Abstract: An apparatus embodiment comprises a voltage source, a measuring device, a controller, and at least two connecting elements. The connecting elements connect to an electrical supply network comprising at least a phase conductor and a neutral conductor. The voltage source applies at least a first voltage and a second voltage to a pair of the conductors of the electrical supply network. The measuring device measures the resistance between the pair of conductors. The controller compares a first resistance measured by the measuring device between the pair of conductors to a predetermined first threshold resistance value and detects that an electric device is connected to the electrical supply network when the first resistance is lower than or equal to the first threshold. When an electric device is not detected, the controller is further configured to perform a higher voltage insulation test of the electrical supply network.