Patent ID: 7737705

Claim:
A state detecting method adapted to an insulation resistance detector comprising: a detecting resistor connected in series to an insulating resistor interposed between a ground and a direct current power supply; a coupling capacitor interposed between the insulating resistor and the detecting resistor; a pulse signal applying member for applying a first pulse signal at frequency 2.5 Hz and a second pulse signal at 4.5 Hz to a series circuit composed of the insulating resistor, the coupling capacitor, and the detecting resistor; a filter for filtering a specific frequency and outputting a node voltage between the coupling capacitor and the detecting resistor; and a reduction detecting member for detecting a reduction of the insulating resistor based on the output of the filter, said method comprising the steps of: calculating a difference between the output of the filter when the first pulse signal having a first pulse width is applied to the series circuit, and the output of the filter when the second pulse signal having a second pulse width shorter than the first pulse width is applied to the series circuit; and detecting the state of the insulation resistance detector based on the calculated difference.