Patent ID: 8198902

Claim:
An insulation state detector that is connected to a primary side of a voltage boosting power source circuit, which boosts a positive electric potential of a DC power source insulated from a ground potential portion and supplies the boosted positive electric potential to a load, comprising: a flying capacitor that is charged with a voltage corresponding to a ground fault resistor by the DC power source; a measurement section that measures a charging voltage of the flying capacitor; a measuring circuit that connects the flying capacitor, which is insulated from the DC power source after being charged by the DC power source, in series between the measurement section and the ground potential portion; a detecting section that detects an insulation state of the voltage boosting power source circuit based on a measurement result of the charging voltage of the flying capacitor obtained by the measurement section; and a reversed-polarity measuring circuit that connects the flying capacitor to the measurement section and the ground potential portion with reversed polarity when an electric potential corresponding to a division ratio of a positive-side ground fault resistor and a negative-side ground fault resistor on a secondary side of the voltage boosting power source circuit exceeds a positive electric potential of the DC power source and the flying capacitor is charged with reversed polarity by a secondary-side positive electric potential at the time of charging of the flying capacitor using the positive electric potential of the DC power source.