State detecting method and insulation resistance fall detector

1. A method of detecting a state of an insulation resistance fall detector that includes: a detection resistance connected in series to an insulation resistance between a vehicle body and a DC power source; a coupling condenser provided between the insulation resistance and the detection resistance; a pulse signal applying unit that applies a pulse signal to a serial circuit constituted by the insulation resistance, the coupling condenser and the detection resistance; a filter that removes a signal component of a predetermined frequency from a voltage of a connecting point between the coupling condenser and the detection resistance; and a fall detecting unit that detects a fall of the insulation resistance on the basis of the output of the filter, the method comprising the steps of:

varying a pulse width of the pulse signal; and

detecting the state of the insulation resistance fall detector on the basis of the output of the filter when the pulse width is varied.

2. The method according to claim 1 , wherein the pulse width of the pulse signal is varied so as to be shorter than a pulse width outputted when a detection is carried out by the fall detecting unit.

3. The method according to claim 2 , wherein a normal state of the insulation resistance fall detector is detected if the output of the filter is considered to change at the time of varying the pulse width so as to be short.

4. The method according to claim 3 , wherein

the pulse signal of a first pulse width is outputted upon detection by the fall detecting unit,

the pulse signal of a second pulse width shorter than the first pulse width is outputted upon detection of the state of the fall detecting unit,

when the output of the filter upon outputting the pulse signal of the first pulse width is not lower than a first predetermined value and the-output of the filter upon outputting the pulse signal of the second pulse width is not higher than a second predetermined value smaller than the first predetermined value, a normal state of the insulation resistance fall detector is detected.

5. The method according to claim 2 , wherein a failed state of the insulation resistance fall detector is detected if the output of the filter is considered to be a third predetermined value or higher and have no variation at the time of varying the pulse width so as to be short.

6. The method according to claim 5 , wherein the failed state of the insulation resistance fall detector is detected if the output of the filter is considered to be not higher than a fourth predetermined value smaller than the third predetermined value and have no variation at the time of varying the pulse width to be short.

7. The method according to claim 1 , wherein a duty of the pulse signal is varied to vary the pulse width.

8. An insulation resistance fall detector for detecting an insulation resistance between a vehicle body and a DC power source, the insulation resistance fall detector comprising:

a detection resistance connected in series to the insulation resistance;

a coupling condenser provided between the insulation resistance and the detection resistance;

a pulse signal applying unit that applies a pulse signal to a serial circuit constituted by the insulation resistance, the coupling condenser and the detection resistance;

a filter that removes a signal component of a predetermined frequency from a voltage of a connecting point between the coupling condenser and the detection resistance;

a fall detecting unit that detects a fall of the insulation resistance on the basis of the output of the filter;

a pulse width varying unit that varies a pulse width of the pulse signal; and

a state detecting unit that detecting a state of the insulation resistance fall detector on the basis of the output of the filter when the pulse width is varied.

9. The insulation resistance fall detector according to claim 8 , wherein the filter includes a low pass filter.