A testing device for testing a high or medium-voltage cable using a very low frequency testing method, comprising: a circuit arrangement with - a test voltage generator that comprises a low-voltage-side earthing input and a high-voltage-side output and is configured to provide a variable test voltage at the high-voltage-side output, - a power connection for connecting the test voltage generator to a supply voltage source, - a test specimen connection for connecting the high-voltage-side output to a conductor of the high or medium-voltage cable, in particular via a measuring connection cable, - a protective earth connection for connecting to protective earth, - a connecting conductor that electrically connects the low-voltage-side earthing input to the protective earth connection, so that the connecting conductor represents a current collection point through which a measuring current flows during the test of the high- or medium-voltage cable, which current is formed between the low-voltage-side earthing input and a shield of the high or medium-voltage cable, which shield is also connected to protective earth, - a low-frequency signal pick-up at the current collection point, at which a low-frequency diagnostic signal is generated based on the measuring current, and - a high-frequency signal pick-up at the current collection point, at which a high-frequency pre-location signal for a measurement of a transient high-frequency oscillation is generated based on the measuring current, and an evaluation electronics that - is connected to the low-frequency signal pick-up for receiving the low-frequency diagnostic signal and is configured for a VLF phase rotation measurement, and - is connected to the high-frequency signal pick-up for recording the high-frequency pre-location signal and is configured for defect distance evaluation. a circuit arrangement with - a test voltage generator that comprises a low-voltage-side earthing input and a high-voltage-side output and is configured to provide a variable test voltage at the high-voltage-side output, - a power connection for connecting the test voltage generator to a supply voltage source, - a test specimen connection for connecting the high-voltage-side output to a conductor of the high or medium-voltage cable, in particular via a measuring connection cable, - a protective earth connection for connecting to protective earth, - a connecting conductor that electrically connects the low-voltage-side earthing input to the protective earth connection, so that the connecting conductor represents a current collection point through which a measuring current flows during the test of the high- or medium-voltage cable, which current is formed between the low-voltage-side earthing input and a shield of the high or medium-voltage cable, which shield is also connected to protective earth, - a low-frequency signal pick-up at the current collection point, at which a low-frequency diagnostic signal is generated based on the measuring current, and - a high-frequency signal pick-up at the current collection point, at which a high-frequency pre-location signal for a measurement of a transient high-frequency oscillation is generated based on the measuring current, and - a test voltage generator that comprises a low-voltage-side earthing input and a high-voltage-side output and is configured to provide a variable test voltage at the high-voltage-side output, - a power connection for connecting the test voltage generator to a supply voltage source, - a test specimen connection for connecting the high-voltage-side output to a conductor of the high or medium-voltage cable, in particular via a measuring connection cable, - a protective earth connection for connecting to protective earth, - a connecting conductor that electrically connects the low-voltage-side earthing input to the protective earth connection, so that the connecting conductor represents a current collection point through which a measuring current flows during the test of the high- or medium-voltage cable, which current is formed between the low-voltage-side earthing input and a shield of the high or medium-voltage cable, which shield is also connected to protective earth, - a low-frequency signal pick-up at the current collection point, at which a low-frequency diagnostic signal is generated based on the measuring current, and - a high-frequency signal pick-up at the current collection point, at which a high-frequency pre-location signal for a measurement of a transient high-frequency oscillation is generated based on the measuring current, and an evaluation electronics that - is connected to the low-frequency signal pick-up for receiving the low-frequency diagnostic signal and is configured for a VLF phase rotation measurement, and - is connected to the high-frequency signal pick-up for recording the high-frequency pre-location signal and is configured for defect distance evaluation. - is connected to the low-frequency signal pick-up for receiving the low-frequency diagnostic signal and is configured for a VLF phase rotation measurement, and - is connected to the high-frequency signal pick-up for recording the high-frequency pre-location signal and is configured for defect distance evaluation.