistance of the outer semi-conducting layer to the resistance of water.