Test object insulation characteristics determn. method for esp. cable or cable system

The electrical variable in time in the frequency range and/or diminishing when flowing through the object is determined, with the variable(s) being measured at a capacitance (Cx), which represents the object under test or a part of it. The electrical variable is then Fourier transformed. The Fourier spectrum resulting from the Fourier transformation is analysed, and esp. the spectrum obtained is compared with a Fourier spectrum obtained previously or with a reference spectrum. The test circuit also includes a transformer (1), a power supply system (3) and a control or regulating unit (4).