A system is provided for verifying the performance of an electrical union. The electrical union provides electrical connectivity across an interface between first and second elements which rotate relative to each other. The system includes a signal source associated with the first element. The signal source is adapted to send an electrical signal over the union to the second element using a first signal frequency. The system further includes a frequency modulator associated with the second element. The frequency modulator is adapted to return the electrical signal over the union to the first element using a different second signal frequency. The system further includes a comparator which compares the signal sent by the data source to the signal returned by the frequency modulator. The comparator provides an output which identifies whether a discrepancy exists between the contents of the sent and returned signals.