Method and test circuit for testing isolated cable sections for earth fault

摘要

In a method and a test circuit for testing isolated cable sections for earth fault, particularly medium- or high-voltage cables which can be open at their end or can be connected by loads, it should also be possible reliably to detect high-impedance and voltage-dependent insulation faults and reliably to prevent the power system voltage from being connected when insulation faults have been found. For this purpose, the cable is continuously charged up by a charging current (IL) to a voltage (UL) the amplitude of which corresponds to that of the power system voltage before the power system voltage is connected. At the same time, a current (IV) proportional to the charging current (IL) charges up a comparison circuit (10), the voltage (UV) of which is continuously compared with a voltage (UM) corresponding to the charging voltage (UL) of the cable. When a nominal measurement voltage (UN) proportional to the nominal power system voltage is reached (UM /= UN) by the voltage (UM), the test process is ended and the connection of the power system voltage is enabled whilst, when the voltage (UM) drops below (UM UV) the reference voltage (UV), the test process is immediately aborted, the connection of the power system voltage is disabled and, at the same time, the existence of an earth fault is indicated. IMAGE