Patent ID: 7994799

Claim:
An insulation resistance detection system comprising: an insulation resistance drop detecting portion comprising: a detecting resistor connected in series with an insulating resistor between a DC power supply and a vehicle body; a coupling capacitor disposed between the insulating resistor and the detecting resistor; and a pulse oscillating circuit for applying a pulse signal to a series circuit including the insulating resistor, the coupling capacitor, and the detecting resistor; changing means for changing a common mode potential which is a potential at a junction between the insulating resistor and the coupling capacitor; a filter applied to a signal at a junction between the coupling capacitor and the detecting resistor to pass the signal within a predetermined frequency range; and failure determining means for determining a failure of the insulation resistance drop detecting portion based on a comparison result obtained by comparing a change of the filter-applied signal before and after the common mode potential is changed by the changing means.