A wiring testing device configured to test the wiring correctness of an inverter is disclosed. The inverter is configured to be connected to a low-voltage three-phase system with a neutral conductor serving as a reference point for grid-side conductor voltages. The wiring testing device includes a test circuit configured to measure the grid-side conductor voltages and determine an average value from the sum of the instantaneous values of the grid-side conductor voltages, and a display unit configured to display an error message if the average value exceeds a threshold voltage for a defined time period. The wiring testing device further includes a capacitor connected between a phase of the three-phase system and the neutral conductor configured to generate a voltage imbalance if the neutral conductor connection is missing, wherein the test circuit is configured to measure the conductor voltages and determine the voltage imbalance.