The device is suitable for measuring the insulation resistance of de-energized devices and systems with rated voltages up to 1000 V, as well as for testing the resistance of grounding conductors, protective conductors, and equipotential bonding conductors, including their connections and terminals. The lamp in the test probe not only illuminates the measurement point but also provides a quick good-bad assessment of the insulation resistance. As long as it is lit, the minimum insulation resistance values according to VDE 0100 are being met.