Patent ID: 8860426

Claim:
An insulation state detection circuit for an ungrounded power source, comprising: a first charging circuit that includes a first series circuit having a first resistor, a second resistor and a capacitor and charges the capacitor in an amount of electric charge according to a power source voltage of a DC power source insulated from a ground potential portion by connecting a positive terminal of the DC power source and a negative terminal of the DC power source through the first series circuit over a predetermined time; a second charging circuit that includes a second series circuit having the first resistor and the capacitor and charges the capacitor in an amount of electric charge according to a ground fault resistor of a negative terminal side of the DC power source by connecting a main circuit wiring of the positive terminal side of the DC power source and the ground potential portion through the second series circuit over the predetermined time; and a third charging circuit that includes a third series circuit having the second resistor and the capacitor and charges the capacitor in an amount of electric charge according to a ground fault resistor of a positive terminal side of the DC power source by connecting a main circuit wiring of the negative terminal side of the DC power source and the ground potential portion over the predetermined time, wherein a charging resistance value in the case that the capacitor is charged in the first charging circuit coincides with a charging resistance value in the case that the capacitor is charged in the second charging circuit when a resistance value of the ground fault resistor of the negative terminal side is a ground fault alarm threshold value of the negative terminal side corresponding to an insulation state of an alarm level; wherein the charging resistance value of the first charging circuit coincides with a charging resistance value in the case that the capacitor is charged in the third charging circuit when a resistance value of the ground fault resistor of the positive terminal side is a ground fault alarm threshold value of the positive terminal side corresponding to an insulation state of an alarm level; wherein at least one of the second charging circuit and the third charging circuit includes an adjusting resistor which is not present in the first charging circuit; and wherein the adjusting resistor is connected in series with at least one of the second series circuit of the second charging circuit and the third series circuit of the third charging circuit.