Rapid and controlled discharge of DC test objects, allowing for a completely safe and non damaging discharge of the test object is especially beneficial in case of test objects having a high capacitance, such as power cables. As the discharging is invoked remotely, no access to the HV area is required. The discharge is controlled by means of optical fibres to ensure reliable and EMC prove operation. With the test of very long cables at very high voltage levels the accumulated charge of the cables might give a health and safety risk in case of an emergency. The DCU provides an emergency discharge function that allows the lab or test operator to push a simple emergency button combination and be assured, that the cable is discharged within seconds, so safe access to the test lab or the test site is secured. DCU discharge units are available for laboratory usage and field usage up to 1200kV and higher.