Ground-fault resistance measurement circuit and ground-fault detection circuit

1. A ground-fault resistance measurement circuit which measures a resistance between a charge section insulated from a conductive frame body of an electrical apparatus and the frame body, comprising:

a capacitor which is connected between the charge section and the frame body;

a first switch which opens and closes a connection path between the charge section connected to the capacitor and the capacitor;

a charging power-source section which outputs a predetermined set voltage for charge;

a charging unit which charges the capacitor, using the predetermined set voltage output from the charging power-source section;

a voltage measurement section which measures a charging voltage of the capacitor;

a discharge control section which closes the first switch after charging the capacitor by the charging unit; and

a calculation section which calculates a resistance value between the charge section and the frame body, based on a change in the charging voltage measured by the voltage measurement section after closing the first switch by the discharge control section.

2. The ground-fault resistance measurement circuit according to claim 1 , wherein the calculation section calculates a resistance value R L between the charge section and the frame body, based on a value Vta indicative of a charging voltage of the capacitor as measured by the voltage measurement section at the time when the discharge control section closes the first switch and a value Vtb indicative of a charging voltage measured by the voltage measurement section at the time when a predetermined set time tb elapses after closing the first switch.

3. The ground-fault resistance measurement circuit according to claim 2 , further comprising:

a first resistance having a first resistance value R 1 disposed on a path from the capacitor to the charge section via the first switch; and

second and third resistances having a second resistance value R 2 and a third resistance value R 3 , respectively, which serve as voltage dividing resistances for dividing a charge voltage of said capacitor, wherein:

said voltage measurement section measures, as the charge voltage, a divided voltage divided by the voltage dividing resistances, and

said calculation section calculates the resistance value R L based on the electrostatic capacity C 1 of the capacitor and from the following expression:

R L =τ×( R 2 +R 3 )/{ C 1 ×( R 2 +R 3 )−τ}− R 1 ,

where τ=tb/(log Vta−log Vtb).

4. The ground-fault resistance measurement circuit according to claim 1 , wherein the calculation section calculates a resistance value R L between the charge section and the frame body, based on a first charging voltage Vta measured by the voltage measurement section at the time when a predetermined first set time ta elapses after the discharge control section closes the first switch and a second charging voltage Vtb measured by the voltage measurement section at the time when a predetermined second set time tb elapses after a lapse of the predetermined first set time.

5. The ground-fault resistance measurement circuit according to claim 4 , further comprising:

a first resistance having a first resistance value R 1 disposed on a path from the capacitor to the charge section via the first switch; and

second and third resistances having a second resistance value R 2 and a third resistance value R 3 , respectively, which serve as voltage dividing resistances for dividing a charge voltage of said capacitor, wherein:

said voltage measurement section measures, as the charge voltage, a divided voltage divided by the voltage dividing resistances, and

said calculation section calculates the resistance value R L based on the electrostatic capacity C 1 of the capacitor and from the following expression:

R L =τ×( R 2 +R 3 )/{ C 1 ×( R 2 +R 3 )−τ}− R 1 ,

where τ=(ta−tb)/(log Vta−log Vtb).

6. The ground-fault resistance measurement circuit according to claim 1 , further comprising:

a clock section which clocks time ta starting from a point of time when the discharge control section closes the first switch until a point of time the charging voltage measured by the voltage measurement section reaches a first predetermined set voltage V 1 or below, and a time tb starting from a point of time when the discharge control section closes the first switch until a point of time the charging voltage measured by the voltage measurement section reaches a second predetermined set voltage V 2 or below, said second predetermined set voltage V 2 being lower than said first predetermined set voltage V 1 ,

wherein,

the calculation section calculates a resistance value R L between the charge section and the frame body, based on the time clocked by the clock section.

7. The ground-fault resistance measurement circuit according to claim 6 , further comprising:

a first resistance having a first resistance value R 1 disposed on a path from the capacitor to the charge section via the first switch; and

second and third resistances having a second resistance value R 2 and a third resistance value R 3 , respectively, which serve as voltage dividing resistances for dividing a charge voltage of said capacitor, wherein:

said voltage measurement section measures, as the charge voltage, a divided voltage divided by the voltage dividing resistances, and

said calculation section calculates the resistance value R L based on the electrostatic capacity C 1 of the capacitor and from the following expression:

R L =τ×( R 2 +R 3 )/{ C 1 ×( R 2 +R 3 )−τ}− R 1 ,

where τ=(tb−ta)/(log V 1 −log V 2 ).

8. The ground-fault resistance measurement circuit according to claim 1 , wherein,

the charging unit includes a second switch which opens and closes a connection path between the charging power-source section and the capacitor, and

the discharge control section closes the second switch so that the charging power-source section charges the capacitor, and thereafter opens the second switch.

9. The ground-fault resistance measurement circuit according to claim 1 , wherein a resistance is disposed on a path from the capacitor through the first switch to the charge section.

10. The ground-fault resistance measurement circuit according to claim 1 , further comprising:

a voltage-division resistance which divides a charging voltage of the capacitor;

wherein the voltage measurement section measures a division voltage obtained from the voltage division by the voltage-division resistance.

11. The ground-fault resistance measurement circuit according to claim 1 , further comprising:

a display section which displays a resistance value calculated by the calculation section.

12. The ground-fault resistance measurement circuit according to claim 1 , wherein:

the electrical apparatus includes a battery insulated from the frame body; and

the charge section is an electrode of the battery.

13. The ground-fault resistance measurement circuit according to claim 12 , wherein:

the electrical apparatus is an electric automobile;

the battery supplies electric power to a mechanical-power motor of the electric automobile; and

the frame body is a body of the electric automobile.

14. A ground-fault detection circuit, comprising:

the ground-fault resistance measurement circuit according to claim 1 ; and

a decision section which decides that a ground fault is generated if the resistance value calculated by the calculation section is below a predetermined threshold value.

15. The ground-fault detection circuit according to claim 14 , further comprising, a notification section which gives notice of a decision result by the decision section.